The Future of Leap Seconds

See also:

• Watch the leap second on your computer!
  The legal number of elapsed seconds since 1972
• Figures and tables showing the reasons for leap seconds
• Figures plotting differences between time scales and the effect of abandoning leap seconds
• A history of precision time scales

Civil time has always been some form of solar time (i.e., time-of-day), but there have been many other ways of reckoning time. During the past few centuries the basis of most civil time has been mean solar time, and this was adopted for legal purposes almost everywhere by international vote in 1884. In 1928 Universal Time became the official name for the quantity which best indicates the mean solar time of the Greenwich meridian (or GMT).

Since their inception early in the 20th century radio broadcasts of time signals have provided a form of mean solar time because that served a dual purpose both for navigation and for civil time. Fractional second leaps were routinely introduced into the broadcasts in order to keep clocks running on mean solar time. In the 1950s atomic resonators became available, and the atomic second was adopted with a different length than the mean solar second. In the 1960s radio broadcasts began to be based on atomic clocks with numerous fractional second leaps in order to track mean solar time. Since 1972 radio broadcasts have provided Coordinated Universal Time (UTC) which serves a further purpose; it always uses atomic seconds and tracks mean solar time by inserting occasional full second leaps. As decades have passed, radio broadcast time signals have become used by an increasing number of systems which need atomic frequency; some of these need elapsed time and others need unique timestamps. Throughout the history of civil time it has always been the case that clocks are reset to agree with the rotation of the earth. That is the distinction between a clock and a chronometer.

The Proceedings of the Colloquium on the UTC Timescale give the best available look at the openly public deliberations. As the process continues it has become clear that some representatives to the ITU-R intend to urge the ITU-R to act unilaterally and recommend the broadcast of an atomic timescale which would continue to be called UTC but which would not have leap seconds. This ignores the results of the colloquium in Torino which recommended that if broadcast time signals are changed to omit leap seconds then the resulting new atomic time scale should have a new name other than UTC. See the 2005-05 Recent Event entry below for links to the details.

It remains unclear whether or how such new broadcasts of purely atomic time would include information about mean solar time. Broadcasts of purely atomic time would presumably be intended to serve the needs of systems, but a change from broadcasting Universal Time would have unknown effects on civil and legal time -- and ultimately on people.

Upcoming events

2008-12-31T23:59:60

The next leap second will occur, and it will trigger an never before seen situation regarding legal time in the US and Quebec and POSIX time.

Recent events

``Curiouser and curiouser!'' cried Alice (she was so much surprised, that for the moment she quite forgot how to speak good English)

-- Lewis Carroll, Alice's Adventures In Wonderland

2008-10-14: ITU-R SG7, Geneva

The meeting of SG7 presumably received a report on the future of UTC (as in ITU-R TF.460) from WP7A.

2008-10-08/13: ITU-R WP7A, Geneva

The meeting of WP7A should have considered the proposed changes for UTC (in ITU-R TF.460). China reportedly objected to any change in UTC.

prior to 2008-10: various national agencies

The delegations to ITU-R from various countries discussed their contributions and responses to the notion of omitting leap seconds in radio broadcast time signals.

2008-09-15/16: The 48th CGSIC meeting, Savannah GA

Tom Bartholomew presented the status of ITU-R WP7A on UTC

2008-09:

The US Naval Observatory conducted a survey about leap seconds which proceeds under the presumption that the underlying broadcast timescale will remain named UTC.

2008-03-31/04-04: ITU-R WP7A, Geneva

The meeting of WP7A discussed escalating the proposed changes for ITU-R TF.460 from WP7A to SG7.

2008-01-28/30: Institute of Navigation National Technical Meeting, San Diego

In session B1 Ron Beard presented The Future of the UTC Timescale. Point 1 on slide number 26 indicated that the Colloquium in Torino did not recommend the creation of a new name for a time scale without leap seconds -- this is not congruent with what actually happened.

2007-10-08/11-02: ITU-R World Radiocommunication Conference (WRC-07)

Changes to ITU-R recommendations were made at this meeting, but not to ITU-R TF.460. (Note that the effective date of the end of leap seconds proposed by the 2004 contribution of the United States was 2007-12-21 which was less than two months after the conference.)

2007-09-24/25: CGSIC 47th Meeting, Forth Worth, TX

Dr. Wlodzimierz Lewandowski of the BIPM gave a presentation on the ITU-R efforts regarding UTC. Indications are that the ITU-R intends to abandon leap seconds by the year 2013.

2007-09-11/14: ITU-R WP7A, Geneva

The most recent meeting of WP7A. There were input documents from Italy, Japan, USA, and BIPM regarding UTC. There was also another new draft of ITU-R TF.460.
With only one objection to the plan to abandon leap seconds in UTC (the UK objected again) the ITU rules allow the proposal to be elevated from WP7A to SG7. Indications are that this will happen during 2008.
The BIPM strongly objected to the proposal from USSG7 (which espoused the notion of giving GPS time, and thus Galileo satellite time as well, an international endorsement). This is in one sense not surprising, for it would, de facto, remove TAI from its ostensibly primary role and effectively transfer control of world time from BIPM to the clocks of USNO and NIST. In another sense it is surprising, for in its 2006 meeting (see pages 19/21) the CCTF described what TAI can and cannot do, and at this WP7A meeting the BIPM discussion includes the phrase "suppressing TAI".

2007-01: US Department of State

Internal discussions occurred about the US position on leap seconds to be represented at the ITU-R WP7A.

2006-10-31: ITU-R SRG 7A

The final report on the future of UTC.

2006-10-30/11-17: ITU Plenipotentiary Meeting (PP-06)

These meetings are held every 4 years and set the overall course of the ITU. It is not clear whether this meeting was relevant to changes in ITU-R TF.460-6.

2006-08-28/09-01: ITU-R WP7A, Geneva

Demetrios Matsakis supplied one of the documents from the meeting to the LEAPSECS list. It appears that the leap second process was not resolved and is still open for input.

2006-08-14/25: The XXVIth General Assembly of the International Astronomical Union

As seen on page 12 of the 2005-08-10 draft 5.1 of USNO Circular No. 179 the IAU working group on UTC chaired by Dennis McCarthy should have produced its final report. The IAU also re-defined the time scale known as TDB.

2005-12-31: IERS Bulletin C 30

There was a leap second. This was the first leap second in seven years. (See why here.) Projected values of UT1 which prompted this announcement are in IERS Bulletin A.
There were some systems which did not handle the event properly, but there were no disastrous results.

2005-11-08/11: ITU-R WP7A, Geneva

The meeting of ITU-R Working Party 7A did not produce a consensus. The contribution from Great Britain objected to any change. The result was to write a letter asking all organizations to observe the upcoming leap second and report to the SRG.

Various times and places until 2005-11

Prior to the meeting of ITU-R WP7A in Geneva the various national and international bodies which are members of the ITU-R held meetings to determine their positions on the issue of abolishing leap seconds. In the United States the responsible body is the USWP7A which is operated by the FCC under the auspices of the Department of State. In the UK the responsible body seems to fall within the Department of Trade and Industry.
The ITU-R web page of contributions to WP7A openly indicates which countries submitted documents for consideration.

2005-10-30: The UK Computer Emergency Response Team published a paper mentioning the possible redefinition of UTC.

2005-10-23/29: XXVIIIth General Assembly of URSI

Session A07 on 2005-10-28 may contain a report on UTC. Alternatively, the business meeting for URSI Commission J should contain a report on the survey which was approved by a resolution at the 2002 GA to be performed by working group J.2.

2005-10-14T16:00 UTC:

Deadline for comment on the Proposed Revised Recommendation ITU-R TF.460-6 from the USWP 7A which was released on 2005-09-19.
Details of the proposal and contact information.

2005-10-04: Meeting of the USSG7, Arlington, Virginia

In contrast to the USWP7A, the meetings of the USSG7 have been announced on Vol. 70, No. 126, page 38216 of the Federal Register ( text and PDF) and in the calendar of the USITUA.

It is not clear to what extent the USSG7 has oversight of the USWP7A or the US policies presented to the ITU-R in conjunction with the United States International Telecommunication Advisory Committee (ITAC-R), but it seems that they do maintain a list of approved documents.

2005-09-20: The UK Royal Astronomical Society issued a press release in support of leap seconds.

