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.
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.
The IAU has had a love/hate/love relationship with MJD. In 1973 the IAU resolved that it should be used. In the mid-1990s the IAU seriously discussed the possibility of recommending that the term MJD should not be used because several other quantities with the same name but different definitions have been in use in various contexts. Finally (?) in Resolution B1 of the XXIIIrd General Assembly in 1997 the IAU recognized that when properly defined the term MJD may be used. (This may have been 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.)
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.
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.
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.
For the only time scale with leap seconds see UTC.
Nevertheless, the practical and historical reality is that
the available forms of GMT have always had leaps. The
leaps happened every time any official clock was reset to
agree with 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.
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.
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 should define the prime meridian. All of the maps of England, however, used the meridian occupied by the previous transit instrument at Greenwich. By strict interpretation of the IMC results, all maps of England suddenly had incorrect values of longitude.
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.
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''.
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.
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
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 the IAU at the Vth GA in 1935. Starting in 1939 the US almanac used ``GCT or UT'' until 1952 when it switched to UT alone.
In 1948 the IAU recommended that UT (and WZ) should be used by astronomers only to designate Greenwich mean solar time reckoned from midnight. Through the 1950s UT was used as the independent variable of the ephemerides.
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''. From 1895 until 1984 the conventional formulae for UT were those of Newcomb which were based on observations of the sun. As pointed out 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. Nevertheless, Newcomb knew that his expressions for the fictitious mean sun would eventually deviate from the position of the actual mean sun.
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 its IXth GA in 1955 the IAU, via its Commission 31 (Time), directed the BIH to publish corrections for the seasonal variations of UT. Although there do not seem to 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.
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.
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.
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.
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,
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).
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 GA in 2006 the IAU approved resolutions which explicitly contain a new definition for the ecliptic, and thus for the equinox.
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.
At the XXIVth GA in 2000 the IAU resolved 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 remain open questions which are even now being considered 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.
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.
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.
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.
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 clocks. 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 XIVth IAU GA 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.
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.
The IAU GA in Prague approved several resolutions including one that re-defined 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.
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.
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.
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.
The name TAI was officially proposed in 1970 and adopted in 1971. Beginning in 1977 TAI has been constructed by steering the frequency of what is now called EAL with an offset. Since 1980 TAI has been a "realization of TT", a coordinate time, in conformance with the definition approved by the IAU in 1991, but TAI does not officially incorporate the clarifications to TT that the IAU adopted in 2000. Guinot (1986) explored the ways in which clocks contributing to TAI measure a proper time.
TAI has always been a statistical combination of the available ensemble of atomic clocks. The differences between TAI and various other atomic time scales are published monthly by 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 TAI can be traced via TT(BIPMxx). The initial results of the meeting about the calculation of TAI held on 2004-03-31 are now online from the BIPM.
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.
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.
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.
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.
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.
Initially the name UTC was not used. The process of broadcasting synchronized signals was called coordination, and the resulting signals were called coordinated. Prior to the advent of atomic clocks it had not been possible to keep the broadcast time scales of widespread systems synchronized to within a millisecond.
The original goal of 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 of UTC 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 UTC.
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.
The archival document from USWP7A proposes that UTC should switch to having leap hours on 2007-12-21.
On the other hand, the leaps which have been introduced in UTC (milliseconds before 1972 and full seconds since 1972) are nothing new for civil time. Prior to atomic clocks 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 astronomical observations. 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.
The possibility of changing UTC to omit leap seconds means ignoring astronomy and disconnecting civil time completely 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.
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 compromise, 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 ``UTC'' be interpreted
simply as a conventionally-available atomically-regulated civil
time, or should it be interpreted as a form of mean solar time?
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.
In 2003 Landon Curt Noll submitted some historical views on how POSIX got the way it is to the LEAPSECS mailing list.
The current standard defines seconds since the epoch ignoring the existence of leap seconds. As a result, the rationale admits that not all POSIX seconds have the same length, and it is also fuzzy about the definition of the epoch.
To put it simply, the POSIX standard is self-inconsistent. A rigorously defined time scale should either be a count of mean solar seconds, or atomic seconds. POSIX tries to be both and as a result it cannot address leap seconds meaningfully.
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 clocks 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 online bibliography page for more ruminations on the POSIX time scale.
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.
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 clocks all practical time keeping was performed in subdivisions of the day. Subsequent to atomic clocks 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 online bibliography page for more ruminations on the Microsoft Windows file time.
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.
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 clock 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 earth rotation angle.
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?
