Time Scales

A definition for the term "time scale":

16th General Assembly of the International Astronomical Union
Grenoble, France (1976)
Resolution No. 4 by Commissions 4 (Ephemerides) and 31 (Time)
a useful time scale is generated by any process which enables dates to be assigned to events

The concept of time has been refined throughout history, and new understanding usually produces a new time scale. The list in this web page aims mostly at describing when and why new time scales were developed. At the end of this web page are links to other web pages which better explain the definitions of various time scales.

Many visits to this page are prompted by APIs used in computer languages that want to handle elapsed time. Terser than the many details here I point to the fundamental problem which confounds POSIX time_t and other computer implementations of time.

I have some plots of some of these time scales.

Earth rotation time
Historically the most obvious indicator of the passage of time has been the diurnal cycle of earth rotation. The day is the fundamental element of all calendars.

The rotation of the earth under the moon and sun produces tides. Reckoning time by tides is sufficiently simple that rocks can do it. Sedimentary rocks called tidal rhythmites contain records of earth paleorotation dating back three billion years.

Apparent Solar Time
Local Time -- sometimes LT

Apparent Solar Time has been used since prehistory. It is reckoned at any location by observation of obvious phenomenon such as sunrise, sunset, or passage through the meridian (noon). Reckoning time by sunrise and sunset is sufficiently simple that plants can do it.

Reckoning time by local apparent noon is sufficiently simple that anyone with a stick in the ground can do it. The ellipticity of the earth's orbit and the obliquity of the earth's equator produce a variation in the duration of a day reckoned by apparent noon. The variation is known as the "equation of time", and it is large enough that a pendulum clock is stable enough to measure it. The "equation of time" is best visualized in combination with the annual variation in the latitude of the sun in order to produce the analemma.

The earliest almanacs tabulated the "equation of time". Until 1930 in the British Nautical Almanac and 1935 in the American Ephemeris and Nautical Almanac the position of the sun was tabulated at apparent noon as well as at mean noon.

Mean Solar Time -- sometimes MST
The principles behind mean solar time and the "equation of time" were known to Ptolemy. By the middle of the 19th century personal timepieces were sufficiently accurate and sufficiently widespread that civilization began to run on mean solar time instead of apparent solar time. In this case "mean" means the average obtained by considering the rotation of the earth in the absence of the annual variations caused by obliquity and eccentricity. Telegraphs were particularly strong drivers of mean solar time because they began to require synchronization of human activities over large distances.
Julian Day Number
A decimal integer count of consecutive days beginning on January 1, 4713 B.C in the proleptic Julian calendar. The Julian Period of 7980 years is a combination of the 19-year Metonic cycle of 235 months, the 28-year cycle of the Julian calendar, and the 15-year cycle of the Roman indiction. The scheme was invented by Scaliger in 1582 who realized that all three cycles were in year 1 during that conveniently prehistoric year. (Note that the ISO 8601 format used elsewhere in this document is inappropriate here because it specifies the Gregorian calendar where the year is always positive.)
Julian Date -- JD in 1849
John Herschel published Outlines of Astronomy suggesting that astronomers (who preferred to use the same date for all of the observations of a single night) should adopt JD as an indication of the number of mean solar days (and decimal fractions thereof) elapsed since JD 0.0 which was at Greenwich mean noon of -4712 January 1 (using the astronomical reckoning of the years in the proleptic Julian calendar). In keeping with Ptolemy, Herschel suggested that the origin of counting should correspond to noon on the meridian of Alexandria.
Julian Date -- JD from 1895/1901
In 1895 Simon Newcomb published his Tables of the Sun. In 1896 the directors of the principal national ephemerides met in Paris and agreed to adopt those conventions and expressions beginning in 1901. Newcomb used Greenwich as the prime meridian for determining noon on the calendar date. Therefore by 1901 there could no longer be any question that the convention for the use of Julian Date had changed to begin at noon of the Greenwich meridian.
Modified Julian Date -- MJD
In keeping with civil usage and the International Meridian Conference of 1884 where days are reckoned from midnight, and also for the sake of convenience of not handling such large numbers, the MJD was defined in the 1950s as (JD - 2400000.5). MJD 0.0 corresponds to 1858-11-17T00:00:00. (Two URLs which attribute the origin of MJD to artificial satellite tracking at SAO are found in Austria and Australia.)

The IAU has had a love/hate/love relationship with MJD.

MJD at the 1973 IAU 15th GA in Sydney
Resolution 4 indicated that MJD should be used to count mean solar days and gave a specific definition with the hope that all agencies would adopt the same usage.
MJD at the 1994 IAU 22nd GA in The Hague
In Resolution C3 Commissions 26, 27, 30, 42 "Do deplore the introduction of the Modified Julian Day system" and "recommend the rescinding of resolution no 4 of the XVth General Assembly of the IAU that established the Modified Julian Day system".
MJD at the 1997 IAU 23nd GA in Kyoto
Finally (?) at the IAU 23rd General Assembly in 1997 Resolution B1 recognized that when properly defined the term MJD may be used. (This looks like another case of capitulation to practical reality akin to that of the 1935 IAU resolution regarding GMT which basically admitted that no action by the IAU could prevent the use of the term.)
Subsequent to the creation of time scales other than Greenwich Mean Time, forms of JD and MJD expressed in those time scales have also been used to indicate elapsed time measured in ephemeris days and in multiples of 86400 SI seconds. As recommended by the IAU in 1997 any current application which requires precision better than one minute, or any historical application which requires precision better than several hours, should take care to indicate which time scale is associated with the use of JD or MJD.

It should also be noted that the use of JD or MJD for the UTC time scale is problematic and ambiguous at the precision of one second. JD and MJD express the elapsed count of some form of "day" as real numbers along a presumably unsegmented, continuous number line. The UTC time scale (and, historically, GMT as used in practical situations before the advent of UTC) contains changes in rate and discontinuities. In particular, there is no obvious way to represent a leap second of UTC (or the smaller leaps present in the available forms of GMT and UTC before 1972) using JD or MJD notation.

Local Mean Time -- LMT
Local Mean Time is the Mean Solar Time for any given longitude around the earth, so it differs at every longitude. Anyone with a stick in the ground, an almanac, a calendar, and a clock can reset that clock to Local Mean Time every day at local apparent noon.
LMT in ancient astronomy
Ptolemy described the principles of mean solar time in Book III, Chapter 9 of his Almagest around year 150. The quarter-hour variation of the arrival of sun at meridian was an impractical detail before the advent of pendulum clocks with good escapements.
LMT in 1665
Recognizing that pendulum clocks had become good enough to be useful for solving the problem of marine navigation Christian Huygens published a treatise on finding longitude with clocks that included a table for the equation of time. The table uses the Gregorian calendar.
LMT in 1669
The treatise by Huygens was translated to English and published by the Royal Society of London. The table of the equation of time uses the Julian calendar.
LMT in 1672
John Flamsteed, the first Astronomer Royal, published his tables for the equation of time. He had created the tables in 1667, but he delayed before publishing. He began using them with precision clocks in the Royal Greenwich Observatory when he took residence in July 1676, and that incepted GMT.
Standard Time
Standard Time is the mean solar time of some conventionally chosen, standard (and hopefully nearby) meridian.

The needs of railroad schedules produced the adoption of standard time zones within which every railway station clock shared the same mean solar time. The UK railways began to adopt Standard Time in 1840, and the US railways adopted Standard Time in 1883.

Daylight Saving Time -- DST
Summer Time
Cynically, daylight saving time is what governments decree to promote productivity by hurrying people off to work earlier so that they don't have time to see what a nice day it is and decide to go fishing instead.
Realistically, the general public almost everywhere greatly enjoys getting up earlier because that allows extra sunlight after the end of the work day.
Cosmic Time
A term used by some of the delegates to the 1884 International Meridian Conference in Washington to designate the time on the prime meridian. Other delegates employed the term Universal Time when describing the same concept.
Greenwich Mean Time -- GMT

The mean solar time of the Greenwich meridian. Sadler (1978) wrote a monograph on the sordid history of GMT which should be read by everyone. Beyond that are a few other details below.

Prior to the long sequence of usages below it is relevant to point out one thing:
GMT does not now have, and never has had, leap seconds.
(Except when what was called GMT did have leap seconds, which was for the first few years starting 1972-01-01 when the radio broadcast time signals switched from the old rubber/elastic seconds version of coordinated time to the new leap second version of coordinated time. The US NBS broadcasts of WWV and WWVB and the UK broadcasts did this. In the case of the US this can be attributed as a misnomer associated with the fact that there had been no official documents giving the name UTC. In the case of the UK the BBC newsreaders still employ the term GMT. For the sordid history of the confusion between GMT and UTC in other nations see preprint 11-662 at the 2011 colloquium "Decoupling Civil Timekeeping from Earth Rotation" and preprint 13-505 at the 2013 colloquium "Requirements for UTC and Civil Timekeeping on Earth")
For the only time scale which really has leap seconds see UTC.
Nevertheless, the practical and historical reality is that the available forms of GMT have always had leaps, just not leap seconds. The leaps happened every time any official clock was reset to agree with calendar days of earth rotation.

Some agencies currently assert that GMT is currently used as a synonym for UTC. Below is a history of meanings which have been applied to GMT. There is no current definition of GMT which is authoritative in all contexts.

GMT started during 1675/1676
On 4 March 1674 ( Julian calendar, Old Style corresponding to 1675-03-14), King Charles II appointed John Flamsteed as the first Astronomer Royal. In June 1675 a warrant founded the Royal Greenwich Observatory, and the foundation stone was laid in August. In July 1676 Flamsteed began residence in the Observatory.

References to GMT before this time cannot have had any contemporary meaning. They are probably better interpreted as indicating what is now known as UT.

in the British Royal Navy until 1805
The time of the Greenwich meridian with the hours reckoned from midnight and days reckoned from noon.
in the British Nautical Almanac from around 1780 until 1833
The data were tabulated according to the apparent solar time of the Greenwich meridian.
GMT starting in 1834
Noting that chronometers were already in use by most ships which relied on astronomical observations for navigation, and in conformity with the report from a committee of "the most distinguished navigators and astronomers of the empire", the British Nautical Almanac began to tabulate phenomenon based on mean solar time.
GMT in 1840-11
The Great Western Railway adopted GMT as its standard time. Other railways and telegraph systems followed suit. Within a generation GMT was legal civil time in the UK.
GMT in civil contexts
The mean solar time of the Greenwich meridian with the hours and days reckoned from midnight.
GMT on 1883-11-18
The US and Canadian railways adopted standard time zones based on the mean solar time of meridians spaced at 15 degree intervals west of Greenwich.
GMT in 1884-10

The mean solar time of Greenwich attained a unique distinction at the International Meridian Conference held in Washington during October. By international vote it became the one time, the cosmic time, the universal time which most nations agreed to use.

The exact wording of the protocols had a curious effect in England because it indicated that the transit instrument then in use at the Greenwich Observatory, the Airy Transit, should define the prime meridian. All of the maps of England, however, used the meridian occupied by the previous transit instrument at Greenwich, the Bradley Transit. By strict interpretation of the International Meridian Conference (IMC) results, all maps of England suddenly had incorrect values of longitude. This change of longitude origin went unrecognized until 1956.

GMT in 1895/1896
In 1895 Simon Newcomb produced expressions which gave a precise definition of the "fictitious mean sun".
Newcomb's expressions were adopted for use starting in 1901 at a conference of the directors of the principal national ephemerides in Paris in 1896.
These expressions were the mechanism for converting sidereal time to universal time until 1984.
GMT from 1903 through 1920
During the late 19th century it had become common for observatories to connect their clocks to telegraph lines and transmit time signals regionally. Early in the 20th century various national time bureaus began to emit radio broadcasts of time signals for the sake of setting the clocks globally, and especially for ships at sea.
In 1911 the Observatoire de Paris began to monitor radio broadcasts of time signals and compare them.
In 1912 la Conference Internationale de l'Heure de Paris recommended the creation of an international time bureau to collect and compare the observations of time broadcasts from all over the world.
The Bureau International de l'Heure (BIH) was created in 1913, but the Great War interfered with international aspects.
The official organization of the BIH occurred in 1919 under the auspices of the new International Astronomical Union (IAU), and the BIH was in full operation during 1920.
The BIH routinely monitored radio broadcast time signals, published their differences, and studied the issues involved in determining longitude and mean solar time.
GMT on 1918-03-19
Although the term GMT was not explicitly used, the standard time for legal purposes in the United States specified by 15USC261 (the Calder Act) was defined to be based on the mean astronomical time of meridians spaced at 15-degree intervals west of Greenwich; i.e., GMT.
GMT in astronomy before 1925
The mean solar time of the Greenwich meridian with the hours and days reckoned from noon.
GMT in the British astronomical almanac before 1925
The mean solar time of the Greenwich meridian with the hours and days reckoned from noon.
GMT in the British astronomical almanac beginning in 1925
The mean solar time of the Greenwich meridian with the hours and days reckoned from midnight.

Within the pages of the British Nautical Almanac and Astronomical Ephemeris the time 00:00:00 GMT had previously meant noon, but starting this year 00:00:00 GMT now meant midnight. Sadler (1978) indicated that the use of the same name "GMT" for a quantity entirely opposite in meaning to the original term was by order of the Admiralty.

GMT according to the IAU beginning in 1925
an ambiguous term

Because the British Nautical Almanac had declared the value of what had once been noon now to be midnight the IAU recognized that the term "GMT" had been rendered useless as a precise term in most archival and practical contexts. Anyone encountering a document containing the term "GMT" could only be certain of its meaning if the document were verified to have originated before 1925. For documents after 1925 there would be no way to be certain whether the author had meant "old GMT" or "new GMT".

GMT according to radio broadcasts starting in 1927
The International Radio Consultative Committee (CCIR) was created to regulate radio broadcasts around the world. Because radio broadcasts were the most readily available form of time signals, the CCIR gained authority over the meaning of the civil time scale.
GMT according to the IAU starting in 1935
In the context of astronomical ephemerides, the mean solar time of the Greenwich meridian with the hours and days reckoned from midnight; i.e., UT.
This was basically capitulation, or more positively, an acknowledgment of the ongoing tendency to continue using the term "GMT" despite its ambiguity.
GMT after 1939
Using observations gathered by the International Latitude Service (ILS) the BIH began publishing the corrections from UT0 to UT1. Initially the corrections were only available for previous years. Eventually the BIH published predictions for the current year and it became possible for the various observatories and time services to adjust their values of UT accordingly.
GMT during World War 2 (1940/1942)
The Airy Transit Circle at Greenwich was taken out of service.
GMT according to navigational almanacs through the 1950s
GMT according to many radio time signal broadcasts through the 1950s

UT as determined by transit observations from the observatory affiliated with the radio station. This was before the seasonal variations of UT2 had been codified, so depending on the observatory the values would initially have been UT0 and later something approaching UT1.

Through this era it was typical to reset the master clocks controlling the radio broadcasts by a fraction of a second whenever it was indicated by the transit observations. These clock resets were, in effect, "leap milliseconds", and they were consistent with the way that mechanical clocks had always been reset in order to keep track of mean solar time. Tabulating the differences between various radio broadcasts had been a principal mission of the BIH from its inception, and only through those records can the meaning of old radio broadcasts be interpreted.