This has triggered significant response from the news media.

2005-09-19: The USWP 7A released the 2005 version of its Proposed Revised Recommendation ITU-R TF.460-6.

This document is open for public comment until 2005-10-14T16:00 UTC.

Details of the proposal and contact information.

The USWP7A operates in conjunction with the United States International Telecommunication Advisory Committee (ITAC-R).

Most meetings of the ITAC-R groups are listed on the United States ITU Association website, but USWP7A meetings are curiously absent from this web page.

2005-08-29/31: Joint meeting of PTTI 2005 and IEEE International Frequency Control Symposium, Hyatt Regency Hotel, Vancouver, British Columbia, Canada

Most of the principals in the effort to modify UTC to omit leap seconds were probably at this meeting.
Unconfirmed reports indicate that because many of its members were present the USWP7A held a meeting. If so, this would mean that a group that represents the policy of the US to an international organization under Department of State guidelines met with no public announcement outside of the boundaries of the US.

2005-08-23: Letter from P. Kenneth Seidelmann.

This letter gives insight into the USWP7A process and the status of the proposal which might be presented to the ITU-R WP 7A in November. (It was originally sent 2005-07-21.)

2005-07-21: Royal Astronomical Society News

The Royal Astronomical Society pointed out the possibility of change to UTC and listed contact information for the UK representatives to the ITU-R.

2005-07-08: Letter from Jean Meeus

Belgian mathematician and astronomer Jean Meeus reacted to the possibility that leap seconds may be abandoned and called it ``a dirty trick''. Meeus is author of numerous books on computational astronomy including Astronomical Formulae For Calculators, Astronomical Algorithms, Mathematical Astronomy Morsels, More Mathematical Astronomy Morsels, and Mathematical Astronomy Morsels III.

2005-07-05: Note to IERS Users

Following the publication of IERS Bulletin C 30 Daniel Gambis sent out this unprecedented note regarding the ITU-R process to abandon leap seconds.

2005-07-04: IERS Bulletin C 30

The IERS announced that there will be a leap second on 2005-12-31. This will be the first leap second in seven years. (See why here.) Projected values of UT1 which prompted this announcement are in IERS Bulletin A.

2005-06: ITU-R SRG 7A meeting

The date and location of this meeting were never announced publicly.

2005-05: CCTF, BIPM

The BIPM published the report from the 16th meeting of the CCTF held on 2004-04-01/02 along with the documents which were presented at that meeting. The documents include the the ITU-R UTC Transition Plan from the ITU-R WP7A SRG 7A which details the response to the colloquium held in Torino in 2003.

The transition plan from the ITU-R representative indicates that ITU-R SRG 7A expected the US WP7A to submit a recommendation to ITU-R WP7A which ignores the advice from the international panel of experts who attended the colloquium held in Torino. (This is indeed what happened at the ITU-R WP7A meeting held during 2004-09-28/10-01.) The transition plan indicates a desire to make the change in UTC by the year 2010.

Pages 15 through 17 of the CCTF report indicate that there was not unanimous assent to this plan, but that the CCTF has no jurisdiction over the definition of UTC. Discussion at this meeting indicated that the European Galileo satellite navigation system would prefer any such change to occur before they become operational in 2008, and subsequent documents from ITU-R WP7A seem to indicate intent to do just so.

There is a precedent for ignoring the requests of international panels of experts with regard to changes in UTC. That precedent was set by the CCIR (the predecessor to the ITU-R) in 1969/1970 when leap seconds were first instituted. In the present case, however, the proposed change would violate the spirit, and possibly the letter, of the existing endorsements for UTC from the IAU and the CGPM.

2005-02: Observatoire de Paris, IERS

Daniel Gambis demonstrated that the effect of the Sumatra earthquake on the position of the earth's pole was not discernible.

2005-01-14: IERS Bulletin C 29

The IERS announced that there will be no leap second on 2005-06-30, which will be six and a half years with no leap. (See why here.)

2004-12-26/28: Sumatra Earthquake

Geophysiscists and seismologists are being quoted about how the magnitude 9.0 Sumatra earthquake may have changed the rotation of the earth. In particular, one estimate says the length of day (LOD) may have changed by 3 microseconds. The IERS provides real-time plots of earth rotation parameters. The plot of LOD for the months of 2004-11/12 shows that the length of day has varied by 300 microseconds. Such variation is typical and is caused by changes in fluid circulation of the oceans and atmosphere -- i.e., the weather. It will not be easy to find an earthquake-caused signal of 3 microseconds when the weather-caused noise is 100 times larger. It is more likely that the earthquake-induced excitation of polar motion (the Chandler wobble seen in the values of x and y from the IERS) will be evident.

2004-10-04, 2004-10-05/08: CIPM Bureau meeting and 93rd meeting

Despite its possible relevance to actions taken at this meeting, the report of the April CCTF meeting has not yet been published.

2004-09-28/10-01: ITU-R WP7A, Geneva

The ITU-R Working Party 7A met and submitted a number of drafts of documents. ITU-R SRG 7A submitted a draft of its final report on UTC transition. The content of the draft ignores and dismisses several results from the 2003 conference in Torino.

As of 2004-09 the index of contributions to WP7A on the ITU website contains a new document from the United States whose title says it is a proposed revision of ITU-R TF.460-6 (the defining document for UTC). Although it is not possible to see the content of that document, it seems likely that it is a revision of this archival document on the FCC website from the United States Working Party 7A that holds the federal charter to interact with the ITU-R.

The archival document from USWP7A proposes that UTC should switch to having leap hours beginning just before the end of calendar year 2007.

Reports have indicated that SRG 7A of the ITU-R expects to continue operating through 2005, which is consistent with producing final recommendations in time for the next World Radiocommunications Conference (WRC) to be held by the ITU-R in 2007. The results of the WRC would typically be foreshadowed at the next general assemblies of the URSI in 2005 and the IAU in 2006, and the subsequent general assembly of the IAU in 2009 would typically respond with further recommendations.

2004-09-20/22: Journees 2004, Observatoire de Paris

The program for the conference showed that Session V on 2004-09-22 addressed the Future of UTC: Consequences in Astronomy.

2004-07: IERS

The IERS announced that there will be no leap second on 2004-12-31, which is six years with no leap. (See why here.)

2004-04-01/02: CCTF

This meeting marked five years since the letter which started the process of reviewing the future of leap seconds. Indications are that this meeting produced an important response to the report of the Colloquium held by ITU-R SRG 7A in Torino.

2004-02-24: ITU-R SRG 7A Torino Colloquium Proceedings published

The Istituto Elettrotecnico Nazionale Galileo Ferraris in Torino has published the Proceedings of the Colloquium on the UTC Timescale held by ITU-R SRG 7A on 2003-05-28/30.

2003-11-27/28: GPS leap week counter overflow

Because there had been no leap seconds for 256 weeks some models of Motorola GPS receivers indicated the wrong date and time for one second, other receivers experienced different effects, some reports are here. The manufacturers and military will likely never reveal whether this might have been a problem for some JDAM smart bombs and other munitions.

2003-10-06/08: ITU-R WG 7A, Geneva

SRG 7A submitted a report based on the Torino colloquium to their parent body.

General/Introductory References

A history of time scales

My own web page which focuses on the history of time scales and has links to other web pages about time scales.

Splitting the Second, A. Jones, 2000-01

A readable history of the development of atomic time which also covers the astronomical aspects.

The leap second: its history and possible future, Nelson et al., Metrologia v38, #6, pp509-529, 2001

This paper is comprehensive and full of useful references. It covers all aspects of time and all issues regarding leap seconds. It is available from the publisher to licensed sites (also via the LEAPSECS archives, see below; and also via Markus Kuhn; and also via URSI). Section 6 makes it clear that UTC has always been a compromise. Sections 7.3 and 7.4 explain the dilemma of UTC.

Duncan Steel, Marking Time: The Epic Quest to Invent the Perfect Calendar, John Wiley & Sons (1999)

A history of the Gregorian calendar which mentions the astronomical aspects of earth rotation. It is especially interesting for its analysis of the reasons which motivated the authorities to make changes in calendars and clocks, and also because it covers the immediate public reactions and long-term social effects that such changes have wrought.

