Time Scales (original) (raw)

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

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

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

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

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

radio broadcast time signals in 1948

The 5th Plenary Assembly of the CCIR (Stockholm, 1948) issued Recommendation 18. It called for study of worldwide standard frequency and time service.

radio broadcast time signals in 1951

The 6th Plenary Assembly of the CCIR (Geneva, 1951) issued Recommendation 70. It called for transmitted frequencies accurate to within 2e-8, and for the governments operating transmissions to send reports to the CCIR as well as the BIH.

radio broadcast time signals in 1953

The 7th Plenary Assembly of the CCIR (London, 1953) approved Recommendation 122 to supersede Recommendation 70. It called for impulses to be separated by one mean solar second synchronized as closely as possible with Universal Time, transmitted frequencies accurate to within 2e-8, and for the cooperation of CCIR, BIH, and URSI to continue.

radio broadcast time signals on 1955-09-03

In Dublin at the 9th General Assembly of the IAU Commission 31 (Time) dictated thatbeginning 1956-01-01 radio broadcast time signals should attempt to provide UT2.

radio broadcast time signals in 1956

The 8th Plenary Assembly of the CCIR (Warsaw, 1956) approved Recommendation 179 to supersede Recommendation 122. It called for impulses to be within 50 ms of UT2, transmitted frequencies accurate to within 2e-8, and for the cooperation of CCIR, BIH, and URSI to continue.

1958-08 radio broadcast time signals in the UK

At IAU Symposium 11 held during the 10th General Assembly in Moscow Essen reported that the UK would broadcast times signals using an offset from the established frequency of cesium.

radio broadcast time signals in 1959

The 9th Plenary Assembly of the CCIR (Los Angeles, 1959) approved Recommendation 319 to supersede Recommendation 179. It called for impulses to be within 50 ms of UT2, transmitted frequencies accurate to within 5e-9, and for the cooperation of CCIR, BIH, and URSI to continue.

1959-08 agreement to "co-ordinate" broadcast time signals

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

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

coordination of UT for broadcasts beginning in early 1960

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

coordination of UT for broadcasts in 1960-09

URSI recommended that the annual changes to the frequency offset from cesium used for radio broadcasts should be chosen by the BIH so that they would all agree while attempting to follow UT2.

coordination of UT for broadcasts in 1961

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

At the 11th IAU General Assembly in Berkeley during 1961-08 the meeting of Commission 31 (Time) referred to the US/UK broadcast time signal experiment. The process of broadcasting synchronized signals was calledco-ordination, the resulting signals were called co-ordinated, and the stated goal was to achieve agreement to within 1 ms.
Commission 31 resolved that the BIH should determine the annual changes to the frequency offset from cesium to be used to co-ordinate the radio broadcasts. At this meeting Commission 31 decided to address the longstanding issue of the conventional longitudes of the observatories around the world. The IAU directed all of the time service bureaus to adopt new, globally self-consistent longitudes for their observatories starting in 1962.

coordination of UT for broadcasts from 1961 through 1971

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

Cautionary note:
During this same period some radio broadcasts were providing SAT (see above). Also, at least until 1967 the Soviet Union and China were broadcasting another form of time coordinated in the USSR independently of the BIH. As seen below, even the US NBS and USNO were not attempting to bring their broadcasts into agreement until 1968-10-01.
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 way to check on the precision is by looking at issues of Bulletin Horaire and finding their tabulations of how poorly various radio broadcast time signals agreed with each other.

coordination of UT on 1962-01-01

Following the direction of the IAU, all observatories in the world adopted new longitudes which were expressed in a globally self-consistent reference frame. On the same date all astrometric observations switched from using the FK3 catalog to using the FK4 catalog, so the bases for both terrestrial and celestial coordinates were changed.

radio broadcast time signals 1963-01-15/02-16

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

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

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

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

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

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

L'échelle intermédiaire TUC (UTC en anglais) sera appelée Temps Coordonné.

This is the earliest known use of the term UTC in any publication.

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

TUC/UTC in 1964-08

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

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

The distinction between time epoch and_time interval_ is not clear to everyone.

along with an explanatory note written by none other than W. Markowitz, H.M. Smith, and L. Essen (see page 16). That note says

The epoch of U.T. is determined by the angular position of the Earth around its axis; it is required for various scientific and technical purposes and for civilian use, sometimes without delay.

TUC/UTC from 1964 on

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

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

which had been spontaneously christened Coordinated Universal Time (UTC)

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

UTC[BIH] in 1965

The BIH started calculating UTC based upon its atomic time scale (which later became TAI).