GMT on 1954-03-30
Gilbert Satterthwaite made the final observation using the Airy transit at Greenwich. After this date there was no longer an operational instrument capable of acting as the authoritative definition of GMT.
GMT according to many radio time signal broadcasts starting in 1956
UT2 as determined by transit observations from the observatory affiliated with the radio station. (Note that many observatories based their time scales and signals on transit observations reduced with conventional values for their longitudes. Before 1962 the conventional longitudes were inconsistent at a level of several hundredths of a second of time, and until 1984 they shifted several times as the BIH adopted new systems.
GMT in 1956
The Meridian Circle Group of the Royal Greenwich Observatory began regular observations with the instruments at Herstmonceux. These observations finally revealed that the Ordnance Survey had continued to designate the zero point of UK longitude from the Bradley Transit at Greenwich even after the International Meridian Conference had designated the Airy Transit as the origin.
GMT on 1956-11-05
R. d'E. Atkinson of the Meridian Circle Group of the Royal Greenwich Observatory wrote an article for The Observatory describing the setup of the new meridian instrument at Herstmonceux and the relationship to Greenwich. In this article he makes the following plain:
  • Although they had been producing GMT for the sake of the UK, for some time the meridian instruments at Greenwich had not had any special status among all the other instruments in the world that measured time.
  • The astronomers at Herstmonceux would not even attempt to ascertain the difference between the new site and the original site at Greenwich. Instead they would simply adopt the longitude which best matched the time scale maintained at the BIH.
  • Not even for the sake of longitude did the original site at Greenwich have any special role as an origin. For the purposes of astronomy, geodesy, and navigation the origin of longitude was already defined by the global averages performed at the BIH.
The effect of these decisions is that, beginning with Herstmonceux, the quantity which had been known as and which was being provided under the name of GMT, had become the BIH time scale known as UT.
GMT according to astronomical almanacs since 1960
The term GMT ceased to be used.
"GMT" according to many radio time signal broadcasts in the 1960s
the earlier form of UTC which used frequency offsets from the atomic second along with annual steps of 50 ms to 100 ms in an effort to match the predicted value of UT2
"GMT" on 1966-04-13
Although the term GMT was not explicitly used, the standard time for legal purposes in the United States specified by 15USC261 was clarified and reaffirmed to be based on the mean solar time of meridians spaced at 15-degree intervals west of Greenwich; i.e., GMT.
"GMT" on 1967-04-28T21:00
In accord with 15USC261 all local times in the US were to be set forward 1 hour on 1967-04-30. Rather than reset the time broadcasts twice annually the US NBS radio broadcasts on WWV and WWVH switched their voice announcements from local time to GMT.
"GMT" in 1968-12
The US NBS radio broadcasts of WWV and WWVH began to employ the time scale that the BIH then called UTC. The voice announcements remained as "GMT".
"GMT" in 1971-08
N.P.J. O'Hora of the Royal Greenwich Observatory wrote another article on the longitude of Greenwich analyzing the current offsets between the BIH prime meridian and the Airy Transit circle. He seemed to lament that the original special purpose of Greenwich had been lost.
"GMT" according to navigational almanacs since the 1970s
"GMT" on 1974-01-01T00:00
The US NBS radio broadcasts of WWV and WWVH stopped using the term "GMT" and began to announce the time scale as "UTC".
GMT according to the IAU since 1976-08/09
The 16th General Assembly of the IAU in Grenoble produced Resolution No. 1 by Commissions 4 and 31 which urged that the term GMT should be replaced by UT0, UT1, UT2, or UTC as is appropriate. Sadler (1978) reports that the wording of this resolution was amended by Commission 31 and that some members of Commission 4 were unaware of the final content until after its adoption.
"GMT" during the 1980s
The determination of time using astronomical observations of stellar transits with meridian circles ceased completely, having been replaced by VLBI and laser ranging. The "conventional longitudes" of the meridian circles around the world, which had been inconsistent by several hundredths of a second of time, became historical artifacts. In the new, globally self-consistent world of coordinates based on VLBI and satellite geodesy the longitude of Greenwich ceased to be zero , and the timekeeping role formerly held by the Greenwich meridian was conceptually assumed by the nearby, but tectonically moving international meridian. For all practical and official purposes, GMT as a precise quantity defined in accord with the 1884 International Meridian Conference had ceased to exist. Nevertheless, anything that might qualify to be called GMT remains within 0.1 second of the values of UT1 or UT2.
"GMT" in computers on 1986-04-14
After several years of efforts by committees of Unix experts the IEEE published the Draft American National Standard titled IEEE Trial-Use Standard Portable Operating System for Computer Environments. This specified that the function time() "returns the value of time in seconds since 00:00:00 GMT, January 1, 1970." The draft did not specify how seconds were to be counted nor how the count of seconds should correspond to a calendar date. This language was changed for the 1988-09-30 publication of the POSIX standard.
"GMT" on 2007-08-09
The president of the United States signed a bill changing 15USC261 to specify that the legal time of the US is based on UTC (as interpreted or modified for the United States by the Secretary of Commerce in coordination with the Secretary of the Navy) instead of the mean solar time at Greenwich.

These many usages for GMT recall a bit of childhood nonsense:

"When I use a word," Humpty Dumpty said, in a rather scornful tone, "it means just what I choose it to mean - neither more nor less."
"The question is," said Alice, "whether you can make words mean so many different things."
"The question is," said Humpty Dumpty, "which is to be master - that's all."
-- Lewis Carroll, Through the Looking Glass

Greenwich Mean Astronomical Time -- GMAT
A new name for the mean solar time of the Greenwich meridian with the hours and days reckoned from noon which was coined by the IAU in 1928. It was to be used when describing the old meaning of GMT which had been in use until 1925. Of course this new term did not alleviate the ambiguity in the meaning of GMT in existing usage.
Greenwich Civil Time -- GCT

In 1928 the IAU approved the use of this term as a synonym for Universal Time. The US almanac used this term from 1925 until 1952 to indicate that days were being reckoned from midnight. The British almanac could not be convinced to use the term GCT, and continued to use the term GMT with the new astronomical meaning.

Part of the argument against GCT was that the legal civil time of Greenwich was set forward one hour in the summer. In 1935 the IAU recognized that the ambiguity caused by the disagreement between astronomical usage and civilian usage would never permit complete adoption of GCT. The term GCT was discontinued by resolution 2 of Commission 3 (Notations) and Commission 4 (Ephemerides) at the IAU 5th General Assembly in 1935. Starting in 1939 the US almanac used "GCT or UT" until 1952 when it switched to UT alone.

Weltzeit -- WZ
In 1928 the IAU approved the use of this term as a German language synonym for Universal Time. In 1948 the IAU recommended that WZ (and UT) should be used by astronomers only to designate Greenwich mean solar time reckoned from midnight. Germany is one of the nations that has adopted UTC as the basis of legal time, and Weltzeit continues to be used to this day with the colloquial meaning often being UTC.
Universal Time -- UT

The congressional act that authorized the 1884 International Meridian Conference (IMC) called for a "standard of time-reckoning throughout the globe". The body of the proceedings contains 72 instances of the phrase "universal day" and 57 instances of the phrase "universal time". The title of the resulting proceedings called this "a universal day." Resolution 5 of the IMC stated "That this universal day is to be a mean solar day", and Resolution 4 stated that the initial meridian for measuring longitude would be Greenwich. Thus the resolutions of the IMC specified the use of the mean solar time of Greenwich, and Universal Time became synonymous with GMT reckoned from midnight.

It is important to note that Universal Time has always been a conventional construct based on measurements of sidereal time. It is not easy to measure the position of the sun with great precision. Even if it were easy the solar measure would still have to be converted to UT by application of some conventional formulae including the "equation of time".

proleptic universal time around 1663
Clocks at observatories became accurate enough to permit L.V. Morrison to re-reduce historic observations of occultations as measures of earth rotation.
universal time in 1895
After overseeing the data reduction of some 60000 observations spanning the years from about 1750 through about 1893 Simon Newcomb of the USNO published his Tables of the Sun. These provided new expressions for the mean longitude of the sun and the Fictitious Mean Sun with an accuracy that exceeded any previous conventional formula for the equation of time. As pointed out much later by Aoki et al. (1982), Newcomb's expressions were designed to give a quantity which incremented uniformly in the reference frame used during the early 1890s. Newcomb understood that his expressions for the fictitious mean sun would eventually deviate from the position of the actual mean sun.
universal time in 1896
The directors of the principal national ephemerides met in Paris. They agreed that all of their publications would adopt the conventions and expressions from Newcomb's tables beginning in 1901.
universal time starting in 1901
From this time until 1984 the conventional formulae for converting observations of sidereal time into UT were those of Newcomb.
universal time in 1925

The proceedings of the 2nd General Assembly of the IAU in 1925 in Cambridge contain discussion and consternation over the change of terminology introduced in the UK Nautical Almanac at the beginning of the year. They were much concerned about the inability to unambiguously interpret whether a document using the term GMT had the old or new meaning.

The General Assembly created a "Temporary Commission on Time" (see page 15) which adopted this conclusion:

Astronomers have not yet reached sufficient agreement on the unique terminology that would be desirable to use for time, and are not prepared to formulate a rule about the subject. It is therefore desirable that astronomers exactly define the time they provide.

Universal Time in 1928
Concluding the work of the Temporary Commission on Time, at the 3rd General Assembly of the IAU in 1928 Commission 4 (Ephemerides) approved the use (see page 5) of the term "Universal Time" as a replacement for the term GMT (which the almanacs had rendered ambiguous in 1925).
The terms Greenwich Civil Time (G.C.T.), Weltzeit (W.Z.) and Universal Time (U.T.) denote time measured from Greenwich Mean Midnight, and are not ambiguous.
Unfortunately the use of the term "Greenwich Civil Time (G.C.T.)" proved to be politically unacceptable.
Universal Time in 1948

Resolution 1 by IAU Commission 4 (Ephemerides) at the 7th General Assembly in 1948 recommended (see page 4)

that the designation 'Universal Time' (Temps Universel; Weltzeit) only be used by astronomers to designate mean solar time, reckoned from midnight of the Greenwich meridian.
Again we see the IAU making UT synonymous with GMT.

Universal Time in the 1950s

Through the 1950s UT was used as the independent variable of the ephemerides.

Universal Time in 1955
Decades of BIH intercomparisons of radio broadcast time signals from around the world had revealed that the variation of latitude caused by polar motion caused the time derived from stellar transit observations to result in different values of UT at different observatories. The BIH had also determined that the seasonal variations in earth rotation were reasonably predictable. At the IAU 9th General Assembly in 1955 Commission 31 (Time) directed the BIH to publish corrections for the seasonal variations of UT. Although there do not seem to be published records, Markowitz (president of IAU Comm. 31) reportedly held further discussions with the BIH and the international time services, and they produced definitions and nomenclature for three distinct versions of UT. The BIH complied in 1956, and the new versions of UT came into common use.
UT0 was the raw measure of UT based on transit observations at a single observatory. Until 1962 these transit observations would have been reduced using the FK3 catalog, and from 1962 until 1984 the FK4 catalog, both of which were based on Newcomb's formulae. UT0 is not corrected for the effects of polar motion which means that different observing stations determine different values. The correction from UT0 to UT1 is at most about 0.035 s. The modern techniques for measuring earth orientation (which became the prevalent means of observation by the 1980s) simultaneously produce UT1 and polar motion directly, so UT0 is effectively not used anymore. The UT0 time scale has no official name, but early mentions of it in publications by the BIH used the term "Temps Universel Classique" (Classical Universal Time). Sadler (1978) points out that UT0 was of little relevance by 1978.
UT1 from 1956 through 1983
UT1 was UT0 with corrections added for polar motion such that, ideally, all observing stations would agree on its value. In practice, the agreement never happened in real time, but only after the fact in the publications of the BIH. The values of UT1 produced through 1984 did not agree with each other because most observatories employed conventional (one might say traditional, or even historic) values for their longitudes. The conventional values of longitude were known to be inconsistent with each other at the level of several hundredths of a second of time. The longitudes had always been that way, and most observatories did not wish to modify their published longitude or introduce a discontinuity in the time scales which they produced. The UT1 time scale has no official name, but early mentions of it in publications by the BIH used the term "Temps Universel, compte tenu du mouvement du pole".

UT2 is constructed from UT1 by adding an empirical formula to remove the effect of the annual seasonal variations in the rotation of the earth. In that sense UT2 is more smooth than UT1, and it provided a better gauge of "mean" for mean solar time. The correction from UT1 to UT2 is at most about 0.035 s. The UT2 time scale has no official name, but early mentions of it in publications by the BIH used the terms "Temps Universel uniforme provisoire" and "provisional Uniform Universal Time".

The CCIR elevated the status of UT2 in 1963 when Recommendation 374 specified that radio broadcast time signals should be in close agreement with UT2. In 1966 CCIR Recommendation 374-1 repeated that specification.

Before the year 1970 measurements of the random variations in the rotation of the earth using the precision made possible by atomic chronometers had revealed UT2 as a dubious exercise, for there are other variations in the rate of UT1 -- some of which are predictable, and some which are not. Nevertheless, because of the the first two versions of CCIR Recommendation 374 it is arguable that UT2 remains the legal time scale for those countries whose legal documents assert that time is based on GMT or the Greenwich Meridian. Sadler (1978) points out that UT2 was of little relevance by 1978.

Universal Time in 1964
At the 12th General Assembly of the IAU in Hamburg the members of the IAU Commissions 31 (Time) and 19 (Rotation of the Earth) discussed the confusion about time among astronomers and physiscists. One statement from the session stands out (see page 304)
The distinction between time epoch and time interval is not clear to everyone.
Commission 31 produced an explanatory note (see page 16) drafted by W. Markowitz, H.M. Smith, and L. Essen in the hope of clarifying the situation which included this quote
The epoch of U.T. is determined by the angular position of the Earth around its axis; it is required for various scientific and technical purposes and for civilian use, sometimes without delay.

Sadler (1978) provided a detailed review of the history of mean solar time, GMT, and all forms of UT before 1984.

During the 1970s new techniques for observing earth orientation were proving to be far more precise and accurate than previous means. These included satellite observations (laser and radio), lunar laser ranging (LLR), and very long baseline interferometry (VLBI). Traditional optical transit measurements of stars had always been limited to observations at night, and certain systematic effects had never been seen.

UT1 from 1984 into 1997
The changes adopted by the IAU in 1976, 1979, and 1982 were implemented in the ephemerides starting in 1984. At the same time the celestial reference system of the FK5 catalog came into use. Since 1984 Newcomb's non-relativistic expressions for UT which had been based on observations of the sun, but which did not exactly track it in the long run, ceased to be used. They were replaced by new expressions for UT1 which no longer refer explicitly to the location of the sun.

Aoki et al. (1982) gave the expression for UT1 which came into use in 1984 and they explain why it is no longer based on the sun.

UT1 starting 1997-02-27 through 2002
At the IAU 22nd General Assembly in 1994 resolution C7 recommendation 3 indicated that the equation of the equinoxes should be amended by the addition of new complementary terms. This change was needed in order to refine the definition of the origin of coordinates from which earth rotation was measured to give accuracy better than a milliarcsecond. In order to avoid any discontinuity in UT1 the change was implemented on 1997-02-27 when the difference between the old and amended expressions was zero. Capitaine and Gontier (1993) demonstrated that the difference has a principal period of 18.6 years and amplitude of 0.176 milliseconds.
UT1 starting 2003-01-01
At the IAU 24th General Assembly in 2000 the IAU 2000 resolutions recommended a complete change in the basis for measuring earth rotation to begin in 2003. Capitaine et al. (2000) gave the expression for UT1 which came into use in 2003. Capitaine et al. (2003) amplify the disconnect from the sun, and they show that the difference from the previous version of UT1 is as much as 2 microseconds over the next few decades and 50 microseconds over the next 200 years. With these new changes UT1 is no longer based on GMST, but rather on ERA (see below).

