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	Coordinated Universal Time was a good articles nominee, but did not meet the good article criteria at the time. There may be suggestions below for improving the article. Once these issues have been addressed, the article can be renominated. Editors may also seek a reassessment of the decision if they believe there was a mistake.
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This article is written in British English, which has its own spelling conventions (colour, travelled, centre, defence, artefact, analyse) and some terms that are used in it may be different or absent from other varieties of English. According to the relevant style guide, this should not be changed without broad consensus.

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Archives

1, 2, 3

GMT is avoided in careful writing?

Really, in the Summer, the UTC time is one hour less that the GMT time. So its ok... 207.6.122.86 (talk) 00:21, 16 March 2011 (UTC)

I don't agree with this statement "GMT is avoided in careful writing" in the introduction. It does not have a citation. On the Misplaced Pages page "Citing Misplaced Pages, "The Harvard Journal of Law and Technology has adopted the following format for citations to articles in Misplaced Pages:"

Misplaced Pages, , http://en.wikipedia/ (as of , GMT). Here is an example: See Misplaced Pages, Bluebook, http://en.wikipedia/Bluebook (describing history and application of the Bluebook) (as of Mar. 21, 2006, 20:50 GMT).

They seem to be partial to GMT. Nly8nchz (talk) 08:04, 20 December 2008 (UTC)

Perhaps it would be better to that that GMT is avoided in technical contexts. Martin Hogbin (talk) 09:32, 20 December 2008 (UTC)

Perhaps it would. As I understand it the Greenwich Meridian is no longer prime but has moved 90 feet or so in order that the 90 degree line should pass through Chicago observatory. Thus it would be wrong either to refer to GMT or the "Greenwich" meridian except in an historical context. —Preceding unsigned comment added by Drg40 (talk • contribs) 09:05, 29 March 2010 (UTC)

Like most things, the real answer is more complicated than that, see Prime meridian. The term GMT is is everyday use in some parts of the world but is best avoided in technical use, as the article currently states. I think this is a reasonable description of the current state of affairs. Martin Hogbin (talk) 10:48, 29 March 2010 (UTC)

Hilarious!! The fact is no one uses UTC in any practical sense (except scientists astrologers etc) UTCis for practical use merely GMT by another name--and very confusing at that-there is of course no such thing as universal time -it has to be measured from somewhere and whether the greenwich meridian is now 100m off centre etc that is where time starts from.Knowing you are gmt plus 5 etc immediately fixes your place on the earth relative to greenwich london.The human brain demands such reference points to work properly. Even when forced to use zulu for example the individual is still mentally thinking of greenwich and not an abstract meaningless number... —Preceding unsigned comment added by 89.132.130.90 (talk) 20:06, 31 October 2010 (UTC)

The second and third sentences currently give indistinguishable definitions of GMT and UT, and maybe they should be merged. The difference is that it encourages the use of UT ("if high precision is not required the general term Universal Time (UT) (without a suffix) may be used") but discourages "GMT" ("generally avoided in technical contexts"). The reference cited doesn't support that. The rationale given -- that it might mean either UT1 or UTC -- would seem to apply equally to UT. How about one sentence describing these synonyms, and if necessary, a second explaining any subtle difference that makes "UT" preferable? 71.139.177.112 (talk) 00:57, 16 February 2009 (UTC)

I think the rational is that while GMT might be used by someone who has not kept up with the terminology, or who is deliberatly "dumbing-down" a text for consumption by the general public, UT is a deliberate indication that for the purposes at hand, the small differences in the various flavors of UT do not matter. I'm not sure if that rational actually works, but I think that is what the passage is trying to get across. --Gerry Ashton (talk) 01:08, 16 February 2009 (UTC)

Really, in the Summer, the UTC time is one hour less that the GMT time. So its ok... 207.6.122.86 (talk) 00:21, 16 March 2011 (UTC)

New comments go at the end of the thread, so I've moved 207.6.122.86's comment. This illustrates what a horrible term GMT is. All the scientific bodies have abandoned it, and if you ask one of them what it means, they'll refuse to answer and tell you to stop using it. The UK considers it to be their legal time, but won't pass a definitive piece of legislation telling exactly how it's defined. The general news media is hopeless on scientific and technical matters, so you can't pay any attention to those folks. What a mess. Jc3s5h (talk) 00:30, 16 March 2011 (UTC)

So are we not right to stick to my suggestion and the current wording that GMT is best avoided in technical contexts. The term is widely used in the UK but it is ill-defined. Anyone who wants to be precise uses UTC but GMT is used in everyday speech. Gerry Ashton expresses it well, GMT is used by people who do not care that much about fractions of a second and correct terminology for precise timekeeping. Martin Hogbin (talk) 18:30, 16 March 2011 (UTC)

I agree and furthermore right at the top of this article it is not correct to say "UTC is synonymous with GMT" because UTC has no British Summer Time offset. Too many people confuse UTC with GMT in GUIs and region configuration screens, even some popular OS builds I've seen! The article on GMT in Misplaced Pages states this clearly right at the beginning. So I feel it is incorrect that this UTC article suggests the opposite; that they are "synonymous" in the first paragraph. I suggest this statement is removed and information added about this common mistake near the top. Right now this is just adding to confusion. Codechief (talk) 20:03, 9 April 2014 (UTC)