UT2C in 1965

Publications from the USNO were calling their time scale by the name UT2C, not UT2 nor UTC.

radio broadcast time signals in 1966

None of the official documents from the Plenary Assembly of the CCIR made use of any form of the term UTC, but they did use UT2. (One of the contributed reports mentioned UTC in the context of its usage by the time service bureaus.)
The 11th Plenary Assembly of the CCIR issued Recommendation 374-1 regarding the broadcast of time signals. The term "UTC" does not appear in the text of the recommendation. The recommendation requires that broadcast time signals be adjusted

to maintain the epoch of the time-signals within about 100 ms of Universal Time UT2

The significant change in 374-1 was to describe (and denote as experimental) the broadcasts of Stepped Atomic Time (with no frequency offset from cesium and many more steps) as a new Best Current Practice.

abject confusion in 1966

Hans D. Preuss of the Department of Geodetic Science at Ohio State University wrote report number 70, "The Determination and Distribution of Precise Time" as part of a contract for NASA. No report does a better job of indicating the chaos of the varied and confusing sources and nomenclature for precise time during the 1960s.

UTC[NBS] on 1967-04-28T21:00

In accord with 15USC261 all local times in the US were to be set forward 1 hour on 1967-04-30. Rather than reset the time broadcasts twice annually the US NBS radio broadcasts on WWV and WWVH switched their voice announcements from local time to GMT.

more confusion in 1967-06

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

the deplorable problem raised by the existence of a great variety of auxiliary time scales, some denoted by a random nomenclature.

and he concluded

the proliferation of artificial derived scales is to be deplored.

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

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

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

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

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

UTC[NBS] in 1968

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

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

UTC[NBS] and UTC[USNO] on 1968-10-01

Through the first half of the 1960s the time scales of the US NBS and the USNO had not tried for agreement better than the 1 ms recommended by the CCIR. By 1967 the two agencies understood that synchronization to within a few microseconds was desirable.

In anticipation of a coordinated coordinate rate for USNO and NBS, on August 24, 1967, the Coordinated Universal Time clock of the Bureau, UTC(NBS), and all UTC transmissions of NBS were advanced by 200 microseconds. This left NBS about 35 microseconds early relative to USNO.

(As seen above, they also provided a unique use of forms of the word "coordinate".) On 1968-10-01 the two scales agreed, and both agencies adjusted the frequencies of their time scales. After that date they steered the frequencies of their time scales to keep the time difference less than 3 microseconds.

UTC[BIH] in 1968-12

The US NBS radio broadcasts of WWV and WWVH began to employ the time scale that the BIH then called UTC. The voice announcements remained as "Greenwich Mean Time".

radio broadcast time signals in 1969

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

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

radio broadcasts of "Coordinated Time" in 1970

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

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

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

radio broadcast time signals on 1970-02-03

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

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

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

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

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

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

UTC[BIH] in 1970-08

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

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

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

UTC[BIH] during 1971-02-17/23

With less than ten months left before the deadline CCIR Study Group 7 met to formulate the detailed instructions for implementing CCIR Rec. 460 in CCIR Report 517 (Question 1/7, Resolution 53).

radio broadcast time signals on 1972-01-01

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

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

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

UTC[BIH] in 1972-06/07

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

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

UTC[BIH] in 1973

The resolutions from the 15th General Assembly of the IAU gave more freely-available public description of UTC:

UTC[NBS] on 1974-01-01T00:00

The US NBS radio broadcasts of WWV and WWVH stopped using the term "GMT" and began to announce the time scale as "UTC".

UTC[BIH] on 1974-07-09/11

The CCDS produced Recommendation S 1 about "Universal Time (Coordinated), UTC". It invited the CGPM to recommend the adoption of UTC as the basis for official time in all countries. It also invited the CGPM to recommend the continued availability of the "additional information provided by the UTC System" -- in particular, DUT1. The explanatory notes contained this quote:

  1. Except where confusion might arise the designation Universal Time (Coordinated), UTC, may conveniently be abbreviated to Universal Time, UT.
  2. The CCDS takes note of the advantages of the use in all languages of the designations UTC and UT, for Universal Time (Coordinated) and Universal Time, respectively.
  3. GMT, which is still in wide use, may be regarded for most purposes as the general equivalent of UT. It is to be hoped that the term GMT will gradually be replaced by the term UT. The CIPM approved CCDS Recommendation S 1 on 1974-09-24.

UTC/TUC in 1974-07-15/26

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

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

UTC on 1975-01-01

CCIR Recommendation 460-1 gave this as the date on which the time scale used for radio broadcasts was to conform to specifications contained within itself.