The current version of UT1 is exactly what is needed for geophysical investigations. It permits evaluation of the length of day (LOD) with a precision that reveals changes due to storm systems and changes in ocean currents. But in continuation of a trend that began over a century ago, there is really no publicly available measured quantity which can indefinitely be used as an authoritative value of mean solar time. Indeed, Fukushima (2001) demonstrated that in about 360000 years the value of UT1 given by the IAU 2000 resolutions will differ by a full day from the count of calendar days experienced by people living on the earth.

UT1R is UT1 after a conventional model for tidal forces has been removed. UT1 indicates the instantaneous orientation of the earth around its axis. There are many periodic tidal forces which perturb the rotation of the earth. The IERS provides on-demand plots of earth orientation parameters. By checking the "Remove tidal variations" box the graph of UT1R becomes visibly smoother than the graph of UT1.
UT2R is UT2 after application of the same tidal model as in UT1R. In effect, this is a measure of the calendar days of earth rotation after all predictable components have been removed. Plots of UT2R give a rough indication of the unmodellable components of UT1; the short period fluctuations are largely due to weather and the long period fluctuations are largely due to the core of the earth.
UT for civil purposes in the far future
According to Aoki et al., the old Newcomb expression (implemented in 1901) for the fictitious mean sun deviated from the true mean sun by 0.020 s/(century*century). It is not clear whether the new FK5-based expressions of Aoki et al. (implemented in 1984), or whether the new NRO-based expressions of Capitaine et al. (implemented in 2003), arrested this acceleration, for that was not the goal of either change. If the new expressions did not remove the acceleration between the fictitious mean sun and the true mean sun then UT1 should continue to indicate mean solar time to about one second for about 1000 years. If the new expressions did remove the acceleration between the fictitious mean sun and the true mean sun then UT1 should continue to indicate mean solar time to within about one second for more than 5000 years. The paper by Fukushima, however, indicates that the IAU 2000 resolutions based on Capitaine et al. will result in a much more significant difference between UT1 and mean solar time within 1/4 of a precessional cycle, which is less than 6000 years.

The goal behind the new expressions defining UT1 has been to ensure continuity of the value and rate of UT1 at the instants of change. These new expressions very closely track the hour angle of the mean sun because they were designed to match a quantity which once did that, but there is no longer a quantity specifically designed to indicate the hour angle of the mean sun. Continuity and uniformity of UT1 has been deemed more important.

Historically no attempt has been made to create expressions for UT which will be valid for more than a few centuries. Demanding a better expression for mean solar time is not immediately reasonable because, in a Clintonesque sense, it depends on what the meaning of the word "mean" means. In particular,

  • Should the "mean" be a mean over a century, a millennium, or ten millennia?
  • Should the zero point of the mean be set at the beginning of that interval or some time during the interval?
  • Should the value of such a hypothetical new mean solar time be matched to the current value of UT at the start?
  • Would that matching require a leap second?
  • How far is it reasonable for a conventional expression for mean solar time to deviate from "true" mean solar time?
  • Given that UT is understood to be nonuniform, should a hypothetical new mean solar time accept that there will be millisecond variations in LOD and abandon the goal of being uniform at that level?

The UTC time scale which currently serves as the basis for all civil time is defined to be calculated from UT1. If civilization continues to desire the use of mean solar time then UT1 will eventually fail to serve that need.

Within the next millennium the IAU will have to consider defining and naming a new version of UT to serve as the quantity which is measured by analemmatic sundials. A name for that time scale might perhaps be Analemmatic Universal Time (UTA).

Sidereal Time -- sometimes ST
The hour angle of the Vernal Equinox. The length of the sidereal day is about 23h 56m of mean solar time. Each tropical year the number of sidereal days exceeds the number of mean solar days by one. The sidereal day differs from the inertial rotation period of the earth because the equinoxes precess around the ecliptic with a period of around 23000 years. Despite being a more fundamental indication of the actual rotation period of the earth, there is no calendar in common use which counts sidereal days.
Greenwich Apparent Sidereal Time -- GAST
The Greenwich hour angle of the true (instantaneous) Vernal Equinox of date, which includes the "nutation in right ascension" or "equation of the equinoxes". Because of nutation GAST was known not to increment uniformly even before variations in earth rotation were known. The equation of the equinoxes can be as large as 1.2 s, which is small enough that many telescope pointing systems ignore it.
Greenwich Mean Sidereal Time -- GMST
The Greenwich hour angle of the mean Vernal Equinox. In this case "mean" means the average obtained by considering the precession of the Vernal Equinox in the absence of nutation, which has a principal period of 18.6 years. Before the variations in earth rotation had been observed GMST was presumed to increment uniformly.
Books on fundamental astronomy historically contained detailed treatments of the formulae regarding GAST and GMST because for most of the past few centuries these were the means by which the calendar days of earth rotation were measured with precision. Measuring the transits of stars with known catalog positions produces values of Sidereal Time, and using the conventional formulae gives values of Universal Time.

From 1895 through 1984 the Vernal Equinox was the location on the non-relativistic celestial sphere defined by Newcomb's expressions. UT1 was determined from GMST using Newcomb's formula. Nevertheless there were several instances of potential discontinuity in UT1 when new fundamental catalogs of stars were adopted.

In accordance with IAU resolution from 1976, 1979, and 1982, in 1984 a new set of astronomical constants came into use along with a switch from the FK4 star catalog to the FK5. From 1984 into 1997 the Vernal Equinox was defined by the expressions in Lieske et al. (1977). UT1 was determined from the expressions in Aoki et al. (1982), who also point out that the new expressions and catalog resulted in shifts of the longitudes of some observing stations.

From 1997-02-27 through the end of 2002 there was an additional correction to the equation of the equinoxes based on IAU 1994 resolution C7 recommendation 3. This correction was required by the vastly increased precision that VLBI had contributed to the measurement of earth orientation.

Based on IAU recommendations in 2000, starting in 2003 the Vernal Equinox was abandoned as the coordinate origin for measurement. A point with some of the original properties has been resurrected by Capitaine et al. (2003). They also point out that the effort to maintain continuity of UT1 had invalidated the classical interpretation of GMST and the equation of the equinoxes.

At the IAU 26th General Assembly in 2006 the IAU approved Resolution B1 which explicitly contain a new definition for the ecliptic, and thus for the equinox, to come into use starting in 2009.

Since 1984 GMST has not been the Greenwich hour angle of the mean equinox, and the equation of the equinoxes has not been the true right ascension of the mean equinox. Beginning in 2003 UT1 is no longer determined from GMST, but from the Earth Rotation Angle (ERA).

Defining the Vernal Equinox to an accuracy of one arcsecond is easy, but the rapid motions of the rotation pole of the earth and the non-planarity of its orbital motion hinder precise definition of the equator and ecliptic. The Vernal Equinox is not well defined at a precision of one milliarcsecond. This ambiguity became evident during the 1970s as VLBI began to contribute to earth orientation measurements, and it became problematic during the 1980s as VLBI and other new techniques replaced traditional optical astrometric measurements. By the year 2000 there were at least three different definitions for the location of the equinox being used for various purposes. VLBI precision is approaching microarcseconds, and it is hard to measure anything when the origin of measurement is not known as well as the measurement itself can be made.

Earth Rotation Angle -- ERA or theta

In 1979 Guinot proposed the non-rotating origin (NRO) as a new and unambiguous scheme for defining earth rotation. Capitaine (1986) gave an early description of the differences between the traditional and NRO-based schemes, and the implications of the NRO were studied through the 1990s. In the literature through the year 2000 the ERA was known as the "stellar angle" (not to be confused with a "sidereal angle", which would refer to an equinox).

At the IAU 24th General Assembly in 2000 Resolution B1.8 directed that the equinox-based scheme for determining UT1 should be replaced by a NRO-based scheme beginning in 2003. This change was sufficiently fundamental that in 2002 the IERS held a workshop entirely devoted to exploring the implications of the change. The terminology and pedagogy for these new elements of fundamental astronomy are very new even after considerations by the IAU Working Group on Nomenclature for Fundamental Astronomy.

ERA is the first officially sanctioned quantity whose name explicitly disavows that earth rotation is time. This reflects a change in the purposes of IAU Commissions 19 and 31 over the course of history. Initially Comm. 31 (Time) was solely concerned with earth rotation, and Comm. 19 was solely concerned with polar motion. Now earth rotation is handled by Comm. 19, Comm. 31 primarily considers atomic time, and Comm. 4 continues its long history with dynamical time.

Fukushima (2001) pointed out that the non-rotating origin (NRO) introduced by the IAU 2000 resolutions for the measurement of ERA and UT1 actually rotates significantly over long time intervals. In particular Fukushima showed that precession of the equinoxes will cause the non-rotating origin (previously referred to as the CEO and now to be called the CIO) to circle entirely around the sky over an interval of approximately 360000 years. (Other papers on the implications of the IAU 2000 resolutions regarding time scales and reference frames are mentioned below at the end of the section on Fully Relativistic Time.)

The above forms of earth rotation time are consistent with the nomenclature that has developed throughout history. In conjunction with its orbital motion, earth rotation naturally produces the days, weeks, months, and years which are counted by calendars. Earth rotation time traditionally and evenly subdivides into 24 hours of 60 minutes of 60 seconds. In the sexagesimal notation used for these subdivisions the time tag 12:00:00 has long been understood to indicate that the sun is nearly overhead.

Earth rotation time is robust across an interruption in civilization because it is derived directly from observation. It is a straightforward matter to recover earth rotation time to a precision of microseconds even after an interruption of a century, and it will always be easy to recover to better than a second.

Although rocks, trees, fishes, bugs, little furry creatures, and most of humanity reckon time by the rotation of the earth, physicists reserve the term for a more uniform concept. The irregularities in the rotation of the earth mean that no form of earth rotation time measures time in the sense used by physicists. All forms of earth rotation time are now better described as "time-of-day". In the vocabularies of most of the languages of the world the long history of the words for "time" has been to mean "time-of-day". In order for "time" to have a simple relationship with a calendar, that "time" must be a form of "time-of-day". In the general population there is not yet a clear understanding of the distinction between the old traditional meaning of time and the new physical meaning.

Dynamical time

Dynamical time means time determined by comparing observations of the motions of objects with physical models that describe that motion. The celestial bodies in the solar system move like hands of a clock. Using a theory of motion for those bodies it is possible to read the hands of the clock and determine what time they are telling. But that dynamical time has no relation to the rotation of the earth.

Non-relativistic dynamical time

At the 5th IAU General Assembly in 1935 Commission 4 (Ephemerides) resolved that all national ephemerides should use the same value for the Gaussian gravitational constant (k) when calculating the motions of the planets. The president of Comm. 4 was E.W. Brown who was renowned for having developed a workable theory of the motion of the moon. Brown was tasked with ascertaining a value of k which was acceptable to all parties and reporting that value at the 1938 General Assembly. Brown died just prior to the 1938 General Assembly, but the agreeable value of k had been determined. At the IAU 6th General Assembly in 1938 resolution 2 of Comm. 4 adopted the value k = 0.017202098950000; this was the original value used by Gauss and later by Simon Newcomb for his tables. The value of k fixes the scale of the solar system in astronomical units, but in conjunction with Newcomb's tables it also fixes the duration of the Ephemeris day. Thus, the rate of what would eventually be called Ephemeris Time was irrevocably fixed in 1938.

Newcomb had suspected that the variations in the motion of the moon were directly correlated with variations in the motions of the inner planets, and Spencer Jones (1939) demonstrated the correlation unequivocally. This was direct evidence that the rotation of the earth varied on a time scale of decades. It meant that Universal Time was not uniform and therefore not suitable for use when calculating ephemerides.

Newtonian Time
A name originally proposed by Danjon (1929) for a time scale defined by Newton's laws of motion.
This name was also used by de Sitter and Brouwer (1938). Without knowing of Danjon's work Clemence (1948) used the same name from de Sitter and Brouwer. This was part of the motivation for the meetings which would soon create Ephemeris Time.
Ephemeris Time -- ET
From 1925 through 1948 various papers had suggested that time might be measured by the motions in the solar system. In 1950 the Conference on the Fundamental Constants of Astronomy held by the CNRS in Paris recommended that the IAU adopt a time scale based on orbital motions as described by Newcomb's tables. The IAU 8th General Assembly in 1952 resolved that Ephemeris Time should be created and used in ephemerides. In 1960 the ephemerides switched from using UT to ET.

Because Newcomb's tables had been based on astronomical observations from the 18th and 19th centuries, the length of the ephemeris day happened to match the length of the mean solar day from around the year 1820.

The difficulty with Ephemeris Time is that it can only be measured in retrospect after reducing observations of the orbital motions of bodies in the solar system. The motion of the sun with respect to background stars is extremely difficult to measure. The motion of the planets, except perhaps Mercury, is too slow to provide a precise measure. As a result, the primary means for determination of Ephemeris Time were observations of the motion of the moon as compared with the theory of Brown.

During the 1960s it became evident that there were deficiencies in Brown's analytical theory of the moon. Several corrections to the analytical theory were named by IAU Comm. 4 during the 1960s, and each correction resulted in a new form of Ephemeris Time.

Named at the IAU 11th General Assembly in 1961 and based on the Improved Lunar Ephemeris (ILE) that was tabulated in the ephemerides from 1960 through 1967. This used the old (1950) system of astronomical constants.
Named at the IAU 13th General Assembly in 1967 and based on the ILE with the new (1964) system of astronomical constants and some corrections to terms in Brown's lunar theory. This version of the ILE was tabulated in the ephemerides from 1968 through 1971. Serious flaws in ET1 were evident by 1970.
Named at the IAU 13th General Assembly in 1967 and based on the the ILE further corrected with new calculations of the solar perturbations. This version of the ILE was tabulated in the ephemerides beginning in 1972. By 1973 ET2 was already known to have deficiencies.

During the 1960s it also became evident that the ever increasing numerical abilities of computers provided a workable alternative to analytical theories. Ephemerides of the motions of bodies in the solar system began to be calculated directly by numerical integration. The numerical integrations incorporated relativistic effects which Newcomb could not have known to include in his expressions. The passage of time on the surface of the earth was being measured much more practically by atomic chronometers. Atomic time agreed well with the time indicated by observations of the solar system compared to numerical integrations. Also, it was not clear whether ET was proper time or coordinate time. This inevitably led to the demise of Ephemeris Time. At the IAU 14th General Assembly in 1970 Guinot suggested that Ephemeris Time could be replaced altogether by using atomic time alone, but by 1973 it was understood that dynamical time and atomic time might differ despite the best of intentions.

The almanacs and ephemerides continued to be based on Ephemeris Time and on the analytic theories rooted in Newcomb's expressions through the end of year 1983.

The definition of Ephemeris Time set a precedent for the nomenclature of time which has created some confusion. The expression used for the mean longitude of the sun in ET was the same expression from Newcomb which was used for the calculation of UT. ET was a strictly uniform time scale not related to the rotation of the earth, and UT was a non-uniform time scale directly related to the rotation of the earth. But the fact that both used the same formulae meant that both were reported using the traditional nomenclature for time which had always been based on rotation of the earth. Values of UT and ET were given using multiples and divisions of days; i.e., calendar years, calendar months, hours, minutes, and seconds.