The fate of the Analemma

Material from the ITU

According to ITU-R TF.460-6, UTC is maintained by the BIPM with assistance from the IERS. According to this web page and other statements the BIPM is responsible for the calculation of TAI. However all statements from the BIPM indicate that they leave the determination of the integer number of seconds between UTC and TAI (i.e., leap seconds) to the IERS. The explanatory supplement to IERS bulletins A and B acknowledges that they are responsible for determining leap seconds, and that they do it in conformance with ITU-R TF.460.

The evolution of ITU-R TF.460 over its past few versions gives some insight into the ongoing process. All of these recent revisions recommend that broadcast time signals conform to UTC. They all define DUT1 as the predicted value of (UT1 - UTC), recommend the broadcast of DUT1, and give a coding scheme to transmit DUT1 for differences as large as 0.8 s.

CCIR Recommendation 460 (1970)

This is presumably the original document defining UTC to be broadcast as atomic seconds with occasional full-second leaps to track UT1 which was approved by the Plenipotentiary meeting of the International Radio Consultative Committee (CCIR) in 1970-01.
The text of this document and its first 3 revisions are not easily available.

CCIR Recommendation 460-1 (1974)

This revision presumably incorporated the advice from the IAU General Assembly in 1973 which indicated that the tolerance between UTC and UT1 needed to be as large as 0.9 seconds.

CCIR Recommendation 460-2 (1978)

CCIR Recommendation 460-3 (1982)

CCIR Recommendation 460-4 (1986)

This revision was renamed ITU-R TF.460-4 in 1992/1993 when the CCIR was reorganized as the ITU-R.

ITU-R TF.460-4 (1986)

This revision predates the activation of the IERS. It parenthetically remarks that ``GMT may be regarded as the general equivalent of UT''. It states that the definitions of terms for earth rotation are available from the glossary in The Astronomical Almanac.

ITU-R TF.460-5 (1997)

This revision predates the discussions about discontinuing leap seconds in UTC. It omits the parenthetical remark about GMT. It states that the definitions of terms for earth rotation are available from the IERS.

ITU-R TF.460-6 (2002)

This revision occurred during the discussion process for discontinuing leap seconds. In addition to DUT1, this revision defines DTAI as (TAI - UTC). It recommends that broadcast time signals provide DTAI, but it does not give any coding scheme for doing so.

There is, however, a major problem with this document as the definition for the distribution of time signals -- it is proprietary. Nevertheless, it is cited as the defining document for UTC, and thus for civil and systems time in many other standards. As a result there is widespread misconception about the content of the document and the rules for inserting leap seconds. There is also widespread ignorance of the history behind the current form of UTC and the problems that form was designed to address. There are also widespread examples of systems which have been designed to think they are using UTC when they are actually incapable of doing so. Finally, as a result of the invention of leap seconds, systems designers and the general public have not had to recognize that time-of-day (universal time) and time interval (atomic time) are two distinct and incommensurate quantities.

Because the text of the document that defines UTC (ITU-R TF.460) is under the purview of ITU-R, the principal action to contemplate omitting leap seconds from UTC appears to have originated in Study Group 7, Working Party 7A.

The names and dates of files in the on-line archives of the ITU are visible to all. In general the content of ITU publications is only available to those who pay the ITU (however the ITU does permit anyone to obtain three recommendations annually free of charge). The following documents seem to be relevant:

[7A/27] Draft new Question ITU-R [TF.qqq] - UTC time scale

This is dated 1999-09-24, and as such it is interesting because it appears to be the earliest document pertaining to the official process. It seems likely that this is the question which resulted in the formation of the ITU-R Working Party 7A Special Rapporteur Group (SRG) that has taken the lead in most of the subsequent proceedings.

Contribution to SRG on the future of UTC - Summary of the Questionnaire about the future of UTC in Japan

The timing of this document makes it seem likely that it is the report from the Communications Research Laboratory in Japan. That document is available to LEAPSECS subscribers via a link below.

Draft proposed decision - Task Group for Question ITU-R [TF.QQQ] - The future of the UTC time scale

The timing of this document makes it seem likely that it is a precursor or draft version of ITU-R 236/7.

ITU-R 236/7: The future of the UTC time scale

The full text is now freely available from this link. This is the question before ITU-R WP7A which has motivated the creation of the SRG.

Reports are available regarding the content of two of the meetings of the SRG. One meeting was in Geneva in 2001-05, and another was in Paris 2002-03. Reports from the Geneva meeting indicate that another meeting may have occurred concurrently with the 2001-11 meeting of PTTI in Long Beach. Reports from the Paris meeting indicated that the only option for change to UTC that the SRG was considering was to discontinue leap seconds altogether after some date yet to be determined. However reports during 2003 seem to indicate that the SRG has modified its opinion; see below.

The Colloquium in Torino, 2003-05-28/30

The Istituto Elettrotecnico Nazionale Galileo Ferraris in Torino hosted a Colloquium on the UTC Timescale called by ITU-R SRG 7A. Various opinions on the future of leap seconds were presented. No branch of the ITU appears to have provided any links to the announcement of this meeting, but links to the text of the announcement were available elsewhere as seen below in the sections on IAU, IERS, and NIST. The advance notices indicated that the SRG would present its consensual opinion on the future of leap seconds, but reports from the colloquium indicate that there was nothing resembling a consensus. A hint regarding the supposed nature of the consensual opinion is visible below in the link to Ron Beard's presentation at the 41st CGSIC meeting.

The agenda considered financial aspects (costs and opportunities) for several disciplines, but not for astronomy.

The LEAPSECS archives also contain the Agenda and Call for Papers to the ITU SRG 7A on the Future of the UTC Time Scale. It has been available to subscribers of LEAPSECS.

Early reports of the colloquium indicate that, beginning in about 20 years, radio broadcast time signals may indeed use a timescale that does not contain leap seconds. The best available summary appears to be that which Markus Kuhn posted to the LEAPSECS discussion list. Inasmuch as I understand the result of the colloquium, I have calculated the consequences of the scheme outlined at Torino.

Material in the LEAPSECS mailing list

Within the archives are copies of many documents from various organizations that are not freely available from any other source. The LEAPSECS list has been invaluable in tracking the proceedings. Without the contributions from its members most of the content of this web page would not be evident.

McCarthy and Klepczynski in GPS World Innovation, 1999-11, pp50-57

Titled GPS and Leap Seconds -- Time to Change?, this appears to have been aimed to introduce the problems and possibilities of changing UTC. It is also available via Richard B. Langley, the author of the magazine column that the article appeared in.

Nelson et al. in Metrologia v38, #6, pp509-529, 2001

This is the paper mentioned above in the general/introductory references (also available from the publisher, see above; and also via Markus Kuhn; and also via URSI). Everyone should read this paper.

A transcript of a panel discussion on leap seconds

This is from the 32nd annual PTTI Meeting in 2000-11. It reveals many players and their motivations. See below for more on PTTI.

A survey by URSI Commission J

From mid year 2000. It is available openly via a link in the URSI section below.

A survey by Communications Research Laboratory, Japan

This is apparently from early year 2002. A majority asserted that the present UTC scheme was not inconvenient. The majority of those who had an opinion asserted that it is better not to change UTC.

Activities Report from the 2001-05 Meeting of ITU SRG 7A

Available as MIME type application/msword and text/plain.

A resolution submitted to the URSI General Assembly (Maastricht, 2002-08)

It resolves to create an URSI-wide committee to poll URSI and prepare another resolution for 2005. This was unanimously approved.

A draft of the questionnaire to be sent to URSI members

This is the action approved by the Maastricht 2002 resolution. It is also available without subscription.

History of IEEE P1003.1 POSIX time

A note about the evolution of Unix time before and after the POSIX standard. It is also available without subscription. This should be read by anyone who is interested in the reasons why POSIX time is the way it is. See also the section below on Unix and POSIX.

An Agenda an Call for Papers to the ITU SRG 7A Colloquium on the Future of the UTC Time Scale

Alas it seems that this message was too long to be included in the Mail Archive.

A summary of the colloquium in Torino by Markus Kuhn.

Also freely available via Mail Archive.

It appears that the contents of the LEAPSECS list have been openly archived beginning in 2003-01. (Particularly large messages and messages with attachments are not archived, and it takes up to a week before messages appear in the index.) This archive has one of the few openly available postings about the colloquium in Torino on 2003-05-28 where the ITU-R WP 7A SRG gathered opinions on the future of leap seconds.