UTC in 1975-05/06

Resolution 5 of the 15th meeting of the CGPM considered that UTC provides both atomic frequency standards and UT (or mean solar time) and endorsed its use for civil time. As a result, legislative bodies insome countries began to adopt UTC as a precisely defined replacement for GMT in the basis of their legal time.

UTC in 1976-08/09

The 16th General Assembly of the IAU in Grenoble produced Resolution No. 3 by Commissions 4 and 31 which recommended that the abbreviation UTC be used for all languages.

UTC on 1977-01-01

Because the rate of TAI was reduced by one part in 1012, the rate of UTC was reduced by the same amount. Therefore, before this date UTC seconds were shorter than they currently are.

UTC on 1978-06-15

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

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

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

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

UTC during 1979-09/12

At the 10 week long World Administrative Radio Conference the Final Acts indicate "UTC is equivalent to mean solar time at the prime meridian" (see page 32), and UTC is used throughout the 984 pages of the document, replacing GMT in earlier versions.

UTC during 1980-11-10/21

The 7th Plenary Assembly of the CCITT accepted UTC "as the time scale for all other telecommunications activities."

UTC in 1982

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

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

This introduction appears to be an early instance of using the phrase "reference time scale" when referring to UTC.

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

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

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

not really UTC in 1986-04

This is the publication date of IEEE Std 1003.1 issued for Trial-Use in April 1986. See the 1988 official publication for details.

UTC in 1986-07

The fourth revision of the document specifying that radio broadcasts of time signals have leap seconds was issued as CCIR Recommendation 460-4.

UTC in 1988

The responsibility for maintaining UTC was split between two agencies. The responsibility for keeping TAI was transferred from the BIH to the BIPM. The responsibility for monitoring earth rotation, determining UT1, and announcing the need for leap seconds was transferred to the newly created IERS. After around a century of operation the BIH and the ILS ceased to exist as these responsibilities were combined into the IERS.

not really UTC on 1988-09-30

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

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

Epoch

as 1970-01-01T00:00:00 Coordinated Universal Time without regard to the fact that at that date there was not general recognition of the existence of a time scale by that name

Seconds since the Epoch

as Coordinated Universal Time with an expression that requires 86400 seconds in one day (and also lacked the 400-year rule that Pope Gregory XIII instituded 4 centuries earlier) without regard to the fact that the rules for UTC do not always have 86400 UTC seconds in one UTC day

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

Epoch

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

not really UTC in 1989/1990

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

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

not really UTC in 1992

The ANSI SQL:1992 (also ISO/IEC 9075:1992) standard allowed for minutes with 59, 61, or 62 seconds. Although it did not use the words, this is another instance of an internationally-approved standards document that erroneously asserted the existence of the "double leap second".

UTC in 1992/1993

The CCIR was reorganized to become theITU-R, and the document requiring leap seconds in radio broadcasts of time signals was renamed ITU-R Recommendation TF.460-4.

UTC from 1995 through 1998

In 1995 aCCTF working group determined that the length of TAI seconds was longer than the SI second because the chronometers contributing to TAI were not corrected for the effects of blackbody radiation. Over the next three years the frequency of TAI was steered to reduce the length of its seconds by about 2 parts in 1014. Therefore the length of UTC seconds was also reduced. This change is evident as the final kink in the plot of TT(BIPMxy).

not really UTC in 1997

At some point after the C language standard erroneously created the notion of "double leap second" it was also incorporated into the POSIX standard. In 2003 Landon Curt Noll described the POSIX process. This error was still present in the 1997 POSIX standard.

UTC in 1997-10

The fifth revision of the document specifying that radio broadcasts of time signals have leap seconds was issued as ITU-R Recommendation TF.460-5. This version deleted the sentence "GMT may be regarded as the general equivalent of UT." It changed the reference for the definitions of the forms of UT from the Astronomical Almanac to the IERS. It removed the option of giving DUT1 by voice or Morse code.

UTC since 1999

Klepczynski publicly suggested discontinuing leap seconds, and the CCTF wrote a letter to various international scientific unions which started the ongoing process of reconsidering the future of leap seconds.

UTC in 2000/2001

In 2000-05 the ITU-R Radiocommunications Assembly tasked WP7A with a Question designated 236/7 (now aka Question SG07.236) "The future of the UTC time scale" in order to begin studies about the possibility of discontinuing leap seconds.