Relativistic dynamical time
At the IAU 16th General Assembly in 1976 resolution 1 adopted the report from Comm. 4 (Ephemerides) which called for sweeping changes to the basis for almost all astronomical calculations. Until this point almost all calculations had been based on the expressions created by Simon Newcomb in 1895. The new system was intended to bring astronomy from the era of Newtonian dynamics built entirely on earth-based observations into the era of relativity, radar, man-made satellites, and interplanetary spacecraft. The IAU adopted a new system of astronomical constants, but the accompanying FK5 star catalog and reference frame was still constructed according to traditional optical astrometric methodologies.

In particular, recommendation 5 from Comm. 4 called for the creation of new time scales which were intended to serve as relativistic replacements for Ephemeris Time. In order to give time for study and implementation of these changes it was resolved that they would not be used in the ephemerides until 1984. Coincidentally this was 100 years after the International Meridian Conference.

Terrestrial Dynamical Time -- TDT
Recommendation 5 in resolution 1 of the IAU 16th General Assembly in 1976 called for the creation of the concept for TDT. The name TDT was adopted by Resolution 5 of Commissions 4, 19, and 30 at the IAU 17th General Assembly in 1979. TDT came into use in the ephemerides in 1984 and was used through 2000. But by 1987 TDT was already deemed to be a misnomer because there is nothing dynamical about it. It was not clear whether TDT was intended to be a measure of proper time or coordinate time. In 1991 the IAU redefined TDT and renamed it TT.
Barycentric Dynamical Time -- TDB
TDB was intended to be the relativistic equivalent of ET for the purposes of calculating planetary ephemerides, but its history has turned out to be far more complex than was expected at its inception.
TDB in 1976
Recommendation 5 in resolution 1 of the IAU 16th General Assembly in 1976 called for the creation of the concept for TDB.
TDB in 1979
The name TDB was adopted by Resolution 5 of Commissions 4, 19, and 30 at the IAU 17th General Assembly in 1979.
TDB in 1984
TDB came into active use as the new expressions and systems for astronomy were implemented in the Astronomical Almanac (which replaced the previously separate ephemerides of the US and UK) and the other ephemerides. In practice the quantity called TDB was actually the independent variable of the DE20x series of planetary ephemerides from JPL.
TDB in 1987
Ongoing discussion of the 1976 definition of TDB had revealed that it was ill defined because it lacked a precise definition for the epoch of its origin, it lacked a metric for transformations to the time experienced by other observers, and because the definition for the rate at which it ticked was not sensible.
TDB in 1991
The IAU defined TCB as a rigorously correct independent variable for planetary ephemerides. The resolution offered that TDB could continue to be used for some purposes despite its inadequately rigorous definition.

Despite resolutions that TCB should be used for all solar system calculations, the ephemerides and many other papers on solar system dynamics continued to use the name TDB for their independent variable.

TDB in 1998
Standish defended the ongoing use of what was being called TDB by publishing a paper which defined Teph and demonstrated that it was a linear transformation of TCB.
TDB in 2005
The IAU Working Group on Nomenclature for Astronomy created presentations and explanations which recommend that the original 1976 definition of TDB be forgotten because it was never used. They agree that the Teph defined by Standish and used in the ephemerides since 1984 has always been what TDB was intended to be. Each ephemeris has its own version of TDB.
TDB in 2006

The IAU 26th General Assembly in Prague approved several resolutions. Resolution 3 produced a new and rigorous definition for TDB.

By so doing TDB has effectively been converted from a Platonic or theoretical time scale into a practical one which is tied directly to JPL DE405.

Guinot and Seidelmann (1988) described the difficulties with the 1976/1979 resolutions and proposed changes that were later implemented.
Rigorously correct relativistic time
In 1991 the IAU resolved that a fully relativistic and inertial reference system should be created, and also defined two new forms of fully relativistic time. In 1994 the IAU specified a list of extragalactic radio sources to be used in defining a reference frame consistent with that reference system. In response the IERS coordinated the International Celestial Reference System (ICRS) and the International Celestial Reference Frame (ICRF). In 1997 the IAU resolved that the ICRS and ICRF should come into use starting in 1998.
Geocentric Coordinate Time -- TCG
TCG was defined by the IAU at the 21st General Assembly in 1991, and its definition was clarified by the IAU at the 24th General Assembly in 2000. It has not explicitly come into use in any ephemerides, but it is defined to have a linear relationship with TT. The unit of TCG is the SI second (9192631770 cycles of the cesium resonance) in a coordinate reference frame that moves with the center of the earth. Because chronometers on the surface of the earth rotate within a gravity well, a chronometer using the SI second of TCG ticks faster than a chronometer using SI seconds of TT or TAI. TCG ticks faster than TDB (and TT and ET) by about 7 parts in 10 billion, or about 20 milliseconds per year. Consequently, the values of physical constants (including the length of the meter) to be used with calculations using TCG differ from the traditional values of physical constants. Various scientific unions have recommended the use of TCG for all measurements made in the terrestrial environment, but the formidable tasks of changing the values of constants in software mean that most such measures continue to use TT.
Barycentric Coordinate Time -- TCB
TCB was defined by the IAU at the 21st General Assembly in 1991, and its definition was clarified by the IAU at the 24th General Assembly in 2000. It has not yet come into use in any ephemerides. The unit of TCB is the SI second (9192631770 cycles of the cesium resonance) in a coordinate reference frame that moves with the center of mass of the solar system. Because chronometers on the surface of the earth move and rotate within several gravity wells, a chronometer using the SI second of TCB ticks faster than a chronometer using SI seconds of TT or TAI. TCB ticks faster than TDB (and TT and ET) by about 1.5 parts in 100 million, or about half a second per year. Consequently, the values of physical constants (including the length of the meter) to be used with calculations using TCB differ from the traditional values of physical constants. Various scientific unions have recommended the use of TCB for all measurements made in the solar environment, but the formidable tasks of changing the values of constants in software mean that most such measures continue to use Teph (often believing that to be TDB).
independent variable of the ephemerides -- Teph
Teph is the name which has been coined to denote the independent variable that has been used in numerically integrated ephemerides since the 1960s. Standish (1998) demonstrated that Teph is a linear transformation of TCB that ticks at a rate equal in some mean sense to the rate of TT. Teph is a realization of the goal that ET and TDB tried to attain.

Each different ephemeris has a different instance of Teph. Typically the values of Teph closely match the values of TT over the interval of the observations which were used to construct the particular ephemeris. Any instance of Teph is a barycentric coordinate time scale which has 0.01720209895 as the value of the Gaussian gravitational constant.

In early discussions of the IAU Working Group on Nomenclature for Fundamental Astronomy it has been suggested that the various instances of Teph might someday be given an official name something like Barycentric Ephemeris Time.

In 2005 the IAUwgNfA has decided that Teph is what TDB was always meant to be, and they are recommending that the two be considered synonymous.

Seidelmann and Fukushima (1992) described TCG and TCB and the reasoning behind them. Vondrak (2002) described the IAU 2000 resolutions which clarified TCG and TCB and the reasoning behind them. Soffel (2002) and Petit (2002) provided further details about the IAU 2000 resolutions which clarified TCG and TCB. Two detailed reviews of the IAU 2000 resolutions and their implications for time and fundamental astronomy are by Seidelmann and Kovalevsky (2002) and Soffel et al. (2003) ( preprint here).

Dynamical time is robust across an interruption in civilization because it is derived directly from observation. As long as the historical record of astronomical observations is preserved it is a straightforward matter to recover dynamical time to a precision of milliseconds even after an interruption of centuries.

Atomic Time

Although there had been previous efforts to build atomic resonators, Atomic Time as we now know it came into being in 1955 when Essen and Parry at the UK NPL created the first workable chronometer using the cesium resonance. By 1958 the cesium resonance had been compared with astronomical observations of the ephemeris second, and the atomic second of 9192631770 Hz was born. Thus 86400 atomic seconds match the length of one ephemeris day, but the mean solar day has been longer than that for quite a while. So atomic time has no relation to the rotation of the earth, which in the long run means it has no relation to a calendar.

Stepped Atomic Time -- SAT

A form of time scale used in long-wave radio broadcast time signals of the US and Germany during the 1960s. The carrier frequencies and most second intervals were atomic seconds, but there were frequent steps of 100 ms to 200 ms inserted in order to match UT2. These steps were the precursor to leap seconds in the later form of UTC. In 1966 broadcasts of this time scale were classified as experimental by CCIR Recommendation 374-1.

In particular, SAT was used by the long-wave transmissions of WWVB in the US and the PTB in Germany. Some devices manufactured by Hewlett-Packard for automatically synchronizing with broadcast time used WWVB, and were therefore synchronized with SAT, not UTC. Complete records of all the steps applied to SAT are not readily available. As a result it can be difficult to determine a relationship with sub-second precision between historical timestamps expressed in SAT and in TAI.

International Atomic Time -- TAI

TAI is the continuation of time scales which began with the first cesium atomic chronometer in 1955.

TAI has always been a statistical combination produced after the fact based on the available ensemble of atomic chronometers. The differences between TAI and various other atomic time scales are published monthly by the Time Department of the BIPM in Circular T.

TAI has not ticked uniformly throughout its history, but once a TAI has been assigned to an event its value is never revised. The variations in the rate of TAI can be traced via the annual publications of TT(BIPMxy).

As seen below in the 2007 entry for TAI, the BIPM disavows that TAI can be available for use in operational systems. This means that no device can provide TAI in real time, and any such claim cannot be based on any authority other than its manufacturer.

1955: Greenwich Atomic -- GA
Essen and Parry constructed the first cesium atomic frequency standard at NPL. The first atomic time scale (and the other early atomic time scales) was constructed by occasionally calibrating quartz chronometers with the cesium frequency.
proleptic "TAI" in 1955-07
The mission of the BIH had always included the monitoring of broadcasts of radio time signals. As some of those signals became calibrated with cesium frequency standards it became possible after the fact to use the BIH data to reconstruct an atomic time scale. This became the basis of the first published atomic time scale.
1956-09-13: A.1
The USNO constructed its atomic time scale based on the cesium chronometer at NRL. In 1959 its epoch was reset such that A.1 equaled the UT2 of the USNO as of 1958-01-01.
1957-10-09: NBS-A
The US NBS in Boulder began an atomic time scale. In 1959 its epoch was also reset equal to UT2 on 1958-01-01.
proleptic "TAI" on 1958-01-01

At this point the various laboratories keeping atomic time scales had only begun to recognize their significance and utility. These atomic time scales were unrelated to each other and only comparable after the fact through laborious computations. Subsequently the various time service bureaus which had then been operating atomic chronometers agreed to set their various atomic time scales to agree with their values for UT2 on this date.

This date was before high-precision international coordination of time had begun, and before the various time bureaus agreed on globally self-consistent values for the longitudes of their observatories. Guinot (2000) points out that because in 1958 the values of UT1 used by various observatories differed by several hundredths of a second of time, so also did their values of UT2. Because these atomic time scales were later incorporated into the atomic time scale of the BIH it can be said, in an approximate sense, that TAI was set equal to UT on this date.

proleptic "TAI" during 1959/1960
During 1959 the various time service bureaus using atomic frequency standards agreed to reset their atomic time scales to their values of UT2 on 1958-01-01 as the common epoch. The resetting of the time scales was finished during 1960.
1960: AM
The BIH Bulletin Horaire began to publish differences between their mean atomic time scale (AM) and the other astronomical and broadcast time scales.
A3, TA
During the 1960s the BIH studied the characteristics of various atomic time scales from monitoring of the radio broadcasts. The BIH revised their scheme for calculating their atomic time scale to use the three best atomic frequency standards and named it A3.
Late in the 1960s more atomic frequency standards were included, the statistical algorithms for combining the contributions were improved, and the name became TA.
atomic time in 1967-08
The 13th General Assembly of the IAU approved Resolution 5 from Commissions 31 and 4 ( English, French) in which the IAU requested that the CGPM should make note of the ephemeris second at their upcoming meeting which would redefine the SI second.
atomic time in 1967-10
Resolution 1 of the 13th meeting of the CGPM redefined the SI second such that it no longer depended on Ephemeris Time but instead was based solely on the hyperfine resonance of cesium. The text of the CGPM resolution did not contain the words requested by the IAU in their recent resolution (c.f. regrettable misunderstandings).
"TAI" on 1970-06-18/19
Under the auspices of the CIPM, the CCDS produced a definition for TAI.
TAI starting 1971-10-04
The 14th CGPM authorized the CIPM to define TAI and work with the BIH to realize the definition.
TAI in 1976-08/09
The 16th General Assembly of the IAU in Grenoble produced Resolution No. 2 by Commissions 4 and 31 which recommended that the rate of TAI be adjusted by one part one part in 1012 on 1977-01-01.
TAI since 1977-01-01
By the mid 1970s it was evident that the scale unit of TAI was not equal to the SI second at sea level. On 1977-01-01 the frequency of TAI was reduced by one part in 1012. Since that time TAI has been constructed by steering the frequency of what is now called EAL with an offset.
Echelle Atomique Libre (Free Atomic Time Scale) -- EAL
Starting 1977-01-01 the un-steered combination of atomic chronometers which had formerly been TAI has been known by the new name EAL.
TAI since 1980
Since 1980 TAI has been a "realization of TT", a coordinate time (in conformance with resolution A.4 recommendation IV from the IAU in 1991). TAI does not officially incorporate the clarifications to TT from IAU resolution B1.9 adopted in 2000. Guinot (1986) has explored the ways in which chronometers contributing to TAI measure a proper time or a coordinate time.
TAI since 1995
By 1995 it was evident that blackbody radiation was affecting the frequency of cesium chronometers, and that the true SI second should be measured at 0 Kelvin. Over the interval from 1995 to 1998 the length of the TAI second was decreased by about 2 parts in 1014 until it corresponded as closely as possible to cesium atoms at absolute zero.
TAI on 1999-04-20/22

At the 14th meeting of the Consultative Committee on Time and Frequency (CCTF) Dr. Dennis McCarthy of USNO presented report CCTF99-18 on The Future of Leap Seconds.

The resulting discussion produced a letter encouraging and recommending that applications which need a time scale without leap seconds use TAI.

An additional output was a circular letter from the Director, BIPM, emphasising the utility of TAI, as opposed to UTC, for systems requiring uniform time, i.e. free from the discontinuities arising from the application of leap seconds.

At a meeting of the Consultative Committee for Time and frequency (CCTF) held here in April 1999, attention was drawn to potential problems that might arise if a number of new, independent, uniform atomic time scales were developed for satellite navigation and electronic communication systems. There is apparently a perceived need for such uniform time scales to avoid problems in navigation systems resulting from discontinuities in UTC produced by the periodic introduction of leap seconds.

There was no consensus within the CCTF for any proposal to change the definition of UTC. Instead, I was asked as Director of the BIPM to draw your attention and that of agencies developing satellite navigation systems, to the option of using TAI which is, of course an international uniform time scale. I remind you of the ITU Recommendation ITU-R 485-2 (1974-1982-1990) in which it is recommended that "time data should be issued wherever possible either with reference to Coordinated Universal Time (UTC) or to International Atomic Time (TAI)". It is clear that if the leap seconds of UTC cause problems in any particular application, the preferred alternative is TAI.