Material from the US Navy

The Earth Orientation Department of the US Naval Observatory is part of the IERS. The provide general introductory background information about earth orientation which includes earth rotation -- i.e., when will it be noon?

The distinction between the fields of expertise represented by these two departments of USNO lies at the core of the dilemma for UTC. The notion of time most evident over the course of human history is the diurnal noon/midnight cycle. A century ago it became evident that earth rotation is not a precise timekeeper. A generation ago atomic time replaced astronomy as the means for practical timekeeping. UTC with its leap seconds is a compromise between these two incommensurate goals.

Material from PTTI

The 31st Meeting, 1999-12-07/09

There was a panel discussion which introduced the possibility and problems of redefining UTC and whether TAI was a viable alternative. This is available as MIME type application/pdf.

The 32nd Meeting, 2000-11-28/30

There was another panel discussion. The transcript is available to LEAPSECS subscribers via the link above. Alternatively, the entire proceedings are visible as a very large file of MIME type application/pdf. The presentation starts on page 235, and the panel discussion starts on page 245.

The 35th Meeting, 2003-12-02/04

The Advance Program indicated that an entire group of sessions considered UTC. The abstracts of many of the papers covered the aspects of UTC as maintained by different national laboratories.

Some particularly relevant papers were

A New Realization of Terrestrial Time, G. Petit

On the calculation of TAI, its likely uncertainties, and comparisons with pulsar observations.

ITU-R Special Rapporteur Group on the Future of the UTC Time Scale, Ron Beard

A summary of the Torino meeting with some pointers toward future directions.

Working Group A: The Future of UTC Summary of the Discussion, Judah Levine

Indications that the legal time of the US might be changed to be UTC rather than mean solar time, and that leap seconds should be abandoned. This mentioned the previous attempt by NIST to change US legal time from GMT to UTC and it foreshadowed the document from the US WP7A which is connected with the FCC, and the Department of State and which seeks to change the definition of UTC to have leap hours starting in 2007.

Material from the IAU

At the 24th General Assembly (Manchester, 2000-08) the IAU passed several resolutions; resolution B2 addresses UTC. This establishes a working group to cooperate with URSI, ITU-R, BIPM, and IERS on leap seconds and UTC.

The responsibility of handling resolution B2 falls to Division I whose main web page is here. The formation and membership of a working group can be seen in section 3.3 of the 1999/2002 Division I Report. The identities of the IAU representatives to ITU-R and to the SRG on UTC are visible on page 36 of Information Bulletin 90.

IAU Division I had one of the few openly available postings about the colloquium in Torino on 2003-05-28 where the ITU-R WP 7A SRG gathered opinions on the future of leap seconds.

IAU Commission 31 (Time/Temps) is part of the Division I effort. Their activities page contains pointers to several relevant documents:

The triennial report for 1996/1999

This contains references to the 14th CCTF meeting of 1999-04 which is covered in its own section below.

The triennial report for 1999/2002

This is available as MIME types application/pdf and application/postscript. Section 2 is about the 15th CCTF meeting (see below). It ends with a cryptic sentence that implies a desire to preserve the leap second, and that seems outside the report of the CCTF itself. Section 5 is about ITU SRG 7A and mentions the surveys performed by the URSI and the IERS.

At the 25th General Assembly (Sydney, 2003-07) Division 1 of the IAU had two meetings on July 17 and 21 where the issue of UTC was on the agenda. These consisted solely of reports and there was no official action.

Material from the URSI

The International Union of Radio Science (URSI) Commission J (Radio Astronomy) has created a Working Group J.2 to address the issue of the leap second. As seen below, this working group appears to have existed by 1999-11-09.

On behalf of URSI Working Group J.2, Demetrios Matsakis of USNO began to send out an initial survey about discontinuing leap seconds on or before 1999-11-09. Judah Levine of NIST was assisting in disseminating this survey. By 1999-12 it appeared in the IERS gazette, and it started a significant discussion in the USENET newsgroup sci.astro.fits.

The web page for Working Group on the Leap Second (J.2) openly contains the text of the survey, along with notes on its distribution. The results of the survey became available in 2000-07. They are available in the WG web page as well as in the archives of the LEAPSECS mailing list via the link above. (Note that the WG web page contains some invalid pointers to the LEAPSECS archive.)

The resolution approved by URSI at the 2002 General Assembly is available as text/html and also to subscribers of the LEAPSECS mailing list via the link above. A draft of the second questionnaire to all URSI members is also available to LEAPSECS subscribers and to anyone via the links above.

The revised website of URSI contains this page with the most current information regarding another survey taken in late 2003.

Material from the BIPM

The BIPM also combines the measurements of an ensemble of atomic clocks around the world in order to produce International Atomic Time (TAI). The BIPM acknowledges that TAI serves as the basis for UTC which is the de facto, and in some localities the de jure, basis for most forms of legal time. They note that in 1975 the CGPM resolved that UTC provides both atomic frequency standards and UT (or mean solar time). However, the BIPM does not clearly advertise the defects of TAI that are evident in their TT(BIPMxx) data which produce the following plot.

Material from the CCTF

The events of the 14th meeting on 1999-04-20/22 were summarized by bodies of the IAU and URSI. The Report of the 14th meeting is available on-line as a zipped file containing two files of MIME type application/pdf (French and English). The English text of section 4 on ``The Future of Leap Seconds'' begins on page 102. It describes report CCTF99-18 by McCarthy of USNO. This report seems to have introduced many of the problem elements seen in more recent presentations by McCarthy that are available via various links in this web page. McCarthy then suggested the formation of a working group. The CCTF did not believe that it had the authority to take action beyond the writing of a letter to be sent to other agencies, and the recommendation of the use of TAI for applications that require a continuous time scale.

The 14th CCTF meeting was held on 1999-04-20/22 (shortly after the 33rd CGSIC meeting at which Klepczynski suggested discontinuing leap seconds in UTC). One result of this meeting was a letter from the president of the BIPM to various concerned agencies (the text of this letter appears to be contained in the URSI 1997/1999 report linked above). This is supported by pages 275 to 276 in the English text of the proceedings of the 21st CGPM meeting (held in 1999-10) which are available as a zipped file containing PDFs in the two languages. It seems likely that the letter sent by the BIPM after this meeting was used as a significant excuse for the ITU activities relating to the redefinition of UTC.

The Report of the 15th meeting in 2001-06 is also available on-line as a zipped file containing two files of MIME type application/pdf (French and English). The English text of section 5 on the ``Redefinition of UTC: Leap Seconds'' begins on page 107. It mentions a report of ITU-R SRG 7A and three general options for the future of UTC. It also contains a poll of the CCTF members that indicates which option each preferred; there was a three-way split with no consensus whatsoever. The documents submitted to the 15th meeting include an early ITU document describing the formation of SRG 7A and the first report of SRG 7A.

Material from the IERS

In 2002-03 the IERS Explanatory Supplement to bulletins A and B briefly mentioned reasons behind the considerations for change in UTC.

From 2002-05 through 2002-06 the IERS performed a survey regarding the use of UTC. The results of the survey were complete by 2002-09. A large majority were satisfied by UTC with leap seconds and a majority thought it would be better not to change UTC. The comments attached to the results give a good indication of how various groups view the possibility of changing leap seconds.

The IERS is has one of the few openly available postings about the colloquium in Torino on 2003-05-28 where the ITU-R WP 7A SRG gathered opinions on the future of leap seconds.

The task of the IERS spans astronomical, geophysical, and meteorological disciplines. The IERS is finalizing a new draft of their conventions. The conventions provide a mathematically detailed and comprehensive view of the models, constants, and standards that underlie measurement of earth rotation. Also of technical interest are the proceedings of a 2002 workshop hosted by the IERS at which the evolution of the understanding of the meaning of the models and constants was discussed. Finally, it should be pointed out that UT1 no longer has any explicit relation to the position of the sun.

Material from the NIST

A NIST document from 1991 explains how the ITU process works and how their recommendations effectively correspond to international law.

NIST had one of the few openly available postings about the colloquium in Torino on 2003-05-28 where the ITU-R WP 7A SRG gathered opinions on the future of leap seconds.