UTC in 2002-02

The sixth revision of the document specifying that radio broadcasts of time signals have leap seconds was issued as ITU-R Recommendation TF.460-6. This version introduced the notion of DTAI = TAI - UTC

not really UTC in 2003

The ANSI SQL:2003 (also ISO/IEC 9075-2:2003) standard still allowed for minutes with 59, 61, or 62 seconds, thus the "double leap second". This means that for more than a decade an international standard was able to contain a notion that had never existed without being corrected. Correcting this level of mistake involves re-educating every human who has read it and re-designing every system which implemented it.

UTC(k)

Approximations to UTC are maintained in real time by various laboratories around the world. For example, the designation UTC(NIST) refers to the approximation to UTC that is maintained at the US National Institute of Standards and Technology, and UTC(NPL) refers to the approximation to UTC maintained at the UK National Physical Laboratory. The differences between the various instances of UTC(k) are published monthly by the BIPM in Circular T.

UTC in 2003-05-28/30

The SRG of ITU-R WP7A held the Colloquium on the UTC Timescale in Torino Italy. The conclusion of that meeting produced the Potential Alternative to the Leap Second. The conclusion called for a broadcast time scale without leap seconds to be given a new name, International Time (TI), which was proposed in apaper contributed by E.F. Arias.

UTC in 2004-04-01/02

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

The conclusion of the SRG was that the creation of a new time scale, to be known as "International Time", was not recommended

From this it was clear that the SRG of ITU-R WP7A had already decided to disregard the advice they had received from the international experts they had gathered to discuss the future of UTC.

Other members of the CCTF discussed in response to Beard.

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

not really UTC in 2004

The POSIX standard no longer contained the notion of "double leap second".

UTC in late 2004

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

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

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

UTC in late 2005

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

UTC in 2010-10

ITU-T SG 15 recommended the use of PTP (IEEE 1588) in ITU-T Recommendation G.8265.1. In many respects this is the ITU-T saying that their systems no longer have interest in following ITU-R Rec. 460. This recommendation tacitly ignores the fact that in 1980-11 the 7th Plenary Assembly of the CCITT (the predecessor organization of the ITU-T) accepted UTC as "the time scale for all other telecommunication activities."

UTC in 2010-12

The ITU-R began to allow open access to recommendations, including TF.460. After 40 years behind a paywall engineers, designers, and code writers could see the document which defines the times signals that are inputs to their systems. Unfortunately this was about 30 years too late for POSIX systems.

UTC in 2011-08

The BIPM produced a special issue of Metrologia entirely on the subject of time scales. In its first article emeritus BIPM director Terry Quinn suggests that the ITU-R should transfer the authority for defining UTC to the CGPM.

UTC on 2012-01-19

The delegates to the Radiocommunications Assembly (RA) had the opportunity to vote on the proposed draft revision of ITU-R TF.460-6. If the delegates had approved the draft then UTC would have stopped having leap seconds 5 years after approval. The effect of approval would be to redefine the word "day" such that it is no longer related to the sun in the sky.
The RA delegates found themselves split into three camps with no consensus. The delegates decided to defer any redefinition of UTC until the RA to be held in 2015.

UTC on 2012-09-13/14

The 19th meeting of the CCTF issued Recommendation CCTF 6 (2012) in response to WRC-15 Agenda Item 1.14 which calls for more study of UTC and leap seconds. It asserted facts that are not congruent with the documented history of statements about time made by the IAU, CCIR, CGPM and other agencies. In particular, fact 9 states that UT1 should not be considered as a time scale, thus effectively asserting that prior to the cesium atomic chronometer there were no time scales. This document is the clearest indication that the ITU-R is dealing with a subject where there has been an ongoing turf war between various national and international agencies who are prepared to distort history in support of their position.

UTC in 2012-10

The 101st meeting of the CIPM considered Recommendation CCTF 6 (2012). They acknowledged that there are "political dimensions" to UTC and the leap second. Their notes included

UTC in current events

Please refer to the sibling document that contains more details onrecent,current, andupcoming events about UTC.

UTC as of 2019-01-01 ?

According to the 2009-06-29 memo from the US Assistant Secretary of Defense this is the earliest date at which DoD systems can be modified to be ready for UTC to have no more leap seconds.

UTC in 2022 ?

According to the Potential Alternative to the Leap Second developed at the Colloquium on the UTC Timescale held by ITU-R SRG 7A in Torino on 2003-05-28/30:
UTC will cease to exist.
UTC will be replaced by the purely atomic time scale named TI which is described above.

The purpose of the leap second is to preserve the international definition of the calendar day duration as one rotation of the earth while allowing the duration of one second to be defined by cesium.

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

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

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

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

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

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

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.