The CCTF recommends, therefore, that in conformity with this ITU Recommendation developers of future satellite navigation systems and electronic communication systems should link their time scales to TAI as the only alternative to UTC and that, insofar as it is feasible, existing systems take steps to align their time scales with TAI. This is in conformity with the CCDS Recommendation S4 (1996) on the "coordination of satellite systems providing timing", in which it was recommended that "the reference times (modulo 1 second) of satellite navigation systems with global coverage by synchronized as closely as possible to UTC". To facilitate the direct use of TAI for satellite navigation systems, the time community is willing to take any steps that are necessary to make TAI easily accessible to users. UTC remains the basis for worldwide timekeeping, but TAI is recommended for those applications requiring uniform time. I urge you to take the necessary steps to inform your constituents of the characteristics of both UTC and TAI so that appropriate use may be made if these international scales. I enclose a few documents that may be of help in this respect.

TAI in 2002

After more than a year of planning, meeting, and writing the Precision Time Protocol (PTP, officially IEEE 1588-2002) was approved as a standard.

PTP time is defined as TAI - 10 s for dates subsequent to 1972-01-01T00:00:00. This means that a POSIX system can set its system clock value of time_t equal to PTP time and use the "right" zoneinfo files to produce human-readable timestamps in UTC.

CAVEAT: A close reading of the 2002 PTP spec indicates that the contents Appendix B may be inconsistent with the rest of the specification. The above paragraph is based on Appendix B and looks like it may be wrong. It may be that PTP is intended to be elapsed seconds of TAI starting on 1970-01-01T00:00:00 TAI. This text will be updated when clarity is certain.

TAI on 2006-09-14/15
At the 17th meeting of the Consultative Committee on Time and Frequency (CCTF) Dr. Beard reported on the process of the ITU-R efforts to redefine the broadcast time scale. The resulting discussion asserted that "TAI is not a disseminated time scale, it is not defined or endorsed for broadcast by the ITU, and the ITU would need to take action to change this."
TAI on 2007-09-04/05

The BIPM contributed document 7A/51-E to the ITU-R WP7A meeting regarding Question ITU-R 236/7. (The letter was reproduced as Document CCTF/09-27 in the Report of the 18th meeting of the CCTF.) The letter says that the CCTF "stresses that TAI is the uniform time scale underlying UTC, and that it should not be considered as an alternative time reference." This appears to indicate that the CCTF and BIPM are not comfortable with the notion that operational systems might be employing TAI as their time scale.

The final sentence of the BIPM document reads:

In the case of a redefinition of UTC without leap seconds, the CCTF would consider discussing the possibility of suppressing TAI, as it would remain parallel to the continuous UTC.
This makes it unclear whether TAI will continue to exist.

TAI in 2008

After another year of effort a revised standard for PTP (officially IEEE 1588-2008) was approved.

PTP time remains defined as TAI - 10 s for dates subsequent to 1972-01-01T00:00:00, but the standard refers to "1 January 1970 TAI" which is a date before TAI had been defined or approved.

CAVEAT: The above paragraph may be wrong. See the warning in the 2002 PTP section above.

Various atomic time scales which contribute to TAI are maintained in real time by various laboratories around the world. For example, the designation TA(NIST) refers to the atomic time scale that is maintained at the US National Institute of Standards and Technology, and TA(PTB) refers to the atomic time scale maintained at the German Physikalisch-Technische Bundesanstalt. The differences between various instances of TA(k) are published monthly by the BIPM in Circular T.
GPS time

The GPS satellites adopted a time scale which was synchronized with UTC on 1980-01-06, and it has been steered in close synchrony with TAI since then. Therefore the difference TAI - GPS has been 19 s to within a microsecond. For practical purposes this means that GPS time strongly resembles the variants of TA(k).

See the current revision of IS-GPS-200 for full technical details. Note that getting precise time from GPS requires that the receiver must apply the polynomial corrections for the SV clocks, the GPS time scale itself, and the ionosphere as detailed in sections and of IS-GPS-200.

Galileo System Time -- GST
The European Union navigation satellite system Galileo has a time scale based on TAI. Early design specifications indicated that GST would be kept within 50 ns of TAI 95% of the time. In effect, this meant that GST would have resembled another variant of TA(k). Before any of the satellites were launched the United States engaged in significant technological diplomacy with the EU in order to ensure interoperability of Galileo and GPS. One result of those negotiations was to change the epoch of GST so that it matches the GPS epoch rather than the TAI epoch. Therefore the difference TAI - GST has also been 19 s to within a fraction of a microsecond.
BeiDou/Compass System time

The satellites for the initial and experimental Chinese GNSS were known as BeiDou-1. They used a time scale which was synchronized with UTC during the year 2000, so the difference between it and TAI was 32 seconds.

The new and operational Chinese GNSS is known as BeiDou-2 or Compass. Its time scale was synchronized with UTC on 2006-01-01, and it has been steered in close synchrony with TAI since then. Therefore the difference TAI - BST has been 33 s to within a few nanoseconds. In effect, this means that BST is another variant of TA(k).

IRNSS Network Time

India is launching satellites known as the IRNSS (Indian Regional Navigation Satellite System). Their time scale is IRNSS Network Time (IRNWT). IRNWT was synchronized with UTC during the year 2006, so the difference between it and TAI is 32 seconds.

International Time -- TI

The name for the radio broadcast time scale which was suggested by Sadler (1978) in his monograph on the sordid history of people and agencies changing the name or characteristics of time scales which were already in use.

The name of the time scale in one of the presentations from E. F. Arias at the ITU-R SRG 7A colloquium in Torino on 2003-05-30 as a likely candidate for radio broadcast time signals at some point in the future. TI would be a purely atomic time scale offset from TAI by a fixed integer number of seconds. In order to avoid discontinuities for systems using radio broadcast time signals the offset would be equal to the offset of UTC at the instant of switching from UTC to TI. The nominal year at which the switch would occur is 2022.

Atomic time is not robust across an interruption in civilization because it depends on the continuous operation of complex equipment. If civilization were disrupted the existing atomic time scales would be lost. The precision to which a new civilization could match new atomic time scales to the current ones would depend upon astronomical observations. If dynamical time were the only available scheme, then the match would not likely be as good as a millisecond.

Terrestrial Time -- TT
TT was defined by the IAU in 1991 as a clarification of what TDT was intended to have been. Seidelmann and Fukushima (1992) described TT and the reasoning behind it. TT has been used in the ephemerides since 2001. TT is intended as a Platonic ideal, for there is no single realization of it. TT is a coordinate time scale for a reference frame which moves with the geocenter, but TT ticks at a rate equal to that of chronometers on the rotating geoid. Therefore TT is a linear transformation of TCG.

TT can be realized by various methods, but the most practically available form is derived directly from TAI. Irwin and Fukushima (1999) have provided the best relationship between TT and TCB. Soffel (2002) and Petit (2002) provided further details about the IAU 2000 resolutions which clarified TT.

TT(TAI) = TAI + 32.184 s

This is TT calculated simply by presuming that TAI has been free of defects since it read 1977-01-01T00:00:00. The value of 32.184 s is the best available estimate of the difference between TDT and TAI on that date.

This time scale is an effort to ascertain corrections to the values of TAI based on retrospective studies of the behavior of the chronometers which have contributed to TAI. The procedure for computing it was first described by Guinot (1987) when TT(BIPM87) was published. The current instance is TT(BIPM14) created in 2015 and plotted here.
Several international scientific unions are preparing to recommend that a new time scale be constructed from radio astronomical measurements of pulse arrival times of an ensemble of pulsars. This time scale would be used to provide an independent means of measuring the defects of TAI inferred from TT(BIPMxy).

Terrestrial Time is not currently robust across an interruption in civilization because its principal realization depends on atomic time. If civilization were disrupted then TT could be reconstructed to a millisecond precision using dynamical time. TT(pulsars) might provide a way to achieve microsecond precision, but this possibility will not be clear until after that new time scale has been in operation for many years.

The forms of dynamical, atomic, and coordinate time above are not based on earth rotation. They have no connection with days in the traditional sense; thus they have no simple relationship with the concept of a calendar. It is important to remember that the 24-hour cycle of tags like 12:00:00 really only makes sense for earth rotation time.

To their credit, the BIPM do report TAI and TT using modified Julian date (MJD) notation. Nevertheless, it remains commonplace to use the traditional calendrical and sexagesimal notations when counting the seconds which are defined by these time scales. (Indeed, for the sake of human cognition this notational convenience is very nearly a requirement.) Unfortunately, that usage leads to even greater confusion about the meanings of time scales.

Denoting TAI with a tag in the form 1958-01-01T00:00:00, and denoting TT, TCG, or TCB with a tag in the form 1977-01-01T00:00:00 should be considered as a convenience only. They are merely counts of elapsed time where one anonymous and indistinguishable second follows another. It is very appropriate to count these forms of time using decimal notation from the epochs represented by those tags. But there is no observable event that happens cyclically at 12:00:00 for dynamical, atomic, or coordinate time -- the entire notion of such a cyclical process is contrary to their uniformly-incrementing conceptual definitions.

Coordinated Universal Time -- UTC
radio broadcast time signals

This category covers two separate concepts: "radio broadcast time signals" and "UTC". As Sadler (1978) pointed out, in recent usage the two concepts have tended not to be distinguished carefully. The history given here attempts to be careful in discerning which has been which.

UTC has always been two things

Universal Time and Atomic Frequency are incommensurate, and this has been recognized in plain language since 12th General Assembly of the IAU in 1964 (see below). As seen in the long story below, over the subsequent 50 years no compromise between these two concepts has ever achieved unanimous agreement.

Starting in 1972 UTC has counted two things

Since 1972 the durations of the UTC day and the UTC second are not related to each other. No other time scale has this property. All other time scales define their notions of second and day with a fixed factor of 86400.

Nothing resembling the name UTC was used prior to 1960, so any claim that UTC can be used before then is inappropriate. The name UTC did not appear in any official context until 1974, so any claim that UTC was used prior to 1974 is almost certainly a reinterpretation of history which does not correspond to anything in contemporary documents.

Radio broadcast time signals and UTC do not fit into any one of the above categories, but they are dependent on them. Since the name "UTC" came into use it has always been both Universal Time and Atomic Time as provided by radio broadcast time signals. As such, UTC has been a practical time scale. The nature of the goals that radio broadcast time signals have been intended to meet and the procedures for meeting those goals have changed over the years.

Prior to the advent of atomic chronometers it had not been possible to keep the broadcast time scales of widespread systems synchronized to within a millisecond. The original goal of what was informally called UTC was for radio broadcasts around the world to be synchronized with each other while providing a time which matched the expected value of UT2 as closely as could be predicted in advance. The length of the second in radio broadcast time signals was adjusted ("elastic seconds" or "rubber seconds") at the beginning of each year, and any accumulated error caused by mis-prediction of the rate of UT2 was corrected by applying occasional steps of 50 to 100 milliseconds to the value of the broadcast time.

One last note before going into the detailed list of the evolution of UTC. The last time that everyone involved in the definition of UTC agreed on the concepts was 1964, the same year that the term UTC first appeared in print. Already by that date the different international organizations involved in regulating radio broadcast time signals were holding as many as three different meetings during a calendar year to discuss what to broadcast. Explaining the concepts of the time scales in the broadcasts, and explaining how it was safe to use those time scales, were less important than who had control over the broadcasts. The result has been that, since the term UTC first appeared in print, no two international specifications have agreed on how to handle the sequence of timestamps in radio broadcast time signals. A flame war erupts almost every time any organization starts a discussion about the technical details of UTC.

radio broadcast time signals in 1951
tde CCIR issued Recommendation 70
radio broadcast time signals in 1953
CCIR Recommendation 122 superseded Recommendation 70
radio broadcast time signals in 1956
CCIR Recommendation 179 superseded Recommendation 122
1958-08 radio broadcast time signals in tde UK
At IAU Symposium 11 held during tde 10th General Assembly in Moscow Essen reported that the UK would broadcast times signals using an offset from the established frequency of cesium.
radio broadcast time signals in 1959
CCIR Recommendation 319 superseded Recommendation 179. It called for time pulses to be within 50 ms of UT2.
1959-08 agreement to "co-ordinate" broadcast time signals

In the UK the agencies of the Royal Greenwich Observatory, the National Physical Laboratory, and the General Post Office, and in the US the agencies of the US Naval Observatory, the Naval Research Laboratory, and the National Bureau of Standards agreed "to co-ordinate the time and frequency transmissions" beginning in 1960 where "the co-ordination plan operates within the framework of the IAU and the CCIR."

The word "co-ordinate" was the same word used to describe the existing collaboration between the US and UK to share the work of producing their national ephemerides. Beginning with the 1960 edition this effort had already resulted in the two countries issuing publications which contained identical sections of astronomical and navigational tables.

coordination of UT for broadcasts beginning in early 1960
An experiment by the US and UK to use the same system for broadcast time signals within the framework of the IAU and CCIR. This was a natural extension of the 1955/1958 collaboration of Markowitz, Hall, Essen, & Parry (1958) where the US radio broadcast time signals of UT2 had been used to calibrate the frequency of the cesium transition and the rate of Ephemeris Time. It used the frequency offset from cesium that Essen had described in 1958. By so doing, the longstanding differences in time broadcasts which had been caused by inconsistencies in the "conventional longitudes" of the observatories were reduced from several hundredths of a second of time to around a millisecond.
coordination of UT for broadcasts in 1960-09
URSI recommended that the annual changes to the frequency offset from cesium used for radio broadcasts should be chosen by the BIH.
coordination of UT for broadcasts in 1961

The report from Commission 31 (Time) to the 11th General Assembly of the IAU mentioned the issues of atomic time vs. universal time, interval vs. epoch, astronomy vs. navigation vs. physics, and the roles of the IAU, CCIR, and URSI. Starting from this point the tensions become visible over the nature of what radio broadcast time signals should be trying to provide.

During the 11th IAU General Assembly in Berkeley the meeting of Commission 31 (Time) referred to the US/UK broadcast time signal experiment. The process of broadcasting synchronized signals was called co-ordination, the resulting signals were called co-ordinated, and the stated goal was to achieve agreement to within 1 ms.
Commission 31 resolved that the BIH should determine the annual changes to the frequency offset from cesium to be used to co-ordinate the radio broadcasts. At this meeting Commission 31 decided to address the longstanding issue of the conventional longitudes of the observatories around the world. The IAU directed all of the time service bureaus to adopt new, globally self-consistent longitudes for their observatories starting in 1962.

coordination of UT for broadcasts from 1961 through 1971

The responsibility for coordinating the frequency offsets and steps was transferred to the BIH. A table of the frequency offsets and steps is available from the Paris Bureau of the IERS, which is the organizational descendant of the earth rotation portions of the BIH.

Cautionary note:
During this same period some radio broadcasts were providing SAT (see above). Also, at least until 1967 the Soviet Union and China were broadcasting another form of time coordinated in the USSR independently of the BIH.
Before assigning absolute meaning, any time-stamp from this era intended to be used with sub-second precision should be subjected to historical scrutiny about the mechanisms of its provenance. This includes the origin of the POSIX epoch at 1970-01-01T00:00:00.

coordination of UT on 1962-01-01
Following the direction of the IAU, all observatories in the world adopted new longitudes which were expressed in a globally self-consistent reference frame. On the same date all astrometric observations switched from using the FK3 catalog to using the FK4 catalog, so the bases for both terrestrial and celestial coordinates were changed.
radio broadcast time signals 1963-01-15/02-16

The 10th Plenary Assembly of the CCIR approved Recommendation 374 to supersede Recommendation 319. Recommendation 374 does not contain any form of the words "coordination", "coordinated", nor the letters "UTC". The text states that the broadcasts of time signals should "be offset to keep the time pulses in close agreement with UT2" and "maintained within approximately 100 ms of universal time UT2" with steps of exactly 50 ms.