On page 3 (page 2569 in the journal) of a 1999 article by J. Levine of NIST in Rev. Sci. Instrum. there is a reference to problems of representing UTC leap seconds which predates any of the official actions to redefine UTC.

Material presented at CGSIC Meetings

The 33rd meeting, 1999-03-17/18

Future of the Leap Second by Bill Klepczynski. He suggested stopping leap seconds temporarily and then having a conference to decide what to do. (Note that Dr. Klepczynski apparently did not espouse this strategy in 1994.) This presentation predated the 1999-04 CCTF meeting which presumably sent out the letters that started the process of redefining UTC, and it is the earliest available reference to the process.

The 36th meeting, 2000-09-17/19

Legal Traceability of Time Signals by Judah Levine of NIST

The 37th meeting, 2001-03-27/29

UTC and Leap Seconds by Dennis McCarthy of USNO. This is probably the best outline of possible options for changes to UTC, but most of them have since been excluded from consideration.

The 38th meeting, 2001-09-09/11

The Future of the UTC Timescale by Ron Beard of ITU-R Working Party 7A SRG on UTC

The 39th meeting, 2002-04-17/19

The Future of UTC by Ron Beard, updating on ITU-R Working Party 7A SRG

The 41st meeting, 2003-03-19/20

The Future of the UTC Timescale by Ron Beard, updating on ITU-R Working Party 7A SRG. Here we learn that there are two contenders for the future of UTC:

• status quo -- continue to insert leap seconds into UTC
• discontinue leap seconds in UTC in favor of leap hours

The option of leap hours is inconsistent with the reports of the 2002-03 SRG meeting in Paris.

The 42nd meeting, 2003-09-08/09

The agenda indicates that Ron Beard reported on the ITU-R colloquium in Torino.

Report on the ITU-R colloquium on the Leap Second by Ron Beard

Most of the previously known options seem to remain open.

Requirements, from seconds to nanoseconds by Ron Beard

This may be part of the research for the SRG report. It contains a partial listing of many agencies and projects, where they get their time, and how accurately they need it.

Practical Relativistic Timing Effects in the GPS and Galileo by Robert A. Nelson

While not directly related to leap seconds, this makes it clear that atomic clocks on satellites have made general relativity into something that cannot be ignored even for processes on earth. There is a Nobel prize lurking in these sorts of investigations.

The 44th meeting, 2004-09-20/21

The summary report is online with links to various presentations. Of particular note is the introductory presentation by W. Lewandowski, principal physiscist at the BIPM time lab. This presentation indicates that a new, uniform civil time would need a new name. This was the result of the ITU-R SRG conference in Torino, but it stands at odds with the most recent draft documents from the ITU-R.

The 45th meeting, 2005-09-12/13

The summary report is online with links to various presentations. Of particular note are the presentations by Ron Beard giving an update on the ITU process and by Nelson and McCarthy giving another overview of UTC and the leap second.

USENET

comp.bugs.4bsd

See the section below on Unix and POSIX

comp.protocols.time.ntp

late 1999

Discussions of the possibility of discontinuing leap seconds in response to the initial activity of the ITU.

late 2000

A long and tedious thread in response to the proposal of UTS by Markus Kuhn (see more on UTS below). This is typical of discussions regarding whether or not the Unix/POSIX standard should handle leap seconds.

mid 2001

A short thread which makes reference to a flame-war in the Portable Application Standards Committee mailing lists over whether the real time clock should be TAI. See below for more on PASC/Unix/POSIX.

2003-10/11 here and here

A thread which makes it clear that the means for handling announcements of leap seconds in broadcast time signals are not well-documented and that the signals themselves have limitations.

sci.geo.satellite-nav

Around the time of each leap second discussions tend to arise regarding how well various receivers handle them. The ITU-R SRG meeting in Torino was announced in a thread that started on 2003-02-13.

2003-11-28

Discussions of the effects observed when the number of weeks since the most recent leap second overflowed the 8-bit counter in the GPS message format.

Curiously, it does not seem to be the case that this newsgroup has yet discussed the impending failure of GPS to be able to provide UTC if leap seconds do continue.

A number of issues of Risks Digest (aka comp.risks) have pondered leap seconds:

From 1988-01 in Risks 6.07

The leap second that ended 1987 triggered an early thread.

From 1996-03 in Risks 17.86

A lone article which is significant because it mentions the increasing occurrence of leap seconds and posits redefining the second.

From 1997-04 in Risks 19.14

A treatise on time measurement.

Starting 1997-05-15 in Risks 19.16

A thread originally about Y2K that forked into another thread beginning with a note that expressed annoyance with leap seconds.

Continuing in Risks 19.17

The leap seconds discussion dredged up relativity (which continued through Risks 19.20).

From 2003-10-09 in Risks 22.94

A reflexive link.

Unix system time and the POSIX standard

Once Unix had become popular it became desirable to standardize the characteristics of the system, including the representation of time. The standards committees decided that POSIX time should be UTC, but the early POSIX standards inexplicably incorporated a concept which never existed in UTC -- the ``double leap second''. This mistake reportedly existed in the POSIX standard from 1989, and it persisted in POSIX until at least 1997. Some Usenet postings that shed more light on this are here, here, and especially here. One can only surmise that none of the authors of those sections of the ANSI C standard or POSIX standard had read the text of ITU-R TF.460, and that may mean that they did not understand the full implications of specifying a system that is supposed to track UTC. Any document with an unqualified reference to an instant with the time tag 1970-01-01T00:00:00 UTC hints that its authors did not understand the history of UTC. (And this also serves as strong example of why an international standard, upon which the operation of almost every system in the world depends, should not belong to an organization like ITU-R which does not openly publish it.)

Subsequent to the early POSIX standards, Berkeley Unix decided that the time_t value in the 4.4BSD kernel should count SI seconds and thus keep TAI, not UTC. This also appears to be the case for FreeBSD. Unfortunately, even this definition of the Unix epoch is subject to interpretation because TAI did not exist by that name until 1971, and because the length of TAI seconds was changed abruptly on 1977-01-01 and again gradually from 1995 through 1998.

The archives of the LEAPSECS discussion list contain an insider's view of the process by which the POSIX time standard evolved as seen by Landon Curt Noll.

The most recent discussions of the POSIX standard have occurred in the context of the PASC discussions mentioned previously, and they have had access to the text of ITU-R TF.460. The PASC archives are not easy to search, but many messages in this search appear to be relevant. A thread about seconds since the epoch started in late 2000. A thread about timestamps picked up in late 2004.

Most of the discussions, and the earlier versions of POSIX, predate the discussions on discontinuing leap seconds in UTC. Nevertheless it is interesting to see that after more than 15 years of effort the Unix communities have not yet been able to produce a consensus on a self-consistent interpretation of system time in the presence of leap seconds. The standard solves the problem in a fashion common to many legal compromises by being unspecific about some aspects of time. 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.

Basically, the standard says that POSIX time is UTC, except when it is not. POSIX seconds are nominally SI seconds, but for practical purposes POSIX time counts seconds of mean solar time. This self-inconsistent fuzziness was inevitable given that the standard must support compatibility with an interface that originated before leap seconds. On the other hand, almost all clocks, wristwatches, and other commonplace items keep time in a similarly fuzzy fashion. In reality they are all modelled on a clock with 24 hours of 60 minutes of 60 seconds of mean solar time. This was the notion of time as refined from the Babylonian era until the advent of leap seconds.

To put it another way, the POSIX standard recognizes that it is not reasonable to suppose that a Unix system clock cannot be reset. If a system clock can be reset, then it is not reasonable to require it to conform to the characteristics of atomic time. Throughout all of history it has been the case that clocks providing civil time are regularly reset. UTC has never been any different in this regard. In the absence of access to a clock which is never reset, it is most reasonable to expect POSIX time to follow the simple rule that there are 86400 seconds in a day.

There are always exactly 86400 mean solar seconds in a calendar day as counted by the POSIX time/calendar functions. There are usually 86400 SI seconds in a calendar day as counted by the POSIX time/calendar functions, but sometimes 86401 and maybe 86399. Only in special cases can it be presumed that POSIX systems do have access to a clock which cannot be reset; for such special cases it may make best sense to define a whole new kind of POSIX time interface. (And it is silliness to expect to extend a time scale with atomic characteristics into the calendar era prior to atomic clocks. Almost any time stamp from before the year 1956 must be interpreted as having the characteristics of mean solar time, not atomic time.)