Recommendation 374 contained the details of what is now informally referred to as the original form of UTC even though the CCIR did not use that name. In particular, 374 described the Best Current Practice of broadcast time signals using a frequency offset from the cesium resonance and monthly small steps, both coordinated internationally.

The US NBS continued to announce their broadcast time signals using the standard time at the radio transmitter, as offset from the mean solar time of Greenwich.

TUC[BIH]/UTC[BIH] in 1964/1965

Editorship of the Bulletin Horaire from the BIH passed from Anna Stoyko into the hands of Bernard Guinot. The first issue produced by Guinot was Series J, Number 1 giving the values of time for 1964 January and February. With the issues from this year the bulletin started to use the name UTC for its time scale constructed from the monitoring of worldwide radio broadcasts. Note, however, that at that time the BIH was more than a year behind in the processing of the time values. Despite being the tabulations for time during 1964-01/02, the content of Bulletin Horaire, Series J, No. 1 contains information about events as late as 1965-05-01, and it was not received by subscribers until late in 1965 June.

On page 2 of that issue Guinot wrote a definition in boldface print

L'échelle intermédiaire TUC (UTC en anglais) sera appelée Temps Coordonné.
This is the earliest known use of the term UTC in any publication.

The bulletin did not make a clear choice of language for the abbreviation. There was not yet a clear precedent for using CUT "Coordinated Universal Time" (in English) nor TUC "Temps Universel Coordonné" (in French), nor UTC (following the pattern that Markowitz of the IAU had worked out with the time service bureaus for UT0, UT1, and UT2).
The canonical ordering UTC is consistent with neither French nor English, but it is consistent with the UT0/UT1/UT2 nomenclature scheme. Despite the canonical ordering, it remains common even today to see usage where the letters are permuted (and worse misnomers such as Consolidated Universal Time, Universal Correlated Time, Universal Coordinating Time, Universal Time Calibrated, Universal Time Code, etc.).

TUC/UTC in 1964-08

The report of Commission 31 to the 12th General Assembly of the IAU noted that most time services had switched to using quartz crystal clocks instead of pendulum clocks.

At the 12th General Assembly of the IAU Commission 31 again resolved that radio broadcasts should use the frequency offset from cesium chosen by the BIH to match UT2. They noted that high frequency radio broadcasts of time signals permitted worldwide synchronization to within about 2 ms, but that transporting cesium chronometers and using satellite telecommunications could attain synchronization to about 1 microsecond. They also produced this regrettably prophetic quote

The distinction between time epoch and time interval is not clear to everyone.
along with an explanatory note written by none other than W. Markowitz, H.M. Smith, and L. Essen (see page 16). That note says
The epoch of U.T. is determined by the angular position of the Earth around its axis; it is required for various scientific and technical purposes and for civilian use, sometimes without delay.

TUC/UTC from 1964 on

For the next few years the term UTC was plainly informal and it appeared in various publications with various different abbreviations. Some authors wrote in English using U.T.C. alongside U.T.0, U.T.1, and U.T.2; others wrote in French with T.U.C, etc. Publications from the USNO used UT2C.

Despite being the first person to use the term in any publication, on page 62 of The Measurement of Time: Time, Frequency and the Atomic Clock Guinot declines to take credit for the name of the time scale

which had been spontaneously christened Coordinated Universal Time (UTC)

In context it is fair to say that the name "Coordinated Universal Time" and its various abbreviations had roughly this meaning:
The kind of Universal Time which we are able to broadcast in radio signals that are internationally coordinated to be synchronized to 1 ms or better.

UTC[BIH] in 1965
The BIH started calculating UTC based upon its atomic time scale (which later became TAI).
UT2C in 1965
Publications from the USNO were calling their time scale by the name UT2C, not UT2 nor UTC.
radio broadcast time signals in 1966
None of the official documents from the Plenary Assembly of the CCIR made use of any form of the term UTC, but they did use UT2. (One of the contributed reports mentioned UTC in the context of its usage by the time service bureaus.)
The 11th Plenary Assembly of the CCIR issued Recommendation 374-1 regarding the broadcast of time signals. The term "UTC" does not appear in the text of the recommendation. The recommendation requires that broadcast time signals be adjusted
to maintain the epoch of the time-signals within about 100 ms of Universal Time UT2
The significant change in 374-1 was to describe (and denote as experimental) the broadcasts of Stepped Atomic Time (with no frequency offset from cesium and many more steps) as a new Best Current Practice.
abject confusion in 1966
Hans D. Preuss of the Department of Geodetic Science at Ohio State University wrote report number 70, "The Determination and Distribution of Precise Time" as part of a contract for NASA. No report does a better job of indicating the chaos of the varied and confusing sources and nomenclature for precise time during the 1960s.
UTC[NBS] on 1967-04-28T21:00
In accord with 15USC261 all local times in the US were to be set forward 1 hour on 1967-04-30. Rather than reset the time broadcasts twice annually the US NBS radio broadcasts on WWV and WWVH switched their voice announcements from local time to GMT.
more confusion in 1967-06

IEEE proceedings V55 #6 published "Some Characteristics of Commonly Used Time Scales" by George E. Hudson. One contemporary sentiment of that era is plainly visible where he wrote

the deplorable problem raised by the existence of a great variety of auxiliary time scales, some denoted by a random nomenclature.
and he concluded
the proliferation of artificial derived scales is to be deplored.

He listed several atomic time (AT) scales including A at US NBS, A.1 at USNO, TA1 at Swiss LSRH, and one at UK NPL. He listed five "slightly different universal time scales" including "UT0, UT1, UT2, universal atomic (UA), and stepped atomic (SA)." He listed the two different schemes approved for radio broadcasts by CCIR Rec. 374-1 using the terms "UTC" (which, as noted above, is not used in 374-1) and "SAT" (for Stepped Atomic Time, and he also notes that WWVB called this NBS(SA) but had previously called it SAB). He further notes that WWV and WWVB actually use NBS(UA) (see 1968-12 below for when NBS switched to following the BIH version of UTC).

UTC[BIH] and UTC[USSR] in 1967-08

The IAU noted the nomenclature "UTC" at its 13th General Assembly in Prague when referring to CCIR Recommendation 374-1 even though the term "UTC" does not appear in the CCIR recommendation. The IAU discussions show that while the radio regulators thought of the broadcasts as being UT2, the astronomers recognized that could not be the case, and they used the internal term of art "UTC" for which there existed no formal recommendation nor resolution.

In the transcripts of the joint meeting of Comm. 31 (Time) and Comm. 4 (Ephemerides) are discussions and resolutions on subjects including

  • the new definition of the SI second
  • the need to compare Ephemeris Time and Atomic Time
  • the fact that there were two different systems of UTC, one run by the BIH and one run by the USSR
  • a recommendation for the BIH to compute an International Atomic Time Scale with value approximately equal to UT2 as of 1958-01-01T00
  • early recognition that the proper time of terrestrial chronometers would differ from the coordinate time of the ephemeris, thus the first hint of the relativistic time scale changes later recommended by the IAU in 1976 and implemented in 1984
  • "No major change in the principles of UTC should be introduced since it has proved useful", a sentiment which was utterly disregarded by the subsequent changes in radio broadcast time signals

In particular, the IAU approved Resolution 6 from Comms. 31 and 4 ( English, French) in which the IAU clearly indicated that "Universal Time" is a proper name for time based on rotation of the earth. In combination with Resolution 5 and the discussions which produced both resolutions it is very clear that the members of of these IAU Commissions and the General Assembly considered purely atomic time scales to be distinctly different than Universal Time.

UTC[NBS] in 1968

US NBS Special Publication 236 first used the name Coordinated Universal Time with the abbreviation UTC as the time scale coordinated by the BIH. It used the terms UT2 and GMT interchangeably, and the terms UT, UTC, and GMT were equated to each other.

By this point it is evident that everyone familiar with the time broadcasts knew them as UTC, but the relationships between UTC, UT2, UT and GMT were expressed in different ways by different people. This is not surprising given that the CCIR still specified the use of UT2 and had never used the term UTC.

UTC[NBS] and UTC[USNO] on 1968-10-01
Through the first half of the 1960s the time scales of the US NBS and the USNO had not tried for agreement better than the 1 ms recommended by the CCIR. By 1967 the two agencies understood that synchronization to within a few microseconds was desirable.
In anticipation of a coordinated coordinate rate for USNO and NBS, on August 24, 1967, the Coordinated Universal Time clock of the Bureau, UTC(NBS), and all UTC transmissions of NBS were advanced by 200 microseconds. This left NBS about 35 microseconds early relative to USNO.
(As seen above, they also provided a unique use of forms of the word "coordinate".) On 1968-10-01 the two scales agreed, and both agencies adjusted the frequencies of their time scales. After that date they steered the frequencies of their time scales to keep the time difference less than 3 microseconds.
UTC[BIH] in 1968-12
The US NBS radio broadcasts of WWV and WWVH began to employ the time scale that the BIH then called UTC. The voice announcements remained as "Greenwich Mean Time".
radio broadcast time signals in 1969

CCIR Interim Working Party 7/1 had been created to resolve issues with broadcast time signals. Despite resolutions from scientific unions calling for an international and interdisciplinary committee to study options for broadcast time signals, in 1969 CCIR Interim Working Party 7/1 unilaterally deemed that radio time signals should broadcast atomic seconds with occasional leaps of full seconds.

IWP 7/1 produced CCIR Plenary Assembly Document VII/1008 which was the draft version that became CCIR Rec. 460. Section 2 of the draft originally included the phrase "or integral multiples thereof". This is the closest that the world ever got to having a "double leap second".

radio broadcasts of "Coordinated Time" in 1970

The report of Commission 31 (Time) to the 14th General Assembly of the IAU covered events of the three years since the previous IAU GA. It appears to have been written in late 1969 or in 1970 before the CCIR meeting. The title of the section on radio broadcast time signals was "Coordinated Time", and it begins with the words "The universal coordinated time scale (U.T.C.)". These are indications that the members of IAU Comm. 31 who wrote the report did not then share a unanimous nomenclature for the time scale in radio broadcasts. It also looks as though some members did not agree that the atomically-regulated broadcasts should be associated with the name Universal Time.

The report of Commission 31 included an unofficial synopsis which is the only publicly-available record of the CCIR efforts to redefine broadcast time signals. The report indicated that changing time signals to have leap seconds might cause difficulties for aircraft collision avoidance systems.

The report also discussed the alternatives for changing radio broadcast time signals in a way that very much prefigured the situation that has been happening in the ITU-R starting in 2000 and continuing through 2015. Alternative (b) in the report matches the draft revision of ITU-R TF.460 submitted to the Radiocommunications Assembly in 2012 by calling it "complete disuse of U.T.C. system replacing it with a coordinated uniform time scale without offsets and steps and therefore not approaching U.T." Thus the IAU is on record opining that UTC without leap seconds is not a form of Universal Time.

radio broadcast time signals on 1970-02-03

Disregarding the requests from various scientific unions the delegates at the 13th plenary session of 12th Plenary Assembly of the CCIR unilaterally decreed that on 1972-01-01 (in less than two years) broadcasts of time signals would begin using the new scheme of leap seconds. This was codified in the text of the new CCIR Recommendation 460. (Most CCIR, and ITU-R, texts were not freely available, but this one was republished by the US National Bureau of Standards on page 31 of Monograph 140, Time and frequency: theory and fundamentals (Byron E. Blair, 1974).)

It was during the Plenary Assembly that the words "or integral multiples thereof" were removed in order to limit the havoc that the change might cause to navigators. The maximum difference DUT1 = (UT1 - UTC) was specified as 0.7 s, which prompted the creation of signals encoding DUT1 which can only indicate values up to 0.7 s.

It is relevant to note that the term "UTC" does not occur in the text of CCIR Recommendation 460. The focus of the discussions was on the technical characteristics of the radio broadcast time signals. The proceedings give no indication that a name for the time scale used in the radio broadcasts was important.

Rec. 460 prescribed that the broadcasts "maintain approximate agreement with Universal Time", but it gave no instructions on how to implement such a scheme.
Rec. 460 prescribed that those instructions were to be specified later.
Rec. 460 also prescribed that the director of the CCIR should transmit its text to the scientific unions, including the IAU.

Rec. 460 was a significant change of practice because it prescribed a new and untested form of broadcast, as opposed to previous CCIR documents which had described existing signals by broadcasters who had a long history of interacting with their users. Whereas the national laboratories had an ongoing means of interacting with their users, the CCIR was not constituted for giving prompt answers to questions about the usage of the signals.

Under Rec. 460 the duration of one second became unrelated to the duration of one day. This satisfied the two demands outlined in 1964 where the physicists required uniform seconds as a measure of elapsed time and the astronomers required that the value in the broadcasts would conform to a measure of the calendar days of Universal Time. Rec. 460 was thus an implicit abrogation of a fact that everyone knew to be true; a day would no longer be the same as 86400 seconds. This abrogation was not clearly communicated.

UTC[BIH] in 1970-08

At the 14th General Assembly of the IAU in Manchester the action of the CCIR to put leap seconds into radio broadcast time signals was discussed at the meetings of Commission 4 (Ephemerides), Commission 31 (Time), and at a joint meeting of 4 and 31. The IAU members used the term "UTC" when discussing the time scale used for radio broadcasts.

At the meeting of Commission 31 it was pointed out that the CCIR had failed to send a letter informing the IAU of the change, so the IAU was unable to respond officially at its 14th General Assembly in 1970. That made it impossible for the IAU to produce an official response before the IAU General Assembly in 1973, which was after the change would be implemented.

The resolutions from the 14th General Assembly of the IAU include

  • Resolution 6 of the entire General Assembly requesting that UTC not differ from UT1 by more than 0.1s without providing a way to give the difference
  • A resolution from Commission 31 with recommended instructions on how the CCIR might implement leap seconds.

UTC[BIH] during 1971-02-17/23
With less than ten months left before the deadline CCIR Study Group 7 met to formulate the detailed instructions for implementing CCIR Rec. 460 in CCIR Report 517 (Question 1/7, Resolution 53).
  • they tasked the BIH to reset the value of time-signal emissions at the end of 1971 such that 1972-01-01T00:00:00 UTC = 1972-01-01T00:00:10 AT
  • they specified that the difference between UTC and UT1 should normally be less than 0.7 s (which proved to be impossible to maintain during 1972)
  • they specified that leap seconds should occur at the end of a UTC month and they produced the 23:59:60 notation for designating positive leap seconds
  • they tasked the BIH with announcing leap seconds at least 8 weeks in advance
  • they tasked the BIH with announcing the value of DUT1 one month in advance, and everyone with circulating that value as widely as possible
  • they noted that "GMT may be regarded as the general equivalent of UT1"
It is relevant to note that during this meeting the atomic time scale (AT, TA) being maintained by the BIH had not yet been officially defined, approved, nor named.
radio broadcast time signals on 1972-01-01

CCIR Recommendation 460 went into effect. At the end of 1971 a special offset of -0.1077580 seconds was applied to step UTC so that ( TAI - UTC ) was exactly 10 seconds. Henceforth the length of the UTC second has matched the length of the TAI second, and the value of UTC has been adjusted via one second leaps to keep it within a second of UT1.

From this day onward the duration of one second has been unrelated to the duration of one day. In radio broadcast time signals the duration of a second has been determined by measuring cesium atoms, and the duration of a calendar day has been determined by measuring the rotation of the earth.