Any system which expects sub-second timing precision needs to be carefully designed taking into account all of the sources of delay and error. The POSIX time interface was not designed with the expectation that it would ever be used for applications which need sub-second timing precision. In more plain language, what the hell are you thinking if you expect to use the system clock in your computer for precision timing?

So what does POSIX time really count?

Prior to the 1950s there was no practical means of marking and counting spans of time other than via earth rotation; all differences of long spans of times were really differences of earth rotation angle. Prior to the 1960s it was inconceivable that any sort of clock could exist that would never need to be reset. It was not until the 1960s that forms of atomic time became widely available and it began to be possible to measure time differences with sub-second accuracies using SI seconds over long intervals. By specifying time_t to be UTC, but ignoring leap seconds, POSIX effectively recognized that the underlying clock in a Unix system cannot be guaranteed to increment in lock step with the atomic clocks of the world, and they chose the traditional meaning of time (of day) rather than the new meaning of (atomic) time.

Therefore POSIX time_t does not explicitly support applications which need a time-stamp in SI seconds, or which need to know the interval between two events in SI seconds. The Realtime and Advanced Realtime extensions to POSIX now offer two separate clocks; one is intended to provide current time-of-day (e.g., UTC), and one is intended to provide elapsed time (e.g., TAI). However this functionality may not be widely implemented for yet a while. Until then applications and users which need purely atomic time must implement their own mechanisms (e.g., LIGO ).

The Wikipedia article on Unix time has evolved into another good reference for the meaning of POSIX time.

Interpretation of the POSIX epoch

It will be very good if the fate of leap seconds is decided well before POSIX has to consider how to store time after that date in 2038.

Note that 1970-01-01 is before the inception of leap seconds in UTC. The origin of the POSIX epoch was during the era of elastic seconds (or rubber seconds) of UTC. The length of UTC seconds from 1966 through 1972 was 3 parts in 10⁸ longer than what was then believed to be the length of one SI second.

1970-01-01 is also before the time scale now called TAI had that name. The name TAI was proposed in 1970 and became official in 1971. Before that time it was simply TA, the atomic time scale of the BIH.

Furthermore, by the mid 1970s the frequency of TAI (and UTC) seconds was deemed to be too large by 1 part in 10¹². So the length of (TAI and) UTC seconds from 1972 to 1977 was shorter than what is currently used as the conventional length of one SI second of TAI. This change on 1977-01-01 is plainly visible in the plot of TT(BIPM04) above.

Finally(?), in 1995 it was deemed that blackbody radiation was affecting the frequency of cesium clocks, and that the true SI second should be measured at 0 Kelvin. So over the interval from 1995 to 1998 the length of the (TAI and) UTC seconds was decreased by about 2 parts in 10¹⁴ until it corresponded as closely as possible to cesium atoms at absolute zero.

For these reasons it is extremely tricky to try to calculate how many SI seconds have elapsed since the POSIX epoch. Indeed, it is probably best not to consider the POSIX epoch to be expressed in UTC at all, but rather as just plain UT. That is to say, although there was atomic time at the inception of the POSIX epoch, and although the time services of many countries in the world were in agreement to millisecond precision, practically all clocks on 1970-01-01 were ticking mean solar seconds and being reset as needed to mean solar time. It is not reasonable to demand that the origin of the POSIX epoch have the characteristics of atomic time scales.

In summary

• It is simple to count mean solar seconds since 1970-01-01 (to within one) because that number is available from the time_t provided by any POSIX compliant system with a correctly set clock.
• Counting the number of UTC seconds since 1970-01-01 is straightforward, for it is the above value plus the number of leap seconds since 1972-01-01.
• Counting the number of TAI (and TA) seconds since 1970-01-01 is somewhat harder, for it requires taking into account the different length of UTC seconds before 1972 and the offsets introduced into UTC.
• Counting the number of elapsed SI seconds since 1970-01-01 is harder still, for it requires correcting for all the changes in the length of TAI seconds which can be seen in TT(BIPM04).

The Past of Leap Seconds

How many leap seconds have happened since 1601-01-01?

The Microsoft Windows file time stamp specifies that it is in UTC. The meaning is necessarily ambiguous because nothing that might be called UTC existed before 1960, and until 1972 UTC used seconds of varying length and steps of milliseconds instead of full leap seconds. Indeed, GMT did not really exist as we know it until the International Meridian Conference in 1884, and GMT did not exist at all prior to the founding of the Royal Greenwich Observatory in 1676.

There are two time scales which can reasonably be extrapolated back to 1601: mean solar seconds of Universal Time (UT) and SI seconds of Terrestrial Time (TT). If these are treated in a fashion consistent with their current usage then it is possible to determine how many leap seconds would have happened since 1601. The answer comes from the astronomical studies of the quantity known as Delta T and Length of Day (LOD).

(See the rest of this answer as a series of plots.)

If the desired answer is the number of elapsed seconds of fixed length equal to one SI second as measured by clocks on the surface of the earth then the time scale is TT. The number of leap seconds elapsed since 1601 is approximately -60 (yes, that is negative 60). This number can be broken down into two components. The number of leap seconds which would have been inserted from 1601 to 1900 is approximately -125. The number of leap seconds which would have been inserted from 1900 until now is approximately 65. This answer is of little practical use because there were no atomic clocks during most of this historical interval, nor even any telescopes at its epoch.

If the desired answer is the number of elapsed seconds of mean solar time then the time scale is UT. The number of leap seconds elapsed since 1601 is zero because UT is a subdivision of calendar days; UT never has leap seconds. This answer is probably most consistent with the intended meaning of civil time stamps in practical use over the entire historical interval. As with POSIX time, however, it necessarily implies that not all seconds have the same length, for mean solar seconds are a measure of earth rotation rather than fundamental physics.

timezone offsets and summer/daylight time

``posix''

This set of zoneinfo database entries considers a Unix time_t value to be equivalent to the value of UTC since 1970. Every day has 86400 seconds. For correct time this requires that the system clock be retarded or reset backward at each leap second. This is the default provided with most Unix systems, and it is most consistent with POSIX.

``right''

This set of zoneinfo database entries considers a Unix time_t value to be equivalent to the count of TAI seconds since 1970. It requires a file of leap seconds to be updated whenever a new one occurs. For correct time this requires that the system clock increment monotonically using SI seconds. Despite its name, this is generally regarded as experimental and inconsistent with POSIX. Nevertheless, this is the branch of code which is used by default in 4.4BSD Unix.

Local civil authorities have demonstrated a tendency to modify the rules with little forewarning; e.g., the US during the energy crisis of the 1970s, much of Australia for the 2000 Olympics. The tz mailing list regularly receives reports that local civil authorities have changed the effective dates for daylight savings time transitions.

It is beyond the scope of the activities of the ITU-R, or any international organization, to dictate the nature of local civil time. If civil time is deemed to be able to tolerate secular excursions of a full half-hour from mean solar time, then implementing the first leap hour seems not much different than omitting a daylight transition. But it is not clear that secular excursions of half an hour are acceptable for legal purposes, and in the long run even leap hours start happening annoyingly frequently.

The most that can be asked of the ITU-R is that radio broadcasts continue to provide Universal (mean solar) time to an accuracy of 1 second or better. It is within the scope of the activities of the ITU-R to recommend that radio broadcasts which switch to a purely atomic time scale should also include sub-modulations which permit a machine to ascertain Universal Time. This would be a significant change, for setting a watch to the correct civil time simply by listening to raw radio signals might no longer be possible for an unaided human. It would be both simple and relatively cheap to create new radio receiving hardware which could use such embedded sub-modulations to reconstruct an audio signal that could mimic the existing broadcasts of UTC. But if civil time were to remain as mean solar time then even this would place a significant burden on the relatively many owners of radio-controlled clocks in favor of the relatively few owners of systems that require atomic time -- the many would have to upgrade their clocks in order to accommodate the few. In the case of most consumer-oriented radio-controlled clocks that means discard the old and purchase anew.