Without explicitly saying so the effect of CCIR Rec. 460 was to abrogate the connection between the concepts of time and date which had been understood by everyone since antiquity. The clock was disconnected from the calendar. Starting in 1972 the leap second is the only way that the concepts of date and time have been kept in sync.

UTC[BIH] in 1972-06/07

J. Terrien (the director of the BIPM who had been at the IAU meetings on radio broadcast time signals since 1964) noted that "the BIPM does no experimental work on the measurement of time and frequency" and apologized that "regrettable misunderstandings, especially between astronomers and physicists, have crept into discussions on time and frequency."

The CCDS declared that UTC can provide a firm basis for an internationally acceptable time system.

UTC[BIH] in 1973
The resolutions from the 15th General Assembly of the IAU gave more freely-available public description of UTC:
  • Resolution 1 from Commissions 4 and 31 pointing out that UTC provides mean solar time and recommending its use for civil time
  • Resolution 4 from Commissions 4 and 31 giving more advice on modifications to the scheme for implementing leap seconds
UTC[NBS] on 1974-01-01T00:00
The US NBS radio broadcasts of WWV and WWVH stopped using the term "GMT" and began to announce the time scale as "UTC".
UTC[BIH] on 1974-07-09/11
The CCDS produced Recommendation S 1 about "Universal Time (Coordinated), UTC". It invited the CGPM to recommend the adoption of UTC as the basis for official time in all countries. It also invited the CGPM to recommend the continued availability of the "additional information provided by the UTC System" -- in particular, DUT1. The explanatory notes contained this quote:
  1. Except where confusion might arise the designation Universal Time (Coordinated), UTC, may conveniently be abbreviated to Universal Time, UT.
  2. The CCDS takes note of the advantages of the use in all languages of the designations UTC and UT, for Universal Time (Coordinated) and Universal Time, respectively.
  3. GMT, which is still in wide use, may be regarded for most purposes as the general equivalent of UT. It is to be hoped that the term GMT will gradually be replaced by the term UT.
The CIPM approved CCDS Recommendation S 1 on 1974-09-24.
UTC/TUC in 1974-07-15/26

The 13th Plenary Assembly of the CCIR approved Recommendation 460-1, the first revision of the document specifying that radio broadcasts of time signals have leap seconds. It incorporated advice from the IAU 15th General Assembly in 1973 and raised the maximum allowable difference of (UT1 - UTC) from 0.7 s to 0.9 s. This was the first instance when the CCIR documents described the implementation rules of leap seconds as used in the time scale for radio broadcasts.

This was also the first CCIR document to be approved by the plenary assembly and the first instance of any CCIR recommendation about radio broadcast time signals which used (in French) the word "coordonné" and the term "TUC" or (in English) the word "coordinated" and the term "UTC". (It was another 4 years until the CCIR issued Rec. 536 to specify a single acronym for UTC.)

UTC on 1975-01-01
CCIR Recommendation 460-1 gave this as the date on which the time scale used for radio broadcasts was to conform to specifications contained within itself.
UTC in 1975-05/06
Resolution 5 of the 15th meeting of the CGPM considered that UTC provides both atomic frequency standards and UT (or mean solar time) and endorsed its use for civil time. As a result, legislative bodies in some countries began to adopt UTC as a precisely defined replacement for GMT in the basis of their legal time.
UTC in 1976-08/09
The 16th General Assembly of the IAU in Grenoble produced Resolution No. 3 by Commissions 4 and 31 which recommended that the abbreviation UTC be used for all languages.
UTC on 1977-01-01
Because the rate of TAI was reduced by one part in 1012, the rate of UTC was reduced by the same amount. Therefore, before this date UTC seconds were shorter than they currently are.
UTC on 1978-06-15

The report of Study Group 7 to the 14th Plenary Assembly of the CCIR contained a number of statements that are even more interesting in retrospect. Its section on UTC started with the words "UTC was introduced by Recommendation 460 in 1972". Those words give no indication that the CCIR recognized any form of UTC previous to 1972, and they omit any mention that the CCIR did not apply any name to the radio broadcast time scale until 1974. The SG7 report asserted that the mean solar time of the Greenwich meridian is Universal Time (UT). The SG7 report acknowledged the 1975 recommendation from the CGPM as the basis for civil time (but did not explicitly reiterate that the CGPM had recognized UTC as a form of mean solar time). The SG7 report also pointed out that some countries had already redefined their legal time to be based on UTC and said "the UTC time scale is the general reference for civil time". (This is the first instance in which the notion of a reference time scale appears. In the aftermath of the 2012 RA and WRC the phrase "reference time scale" had become jargon.) So SG7 recognized in report to the CCIR plenary that the use of UTC had already accumulated political implications without clearly realizing that those would eventually compete with technical considerations. Finally it indicated that the implementation of UTC was complete and there was no further need for Interim Working Party 7/1. Thus, just as the world of computing systems was beginning to recognize a need for what would become POSIX, the CCIR decided to abolish the working party which was best suited to answering questions about operational details.

The 14th Plenary Assembly of the CCIR approved working document 7/1007 as the second revision of the document specifying that radio broadcasts of time signals have leap seconds in CCIR Recommendation 460-2. (Once again, the revision to Rec. 460 was mostly to incorporate the words and actions of other agencies, in this case CCDS Recommendation S1 (1974)). The Plenary Assembly requested the views of the Observer for the International Astronomical Union regarding the sentence "GMT may be regarded as the general equivalent of UT." (That is in Annex I section A about the UT1 variant of the time scale Universal Time.) The IAU observer "said that the use of GMT was neither authorized nor approved, but merely noted, by his organization." The Chairman of Study Group 7 (G. Becker of FRG) proposed putting parentheses around the GMT sentence, and the assembly approved Rec 460-2 with that amendment.

The Plenary Assembly also approved working document 7/1008 as CCIR Recommendation 535. The aim was to recommend the usage of the term UTC to replace GMT in all international telecommunications activities and documents. This recommendation was explicitly based on consideration of the 1975 action of the 15th CGPM and the 1973 action of the 15th General Assembly of the IAU, both of which had acknowledged UTC as a form of mean solar time. In particular this recommended that UTC replace GMT in the Radio Regulations of the CCIR itself, and the resulting wording made it clear that this usage applied not merely to time, but to calendar dates as well. In combination with the words in the SG7 report that distinguish UTC from the mean solar time of them Greenwich meridian this resulted in a longstanding confusion because Radio Regulation 2.5 defines the date using the concepts of longitude and earth rotation where UTC itself is only connected to those concepts because there are leap seconds.

The Plenary Assembly also approved working document 7/1009 as CCIR Recommendation 536. The aim was to introduce "language independent time-scale notations". Explicitly based upon consideration that in 1971-10 the 14th CGPM had defined International Atomic Time "using the designation TAI" and that in 1975-05 the 15th CGPM had recommended the use of Coordinated Universal Time "using the designation UTC" this recommended the use of those abbreviations. Thus the CCIR did not take credit for creating the abbreviations TAI and UTC, but rather accorded that to the CGPM. Finally, Recommendation 536 explicitly recognized UTC as a form of Universal Time, and the CCIR recognized UT as mean solar time.

UTC during 1979-09/12
At the 10 week long World Administrative Radio Conference the Final Acts indicate "UTC is equivalent to mean solar time at the prime meridian" (see page 32), and UTC is used throughout the 984 pages of the document, replacing GMT in earlier versions.
UTC during 1980-11-10/21
The 7th Plenary Assembly of the CCITT accepted UTC "as the time scale for all other telecommunications activities."
UTC in 1982

The introduction by the chair of SG7 starts off with a paragraph that is, in retrospect, astonishing for the way it crows about how UTC with leap seconds has been acepted by many agencies as solving all problems:

The year 1982 will see the tenth anniversary of the system of Coordinated Universal Time (UTC) which was introduced in essentially its present form on 1 January 1972 by Recommendation 460. In the past ten years the UTC system has gained widespread acceptance as the reference time scale for timekeeping purposes throughout the world, not only for technical and scientific applications but also for time in everyday affairs. It is particularly gratifying to the Study Group that the World Administrative Radio Conference in 1979 (WARC-79) accepted the terms of Recommendation 535 and adopted UTC as the reference time scale for all radiocommunication activities. Likewise, UTC was also accepted in 1980 by the VIIth Plenary Assembly of the CCITT as the time scale for all other telecommunications activities. Together, these two actions constitute a powerful endorsement of the careful work and the intensive discussions extending over more than one plenary period which led to the present UTC system, as specified in Recommendation 460.
This introduction appears to be an early instance of using the phrase "reference time scale" when referring to UTC.

In contrast to the introduction in 1978, this introduction refers to the existence of "the UTC system [...] prior to 1972" even though no document approved by the Plenary Assembly had used that name until 1974. So again we see a CCIR document asserting facts that are inconsistent with the recorded history of the CCIR.

The 15th Plenary Assembly of the CCIR approved the third revision of the document specifying that radio broadcasts of time signals have leap seconds as CCIR Recommendation 460-3.

In retrospect, the absence of publications containing clear and consistent specifications makes it no surprise that external agencies (e.g., IEEE, ANSI, ISO, POSIX) later produced standards for programming interfaces describing something that they called UTC as a time scale with properties that differed from the UTC in radio broadcasts. It is amazing to see the extent to which SG7 failed to foresee the the problems with leap seconds that started to appear within the next decade.

not really UTC in 1986-04
This is the publication date of IEEE Std 1003.1 issued for Trial-Use in April 1986. See the 1988 official publication for details.
UTC in 1986-07
The fourth revision of the document specifying that radio broadcasts of time signals have leap seconds was issued as CCIR Recommendation 460-4.
UTC in 1988
The responsibility for maintaining UTC was split between two agencies. The responsibility for keeping TAI was transferred from the BIH to the BIPM. The responsibility for monitoring earth rotation, determining UT1, and announcing the need for leap seconds was transferred to the newly created IERS. After around a century of operation the BIH and the ILS ceased to exist as these responsibilities were combined into the IERS.
not really UTC on 1988-09-30

This is the publication date of IEEE Standard Portable Operating System Interface for Computer Environments (IEEE Std 1003.1-1988) also known as POSIX.

Chapter 2 (Definitions and General Requirements) section 2.3 (General Terms) defines

as 1970-01-01T00:00:00 Coordinated Universal Time without regard to the fact that at that date there was not general recognition of the existence of a time scale by that name
Seconds since the Epoch
as Coordinated Universal Time with an expression that requires 86400 seconds in one day (and also lacked the 400-year rule that Pope Gregory XIII instituded 4 centuries earlier) without regard to the fact that the rules for UTC do not always have 86400 UTC seconds in one UTC day

Appendix B (Rationale and Notes) section B.2.3 (General Terms) explains

  • leap seconds are ignored for the sake of an "easy" method
  • "most systems are probably not synchronized to any standard time reference"
  • "it is inappropriate to require that time" ... "precisely represent the number of seconds"
  • "The standard is more concerned about the synchronization of time between applications of astronomically short duration"

For POSIX the choice of goals was that simplicity and calendar days were more important than precise time. The result of this standard has been that a machine cannot handle precise time with leap seconds and also be a Conforming Implementation of POSIX.

not really UTC in 1989/1990

The C programming language was standardized as ANSI X3.159-1989 and then as ISO/IEC 9899:1990. Some time between the drafts in 1988 and the final standard the section on time.h with the definition of struct tm changed the allowed values of element tm_sec to the range [0-61] with the explanation that this was to handle a "double leap second". There has never been such a thing as a "double leap second". In 2001 Mark Brader claimed that this was his fault.

This erroneous notion of "double leap second" highlights that ITU-R recommendations are expected to be implemented by everyone, yet they are not freely distributable so their content is not widely known. As seen in this history of UTC, ITU-R recommendations may be approved in the absence of any consensus, rationale, implementation details, or interoperability tests.

not really UTC in 1992
The ANSI SQL:1992 (also ISO/IEC 9075:1992) standard allowed for minutes with 59, 61, or 62 seconds. Although it did not use the words, this is another instance of an internationally-approved standards document that erroneously asserted the existence of the "double leap second".
UTC in 1992/1993
The CCIR was reorganized to become the ITU-R, and the document requiring leap seconds in radio broadcasts of time signals was renamed ITU-R Recommendation TF.460-4.
UTC from 1995 through 1998
In 1995 a CCTF working group determined that the length of TAI seconds was longer than the SI second because the chronometers contributing to TAI were not corrected for the effects of blackbody radiation. Over the next three years the frequency of TAI was steered to reduce the length of its seconds by about 2 parts in 1014. Therefore the length of UTC seconds was also reduced. This change is evident as the final kink in the plot of TT(BIPMxy).
not really UTC in 1997
At some point after the C language standard erroneously created the notion of "double leap second" it was also incorporated into the POSIX standard. In 2003 Landon Curt Noll described the POSIX process. This error was still present in the 1997 POSIX standard.
UTC in 1997-10
The fifth revision of the document specifying that radio broadcasts of time signals have leap seconds was issued as ITU-R Recommendation TF.460-5. This version deleted the sentence "GMT may be regarded as the general equivalent of UT." It changed the reference for the definitions of the forms of UT from the Astronomical Almanac to the IERS. It removed the option of giving DUT1 by voice or Morse code.
UTC since 1999
Klepczynski publicly suggested discontinuing leap seconds, and the CCTF wrote a letter to various international scientific unions which started the ongoing process of reconsidering the future of leap seconds.
UTC in 2000/2001
In 2000-05 the ITU-R Radiocommunications Assembly tasked WP7A with a Question designated 236/7 (now aka Question SG07.236) "The future of the UTC time scale" in order to begin studies about the possibility of discontinuing leap seconds.
UTC in 2002-02
The sixth revision of the document specifying that radio broadcasts of time signals have leap seconds was issued as ITU-R Recommendation TF.460-6. This version introduced the notion of DTAI = TAI - UTC
not really UTC in 2003
The ANSI SQL:2003 (also ISO/IEC 9075-2:2003) standard still allowed for minutes with 59, 61, or 62 seconds, thus the "double leap second". This means that for more than a decade an international standard was able to contain a notion that had never existed without being corrected. Correcting this level of mistake involves re-educating every human who has read it and re-designing every system which implemented it.
Approximations to UTC are maintained in real time by various laboratories around the world. For example, the designation UTC(NIST) refers to the approximation to UTC that is maintained at the US National Institute of Standards and Technology, and UTC(NPL) refers to the approximation to UTC maintained at the UK National Physical Laboratory. The differences between the various instances of UTC(k) are published monthly by the BIPM in Circular T.
UTC in 2003-05-28/30
The SRG of ITU-R WP7A held the Colloquium on the UTC Timescale in Torino Italy. The conclusion of that meeting produced the Potential Alternative to the Leap Second. The conclusion called for a broadcast time scale without leap seconds to be given a new name, International Time (TI), which was proposed in a paper contributed by E.F. Arias.
UTC in 2004-04-01/02

At the 16th meeting of the CCTF Ron Beard reported on the 2003 UTC colloquium at IEN in Torino.

The conclusion of the SRG was that the creation of a new time scale, to be known as "International Time", was not recommended
From this it was clear that the SRG of ITU-R WP7A had already decided to disregard the advice they had received from the international experts they had gathered to discuss the future of UTC.

Other members of the CCTF discussed in response to Beard.

Dr Boulanger expressed the opinion that it was wrong to change the definition of the (UTC) timescale without changing its name as it could lead to a diminution of trust. Dr McCarthy pointed out that there was a precedent for this. The frequency steps which were once part of UTC has been dispensed with. Dr Arias said that the word "universal" would no longer be appropriate for UTC. Dr McCarthy recalled that the word "coordinated" was chosen to reflect the coordinated change in the different timescales, then in use in the UK and the USA, to the (new) UTC. Dr Arias clarified her earlier point; she believed that the word "universal" was appropriate only for a timescale that was "linked" to the rotation of the Earth.

not really UTC in 2004
The POSIX standard no longer contained the notion of "double leap second".
UTC in late 2004

As of 2004-09 the index of contributions to WP7A on the ITU website contained a new document from the United States whose title said it was a proposed revision of ITU-R TF.460-6 (the document requiring leap seconds in radio broadcasts of time signals). Although it was not possible to see the content of that document, it seems likely that it was a revision of document nc1893wp7a on the FCC website from the United States Working Party 7A that holds the federal charter to interact with the ITU-R.

Document nc1893wpa from USWP7A proposed that UTC should switch to having leap hours on 2007-12-21. It is relevant to note that on this date the legal time of the US was mean solar time, so a document which did not include some kind of leap would have been contrary to the will of congress.

At the meeting of ITU-R WP7A on 2004-09-28/10-01 the other delegates scoffed at the notion of leap hours.

UTC in late 2005

On 2005-09-19 the USWP 7A released the 2005 version of its Proposed Revised Recommendation ITU-R TF.460-6. The document was not approved by the US State Department, but its text would have ceased adding leap seconds as of 2012-12-21. The fact that this date would have been the same as the end of the Mayan calendar long count was a source of some amusement.
At the meeting of ITU-R WP7a on 2005-11-08/11 there was no consensus for change.

UTC in 2010-10
ITU-T SG 15 recommended the use of PTP (IEEE 1588) in ITU-T Recommendation G.8265.1. In many respects this is the ITU-T saying that their systems no longer have interest in following ITU-R Rec. 460. This recommendation tacitly ignores the fact that in 1980-11 the 7th Plenary Assembly of the CCITT (the predecessor organization of the ITU-T) accepted UTC as "the time scale for all other telecommunication activities."
UTC in 2010-12
The ITU-R began to allow open access to recommendations, including TF.460. After 40 years behind a paywall engineers, designers, and code writers could see the document which defines the times signals that are inputs to their systems. Unfortunately this was about 30 years too late for POSIX systems.
UTC in 2011-08
The BIPM produced a special issue of Metrologia entirely on the subject of time scales. In its first article emeritus BIPM director Terry Quinn suggests that the ITU-R should transfer the authority for defining UTC to the CGPM.
UTC on 2012-01-19
The delegates to the Radiocommunications Assembly (RA) had the opportunity to vote on the proposed draft revision of ITU-R TF.460-6. If the delegates had approved the draft then UTC would have stopped having leap seconds 5 years after approval. The effect of approval would be to redefine the word "day" such that it is no longer related to the sun in the sky.
The RA delegates found themselves split into three camps with no consensus. The delegates decided to defer any redefinition of UTC until the RA to be held in 2015.
UTC on 2012-09-13/14
The 19th meeting of the CCTF issued Recommendation CCTF 6 (2012) in response to WRC-15 Agenda Item 1.14 which calls for more study of UTC and leap seconds. It asserted facts that are not congruent with the documented history of statements about time made by the IAU, CCIR, CGPM and other agencies. In particular, fact 9 states that UT1 should not be considered as a time scale, thus effectively asserting that prior to the cesium atomic chronometer there were no time scales. This document is the clearest indication that the ITU-R is dealing with a subject where there has been an ongoing turf war between various national and international agencies who are prepared to distort history in support of their position.
UTC in 2012-10
The 101st meeting of the CIPM considered Recommendation CCTF 6 (2012). They acknowledged that there are "political dimensions" to UTC and the leap second. Their notes included
  • UTC will continue even without the leap second.
  • Other communities will be interested in changes involving the leap second due to the impact on standards.
  • It is the responsibility of the CGPM to define time scales, not the ITU.
UTC in current events
Please refer to the sibling document that contains more details on recent, current, and upcoming events about UTC.
UTC as of 2019-01-01 ?
According to the 2009-06-29 memo from the US Assistant Secretary of Defense this is the earliest date at which DoD systems can be modified to be ready for UTC to have no more leap seconds.
UTC in 2022 ?
According to the Potential Alternative to the Leap Second developed at the Colloquium on the UTC Timescale held by ITU-R SRG 7A in Torino on 2003-05-28/30:
UTC will cease to exist.
UTC will be replaced by the purely atomic time scale named TI which is described above.

UTC has always been a hybrid of the needs for atomic frequency and time interval and the needs for the counting of calendar days. This is because the intended purpose for UTC is to satisfy two separate goals: seconds of uniform length, and days that match the calendar progression of earth rotation. As a result

On the other hand, the full second leaps which have been introduced in UTC since 1972 (and the millisecond leaps in radio broadcast time signals before 1972) are nothing new for civil time. Prior to atomic chronometers there were no practically available clocks which were as stable as earth rotation. Even the best temperature-controlled quartz crystal clocks needed to be reset -- i.e., leaped -- regularly to agree with the astronomical observations measuring calendar days. Throughout the long history of developing clocks for timekeeping it had always been that way. Prior to 1960 the leaps had been applied individually, by each local timekeeping agency, for its own set of time signals, as deemed necessary by the astronomers running the transit instrument at the local observatory. The only real change that happened in 1972 was to agree that the leaps should be full atomic seconds, coordinated internationally, and that there should be a nomenclature scheme for referring to those leap seconds.

What happened in 1972 was that the clock was disconnected from the sun. Each individual second became disconnected from earth rotation, but the leap seconds allow the individual days of the calendar to remain connected with the sun.

The possibility of changing UTC to omit leap seconds means ignoring astronomy and disconnecting the calendar from the rotation of the earth. It would make the progression of civil time predictable, but time tags such as 12:00:00 would be completely unrelated to having the sun overhead at noon. Over the passage of centuries the difference between 12:00:00 and noon would become increasingly obvious.

Reversing what Julius Caesar did 2000 years ago, it would remove the calendar from the hands of astronomers and put it back into the hands of politicians. It would change the meaning of all contracts based on days. Bank interest would effectively be accumulated per second rather than per day. Midnight debut showings of blockbuster movies would be based on cesium atoms and political decrees with no reference to sunrise or sunset.

Note also that if UTC is redefined without being renamed, the result will be the same sort of archival chaos as was created by the British Admiralty when it redefined GMT in 1925. Because the definition of UTC has always been a hybrid, anyone finding the term UTC in a document will not be sure of its intended meaning even if the document originated while UTC had leap seconds. After such a change it will be unclear whether authors intended the use of "old UTC" or "new UTC", or whether the difference is important at all. This question will have to be asked:

Should an instance of the term "UTC" in a document be interpreted simply as specification of a conventionally-available atomically-regulated civil time, or was it originally intended to specify a form of mean solar time and related to calendar days?
Persons reviewing computer source code and documentation will have to ascertain whether the original intent of authors and system operations involving any quantity called UTC require a time scale with properties that match mean solar time or not. Legal systems which have relied on the fact that UTC is a form of mean solar time may require revision. Legal and operational systems which seem to work at first may fail later as the difference between UTC and UT1 grows. Changing the characteristics of UTC without changing the name may seem easy at first, but it has widespread consequences of potentially considerable cost which must eventually be paid.

UTC without leap seconds would be inconsistent with the nomenclature established by resolution V of the 1884 International Meridian Conference and adopted by the IAU, and also inconsistent with the 1975 CGPM resolution on UTC.

The robustness of UTC across an interruption in civilization is schizophrenic specifically because UTC is a combination of two incommensurate concepts. Inasmuch as UTC is a vehicle for communicating mean solar time, UTC is robust because by its own definition it admits that the conventional value of mean solar time need not be accurate to much better than one second. Inasmuch as UTC is a vehicle for atomic time, UTC cannot be robust because atomic time is not.

POSIX time
The current standard for this time scale is IEEE P1003.1 or POSIX.

In 2003 Landon Curt Noll submitted some historical views on how POSIX got the way it is to the LEAPSECS mailing list.

The standard defines seconds since the epoch ( 2004, Base Defs sec 4.14, 2008, Base Defs sec 4.15) as ignoring the existence of leap seconds. As a result, the rationale (2004, Rationale A.4.14, 2008, Rationale A.4.15) admits that not all POSIX seconds have the same length, and it is also fuzzy about the definition of the epoch ( 2004, Rationale sec A.3, 2008, Rationale sec A.3).

To put it simply, the POSIX standard is self-inconsistent. A rigorously defined time scale should either be a count of mean solar seconds (which are subdivisions of calendar days), or atomic seconds (which are unrelated to calendar days). POSIX tries to be both and as a result it cannot address leap seconds meaningfully.

The link in this sentence leads to my web page with javascript that plainly demonstrates the insoluble problem posed by the past history of POSIX time.

POSIX time shares most of the caveats of Julian Date (above). There are two main distinctions between POSIX time and the forms of Julian Date. First, POSIX time is an integer count of seconds instead of an arbitrary-precision real number of days. Second, the quality of chronometers and the representations of integers in computing hardware place severe restrictions on both the precision of instants of time and the useful span of dates which can be represented by POSIX time.

See the related calendar and bibliography page for more ruminations on the POSIX time scale.

There is no way to go back and make POSIX time retrospectively self-consistent. This was tried with the "right" zoneinfo files, and the result was confusing and somewhat chaotic.

There is a way to make POSIX time self-consistent starting at some future date -- even if UTC retains leap seconds. POSIX time (as well as the NTP which usually underlies it) can become self consistent if the available broadcast time scale stops having leap seconds. This could be accomplished by changing the name of the broadcast time scale, omitting leap seconds from the newly-named broadcast time scale, and letting UTC become a time zone in the zoneinfo files. This is effectively the strategy espoused in the conclusion of the 2003 Colloquium on the future of UTC held in Torino. See the prescription on this page for details.

Microsoft Windows file time
Microsoft Windows defines its file time as "the number of 100-nanosecond intervals that have elapsed since 12:00 A.M. January 1, 1601 Coordinated Universal Time (UTC)."

The year 1601 is prior to the development of reasonably accurate escapements for clocks. The year 1601 is prior to the development of the telescope. The year 1601 is 75 years prior to the establishment of the Royal Greenwich Observatory, so not even GMT makes sense.

As noted above, nothing that could possibly be called UTC existed prior to 1960. UTC has always been an atomically-regulated time scale. Prior to atomic chronometers all practical time keeping was performed in subdivisions of the day. Subsequent to atomic chronometers it becomes necessary to decide whether a time scale is counting calendar days or SI seconds.

For values subsequent to 1972 trying to do both means that Microsoft Windows file time faces the same problem as POSIX time. For values prior to 1956 there is no possible way to attribute an unambiguous meaning to a Microsoft Windows file time. The whole notion is utter bilge.

See the related calendar and bibliography page for more ruminations on the Microsoft Windows file time.

Also note that Microsoft has rejected the notion that Windows is capable of noticing leap seconds or their absence because

The W32Time service cannot reliably maintain sync time to the range of 1 to 2 seconds. Such tolerances are outside the design specification of the W32Time service.
(It is likely the case that any other OS running on motherboards with piece-of-crap hardware clock implementations would experience the same issue.)

Microsoft .NET Framework DateTime Structure

The .NET framework from Microsoft takes the above notion even farther into fantasyland.

According to the docs on msdn the DateTime structure "represents dates and times with values ranging from 12:00:00 midnight, January 1, 0001 Anno Domini (Common Era)" using "100-nanosecond units called ticks, and a particular date is the number of ticks since 12:00 midnight, January 1, 0001 A.D. (C.E.) in the GregorianCalendar calendar".

According to calendar historians, at that date the Roman empire under Augustus was probably still engaged in the process of having no leap years at all (in order to correct for having one every three years starting in the year Julius died). This is not to mention the fact that the Common Era would not be defined for another millennium, the Gregorian Calendar for another half millennium after that, and the atomic chronometer another four centuries after that.

Network Time Protocol (NTP)

Dr. David Mills at the University of Delaware began implementing the Network Time Protocol (NTP) in the early 1980s. The current versions of NTP are able to keep computers all around the Internet synchronized to better than one millisecond. Dr. Mills has written this book on NTP as a definitive reference.

Principal documentation for NTP can be found at the web site of Dr. David Mills and at the NTP project web site. The IETF has an active working group on NTP and provides status of the effort.

It is almost certainly a mistake for me to write anything further about NTP, but its characteristics place it on this page.

The NTP timescale is kept and exchanged via an unsigned 64-bit fixed point integer where the upper 32 bits represent integer seconds and the lower 32 bits represent fractions of seconds to a resolution of around 200 picoseconds. The origin of the current NTP era is 1900-01-01T00:00:00 UT, and the NTP counter will wrap around in the year 2036. Given that UTC with leap seconds originated in 1972, and that atomic time did not exist before 1955, it is not clear that any meaning dare be attributed to the fractional bits of the NTP chronometer during most of the first half of the present NTP era.

The NTP counter ignores leap seconds. As such, its practical properties are very similar to POSIX time. NTP ticks in SI seconds, but its counter accumulates mean solar seconds. At the sub-second level NTP time corresponds directly with TAI or UTC since 1972. At the resolution of one second NTP corresponds to mean solar time. Differences NTP over long spans of time correspond to the historical tradition where "time" means calendar days of earth rotation.

Local Civil Time -- sometimes LCT
Local civil time is that which is decreed by the local authorities. No decision by the IAU, ITU, IERS, BIPM, CGPM, URSI, or any other international organization can dictate the time scale used for legal purposes in any jurisdiction. The laws of many localities have not changed since GMT was the basis for worldwide time. Other localities changed their laws to adopt UTC as the basis for legal time after the IAU, CGPM, and other international organizations recommended its use.

The properties that are suitable for LCT remain an open question. Some kind of solar time has been the basis for most calendrical schemes since the dawn of history. Any form of solar time is guaranteed to be non-uniform, and hiding that non-uniformity has been a goal for centuries. The non-uniformity is too subtle for humans to notice, but the number of human-created systems which can detect that non-uniformity continues to increase.

Atomic time is necessary to many systems used by civilization. Current applications for navigation require that everyone be able to agree on its value to about 10 nanoseconds. Mean solar time is traditional as civil time and necessary in order to construct a calendar that counts days. Current applications for civil time require that everyone be able to agree on its value to about 10 milliseconds. There is little dispute that it is far more important for everyone to be able to agree on the value of civil time to within 100 nanoseconds than it is for civil time to have a value that tracks mean solar time to within 100 milliseconds.

Humanity could adopt perfectly uniform atomic time now at the expense of letting our descendants figure out how to return to mean solar time if they so desire.
We could admit that both mean solar time and atomic time are important at the expense of retrofitting all timekeeping systems, legal systems, and the general populace with the understanding that there are two different kinds of time.
The relative merits of these and other options are not yet clear.
Perhaps by forcing the question might we trigger civil timekeeping chaos where different jurisdictions choose different answers to the questions?

For those who are designing operational systems that rely on stable inputs of time scales the many changes in the definitions, meanings, names and uses of time scales evoke another pop-culture quote:

I am altering the deal. Pray I don't alter it any further.
-- Darth Vader, The Empire Strikes Back

Bibliography of works not directly cited above

Other web pages on time scales

Thanks go to John Seago for providing some references above.
Steve Allen <sla@ucolick.org>
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