Other Unix Discussions

comp.bugs.4bsd from 1988-01-07

This thread on including leap seconds in the time functions of the C library was started by Bradley White, who later wrote the ``right'' code into the Olson library.

comp.bugs.4bsd from 1988-01-12

This thread picked up after contact with David Mills who wrote the NTP software that is widely used to synchronize computers with UTC.

Markus Kuhn proposes UTS as a smooth option for Unix-like systems and other system clocks which do not like leap seconds. He has also proposed an API for ISO C which accommodates leap seconds.

David R. Tribble wrote a detailed proposal for extended range time types in the C and C++ languages. The proposal has not been adopted.

D. J. Bernstein has various notions about on UTC, TAI and UNIX Time . He also provides libtai, a library for storing and manipulating 64-bit dates and times that can give results for an interval of hundreds of billions of years. This is generally regarded as experimental.

Network Time Protocol

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 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.

Version 4 of NTP includes a mechanism for transmitting the historical table of leap seconds, which means that NTP can be used to transmit TAI. However there is no programming interface for permitting a system running NTP to make use of this table of leap seconds, and the current uncertainty in the future of UTC is probably not helping to motivate the development of one.

Legal Time

Some countries have adopted UTC as their legal time. Among them appear to be France, Germany, Hong Kong, Korea, The Netherlands, New Zealand, Sweden, and Switzerland.

Many of the above links, however, must be taken with a grain of salt. Whereas UTC is currently the basis for legal time de facto almost everywhere, it is not always the basis of legal time de jure. Nevertheless it appears to be common for national standards bodies to claim that UTC is the legal time even if their laws refer to Greenwich Mean Time. In particular note the case of Australia as a whole vs. its province of New South Wales.

Legal time in the United Kingdom is based on GMT, not UTC. See the detailed history by Joseph S. Myers who also pointed out that the most recent attempt to make UTC the legal time of the UK ``failed for lack of time''.

Legal time in the United States is also based on GMT, not UTC. See the US Code here or at Cornell. Despite the US Code, the publications of the NIST give the impression that UTC is actually the legal time, and they justify this by claiming that GMT no longer exists. (Whereas this is true in a pragmatic/technical sense, it is disingenuous to assert that the original meaning cannot be recovered.) On 2002-11-19 bill S 3177 was introduced in the senate. It did not get past committee, but it contained language that would have modified 15 USC 261 to define standard time zones in terms of UTC instead of Greenwich mean time.

The laws of many other countries refer to GMT as the legal time. Among these appear to be several Canadian provinces (Ontario, Saskatchewan, and Quebec), Ireland, Namibia, and the European Union (text/html or application/pdf).

As a result, in the United States and the United Kingdom, if a contractual agreement has clauses which specify time, but do not specify the time scale to be UTC, then for legal purposes the time would default to be based on mean solar seconds of GMT where leap seconds do not exist. In France, Germany and any country where the legal time scale is based on UTC, then the time clauses in the contract would default to be based on SI seconds of UTC, and leap seconds would count. In any location, however, a contractual obligation which is actually dependent on sub-second resolution had better be specific about the time scale.

It is reported that the national time authorities of 49 nations base their legal time scales on UTC. F. Pollastri provides a link with an old partial list of national time authorities. S. Young and LLRX.com provide a link with a historical analysis of legal time in the United States.

John Seago and Kenneth Seidelmann have written National Legal Requirements for Coordinating with Universal Time which takes a detailed look at the corpus of existing legislation on time. This paper was available at the Torino colloquium.

Legal time on the Internet is UTC. (However, as noted above in the POSIX section, most computers do not keep true UTC, so the timestamps of Internet events near leap seconds may be imprecise or ambiguous.) UTC is specified by name in a great many other standards documents which span a wide range of disciplines. Even though Universal Time is not appropriate for the operations of many complex systems, the name UTC is thoroughly entrenched in systems that underline modern society.

Timestamps specified by UTC effectively have to be communicated in ASCII. Some archival data systems such as EOSDIS and CCSDS have explicitly recognized the problems posed by timestamps in UTC. Their standards documents distinguish between ASCII and binary representations of time, and they recommend the use of timestamps in both UTC and atomic time when necessary.

In summary, because UTC is internationally recognized as a form of mean solar time in accord with the International Meridian Conference of 1884 legal time everywhere is currently based on the rotation of the earth. This basis is a requirement if time is to be related to a calendar that counts days.

As detailed by links within this web page there are, nevertheless, ongoing efforts by individuals in the US NIST, the US Navy, the US Department of State, the US FCC, the ITU-R, the BIPM, the IERS, and other national and international organizations to break the connection between clock time and calendar date that was established by international vote in 1884 and thus change the basis of legal time everywhere from earth rotation to atomic oscillations.

UT2 was the best effort at GMT

The increasing ease and precision with which earth rotation could be measured obsoleted the entire notion of UT2. Therefore in 1972 when UTC was changed from mean solar seconds to SI seconds with leaps, it also began to be defined with respect to UT1. To a physicist it no longer makes sense to think of any form of mean solar time as time. For practical purposes of the measurement of intervals, time in the sense that relates to the evolution of physical processes is now most nearly measured by atomic time. UT in all its forms is actually a measure of time-of-day, which is really the earth rotation angle. For practical purposes of indicating the time-of-day for civil events, the differences between GMT, UT0, UT1, UT2, UT1R and UTC in its current form are insignificant (less than 0.9 second). All of them are versions of the mean solar time on the Greenwich meridian.

Distinguishing the two different meanings for time

Here is the question for humanity: Which is the more fundamental unit of time, the day or the second?

Mean solar days are the fundamental element of modern calendars and thus of most legal systems of time. Universal Time has always been a measure of time-of-day expressed as fractions of mean solar days. UTC with leap seconds has been a practicable form for distributing that quantity.

SI seconds are a fundamental element of modern telecommunication and navigation systems. Atomic time has always been a measure of time interval expressed as SI seconds. Before the GPS satellites there was not a globally available means of distributing that quantity.

UTC in its current form with leap seconds will continue to work for over 1000 years. Atomic time without leap seconds will be off by an hour in 1000 years. If the ITU-R were to recommend that radio broadcasts provide only a form of leap second-free atomic time then the legislatures of the world would have to ponder whether to recognize that as a form of legal time, the grandfather clock owners of the world would wonder why having names for the aeons-old concept of time-of-day (i.e., 12:00 for noon and midnight) was no longer to be relevant to human society, and analemmatic sundial owners would be out of luck.

Systems that need atomic time are, of course, intended to serve the needs of people. The task that the ITU-R SRG 7A has tackled implies determining the nature and accuracy requirements for time as used by almost all the systems and peoples of the world. This is not an enviable task, but the SRG includes some of the smartest people in the world. The attempt of UTC to provide two different things and the broad misuse and misunderstanding of UTC with leaps may require the demise of that name in favor of two replacements: one that provides mean solar time of the origin meridian for civil purposes, and another that provides purely atomic time for systems. The ultimate solution may require almost everyone who deals with time to adopt new hardware and software that recognizes both time-of-day (in mean solar seconds) and elapsed time interval (in SI seconds).

Media Coverage

2000-01-12: Agilent Technologies Technology Forum (Agilent is the part of the company formerly known as Hewlett-Packard which continues to make commercial cesium atomic clocks)

2002: Joe Celko, Killing Time, dbazine.com

2003-04-18: Slashdot

2003-06: The American Astronomical Society Newsletter

2003-06-12: Nature ( text/plain or application/pdf available by subscription)

2003-06-17: Wissenschaft.de

2003-06-17: Heise.de

2003-06-17: Symlink.ch

2003-06: ORF.at

2003-06-19: Le Nouvel Observateur

2003-06-26: The Guardian (also in its Education section)

2003-06-29: Welt am Sonntag (expired)

2003-07-13: Neue Zuricher Zeitung (by subscription, go here and search for ``schaltsekunde'').

2003-07-22: CNN (also here)

2003-08-19: The Age, Australia (a reprint of the Guardian article)

2003-08-25: Der Spiegel (expired)

2003-09: Worthing Astronomical Society September newsletter

2003-09-02: Sueddeutsche.de (expired)

2003-10: Physics Today (by subscription here)

2003-11: Wired magazine

2003-11-10: ITworld.com

2003-11-22: New Scientist p. 30 of the print edition.

2003-12-13: letters to the editor showing that there is still no consensus among experts

2003-12-14: New York Times Magazine

2003-12-26: The Age and The Sydney Morning Herald

2003-12-30: Boulder Daily Camera

2003-12-30: Slashdot commenting on a widespread and imprecise AP story. (Why the story is so wrong.)

2003-12-31: Discovery Channel

2003-12-31: Telegraph (c.f. New Scientist above)

2004-01-01: CNN as one, likely long-lived, representative of the confusing AP story

2004-01-01: Slashdot again, on the same misleading AP story

2004-01-04: Astronomy Magazine

2004-02-27: Tages-Anzeiger

2004-03: Discover Magazine

2004-05: a letter and a reply to dispel a misconception (the answer is explained more pictorially here)

2004-05-13: The Angry Coder

2004-06: The American Astronomical Society Newsletter

2004-10: COTS Journal: Does anybody really know what time it is?

2004-12: Think Sync: The synchronization of the world ? 120 years ago

2005-02-22: ABC Science Online: In 2005-09 Australia will switch its legal time scale from GMT to UTC

2005-03: The American Astronomical Society Newsletter contains a letter by McCarthy, Fliegel, and Nelson promoting UTC without leap seconds.

2005-03: The Compendium (the quarterly journal of the North American Sundial Society) mentions the LEAPSECS issues.

2005-06: The American Astronomical Society Newsletter contains a letter by Seaman and myself promoting UTC with leap seconds. For reference until it becomes available online from the AAS it is here

2005-07-06: Nature has an interview with Markus Kuhn about the upcoming leap second.

2005-07-11: Bayerischer Rundfunk

2005-07-29: The Wall Street Journal covered the ITU-R process.

2005-07-29: The Pittsburg Post-Gazette picked up the WSJ article.

2005-07-30: Slashdot picked up the WSJ article.

2005-08-05: The Inquirer enlarges on the WSJ article.

2005-08-08: The Baltimore Sun gives the impression that it's a war between France and the US.

2005-08-15: The Republican of Springfield, Mass. has an editorial.

2005-09-08: The Swiss Federal Office for Metrology and Accreditation published its Journal of Metrology metINFO. A good article about leap seconds begins on page 24.

2005-09-22: The Scotsman covered the UK RAS press release in some depth.

2005-09-27: The BBC covered the UK RAS press release with mention of the UK response.

2005-09-29: The University of Wisconsin's Why Files covered the UK RAS press release.

2005-10-01: The New Scientist covered the UK RAS press release including an interview with Robert Nelson.

Here the journalism of the New Scientist deserves some inspection.
Nelson is quoted ``The European Space Agency doesn't launch rockets in months when leap seconds are to be inserted, because they can't be certain of the effect on navigation systems if the process doesn't go smoothly''.
The ESA has a web site which includes a list of launches.
On 1998-12-05 flight 114 launched SATMEX 5, and on 1998-12-21 flight 115 launched PAS 6-B.
1998-12-31T23:59:60 was a UTC leap second.
On 1997-06-03 flight 97 launched INMARSAT-3F4 and INSAT-2D, and on 1997-06-25 flight 96 launched INTELSAT 802.
1997-06-30T23:59:60 was a UTC leap second.
Tsk, tsk to the New Scientist.

2005-10-05: The Neue Zurcher Zeitung has a very good article on the current state of leap seconds.

2005-10-27: EDN has a brief article on the dilemma.

2005-10-27: Die Zeit has an article (also available via Wissenschaft Online) which interviews Andreas Bauch of PTB.

2005-10-28: Die Welt has an article.

This article also interviews Andreas Bauch of PTB, and he is quoted ``Schlampig programmierte Software ist das Problem, nicht die Schaltsekunde'' (Sloppily programmed software is the problem, not the leap second).
The article also mentions the date when the Russian GLONASS navigational satellite system has been hinted to have gone offline because of a leap second. This is another example of journalism which deserves inspection. The web page of Notice Advisory to GLONASS Users for 1997 July very clearly states that GLONASS was offline during the leap second, but not because of the leap second.

2005-11: Scientific American has a three page article.

2005-11-05: The New York Times (or here with the cartoon) has an op-ed piece on the ITU-R WP7A meeting.

2005-11-06: The International Herald Tribune picked up the op-ed piece from its sibling.

2005-11-06: An unusually beautiful blog entry on leap seconds (in Portuguese).

2005-11-07: The Boston Globe has an article.

2005-11-08: Axel Nothnagel of the University of Bonn issued a press release about the upcoming leap second and possible leap hour.

2005-11-09: The German press release was picked up by more outlets than I can link. Notable among them are Technology Review and Heise Online (which is also available in an English version).

2005-11-09: The press release from Bonn made it into the Spanish press, e.g., elmundo.

2005-11-09: The BBC has an article on the WP7A meeting in Geneva.

By 2005-11-15 this article had completely changed its content and was reporting on the outcome of the WP7A meeting.

2005-11-09: The BBC has an article on the first EU Galileo navigation satellite.

To see why this is related to leap seconds, read this panel discussion from the 2004 PTTI meeting.

2005-11-10: The Independent has an article on the WP7A meeting in Geneva.

2005-11-11: Frankfurter Rundschau opines about leap seconds.

2005-11-12: The Daily Telegraph gives the first hint about what the ITU may have said after the WP7A meeting.

2005-11-12: Hamburger Abendblatt has a story.

2005-11-12: Guardian Unlimited asks why the Americans want to get rid of GMT.

2005-11-13: The Boston Herald confuses ITU-R WP7A with the ITU WSIS meeting in Tunisia.

2005-11-14: Scientific American openly published their story.

2005-11-15: South London Press has a hint about what may have happened at the WP7A meeting in Geneva.

2005-11-16: The Royal Astronomical Society indicates what happened at the WP7A meeting in Geneva. They kindly provide this press release from ITU-R WP7A.

2005-12-20: Sky & Telescope published a press release about the leap second including a link to a PDF file of an editorial by Christian Steyaert from the December issue.

2005-12-23:

• l'Observatoire de Paris issued a press release about the impending leap second (also available in English).
• The Royal Astronomical Society issued a press release about the impending leap second.
• The US Naval Observatory issued a PDF file with a press release about the impending leap second.
• The Toronto Globe and Mail has an article on the upcoming leap second.

2005-12-24: The Scotsman has an article which mentions the potential of legal problems if leap seconds cease.

2005-12-25: The Washington Post has a good article on the leap second and debate.

2005-12-26:

• Deutschland Radio has a quite poetic 16 minute program which interviews the principals and mentions the US efforts in several interviews.
• Scripps Howard News Service has an article on the leap second with quote from McCarthy.

2005-12-27: The Denver Post has a good article on the leap second and debate.

2006-03: Physics Today has a note by Daniel Kleppner of MIT on ion trap clocks that may be too good to be true.

2006-03-30: The International Herald Tribute has an article on the leap second debate.

2006-04-22: Science News has an article on the leap second debate.

2006-11-01: Physics Today has an op-ed by Brian Luzum on the pros and cons of leap seconds which is available to subscribers.

2006-12: The December issue of Harper's has a report by Michelle Stacey on the effort to abandon the leap second.

2008-12-17: New Scientist has an article on scrapping the leap second.

2008-12-17: The Times has an article on GMT and the Royal Observatory along with article on the ITU and leap seconds and commentary on the status of GMT.

2008-12-18: The Telegraph has an article on the leap second and possible loss of GMT.

Miscellaneous

• Markus Kuhn has another compendium of leap second information.
• An anecdotal report of leap second troubles from mid 1997.
• Rob Seaman proposes that systems handling UTC would be more robust if leap seconds occurred more often.
• Ed Davies proposes a scheme for using DNS on the Internet to distribute the historical table of leap seconds.
• Tom Van Baak demonstrates what kind of clock is required before the occurrence of a leap second is noticeable.
• Of course, all of this might just be part of some vast conspiracy. Not.
• During the past two centuries about 3 dozen new time scales have been created, of those 2 dozen in the past 50 years, and of those one dozen in the past 20 years.
  Would discontinuing leap seconds really prevent further proliferation?
• Long before leap seconds were at risk I had been known to pontificate on time and earth rotation...
• ...and I have vested interests and opinionated calculations on how UTC should be.