User:Codechief, do you contend that GMT advances in (northern hemisphere) spring and falls back in autumn? If so, I have seen many uses of GMT that never observe any daylight saving adjustment. If you can provide a reliable source that GMT includes a daylight saving adjustment, we might be able to edit several articles to state that GMT is hopelessly confused and therefore totally useless. Jc3s5h (talk) 20:22, 9 April 2014 (UTC)

Just to confirm, GMT is never used to refer to daylight savings time, which is called BST in the UK. Martin Hogbin (talk) 10:00, 10 April 2014 (UTC)

Most Recent Leap Second

The most recent Leap Second just occurred Dec 31, 2008. Should this be in the article (or is it and I missed it)? Pointless trivia that should not be included: 2008 was adjusted to be 366.000012 days long.wcf Facts are stubborn. Comments? 21:36, 11 January 2009 (UTC)

The most recent Leap Second just occurred June 30, 2012. Georges Theodosiou email: chretienorthodox@hotmail.fr

page clock

on some refreshes this does not work and shows some random time (it does have some preference for 10:50) and why is the date in YYYY-MM-DD ? Machete97 (talk) 11:21, 3 February 2009 (UTC)

For me it shows in 12 hour form without AM/PM. At the moment it reads 07:01 when it should be 7:01PM or 19:01 80.47.218.9 (talk) 19:05, 19 October 2013 (UTC)

Just tried it myself. When I loaded the page, it gave me an 07:09, when it was 07:19 UTC. When I clicked the refresh, the correct time came up. That may be a cache dependent issue.Wzrd1 (talk) 20:29, 19 October 2013 (UTC)

dubious fact

Just to note that I've tried to clear up some loose/inaccurate language which seemed to be all that made the tagged 'fact' 'dubious' (see 1st para under 'History', about Greenwich and meridian conference). Terry0051 (talk) 01:13, 26 February 2009 (UTC)

Bogus example

I can't find any guarantee that the magic word CURRENTTIME, used below, is UTC. It seems likely that it would depend on the server's locale. As the locale is not necessarily UTC, and this example does not illustrate another part of the UTC standard, I have taken it off the article:

Gyro Copter (talk) 14:54, 25 March 2009 (UTC)

• I just checked, according to , the time generated is determined by the user preferences, defaulting to UTC. This cannot be trusted to always give UTC (presumably some users change their preferences). Gyro Copter (talk) 14:58, 25 March 2009 (UTC)

• This was a useful utility. Could it be restored with a disclaimer that the time is dependent upon the user's preferences being accurately referenced to UTC? AusJeb (talk) 21:54, 28 April 2009 (UTC) (Well assuming my preferences are correct, and really assuming that any of this is correct, because ultimately, isn't this an arbitrary reference to an arbitrary reference used to mark the passage of time)

• I oppose presenting potentially false information, even with a disclaimer. --Jc3s5h (talk) 22:08, 28 April 2009 (UTC)

Describe UCT

The article doesn't mention the abbreviation UCT. It's a common (mis?)abbreviation used when discussing time zones. Searching for UCT time zone on Misplaced Pages or Google both refer to this page, I think the page should describe, or at least mention it.

Chei (talk) 15:38, 9 April 2009 (UTC)

Daylight saving time

I am not sure if it is mentioned, I may have missed it - but :

Is time zone, as the basic one, a subject of changing with Daylight Saving Time, or is it (like I prefer to believe) locked to Standard time / winter time ? --TorSch (talk) (of Misplaced Pages.no) 12:14, 19 July 2009 (UTC)

I'm not sure I understand the question. If you mean does UTC "spring ahead and fall back" as with daylight savings time, no, UTC is never affected by daylight savings time in any way, shape, or form. --Jc3s5h (talk) 17:55, 19 July 2009 (UTC)

OK, then it was like I believed it was. Maybe the line you wrote above, UTC is never affected by daylight savings time in any way, should be found in the article text, to make sure there will never be any doubt? TorSch (talk) (of Misplaced Pages.no) 18:52, 21 July 2009 (UTC)

Aren't the acronym for most time zones changes when they are taken to their spring time, such as by replacing one of their letters with the letter D? Doesn't that suggests that all time zones never affected by daylight savings time in any way? anonymous 1:48, 8 Feb 2015 (UTC) — Preceding unsigned comment added by 217.132.101.41 (talk)

If you look at Time in the United States you will see that time zones and daylight saving time are specified in different parts of the law. I've read that it's even more complicated in the European Union; there is EU legislation that controls the dates when the daylight saving change is made, but the time zone and amount of the change is controlled by laws in each member nation. So I think it's useful to keep the idea of time zone separate from the idea of daylight saving time. Jc3s5h (talk) 02:01, 8 February 2015 (UTC)

The claim in the article still need to be referenced.....

even the info is within the article such as from the table of Compromise abbreviation--222.64.25.204 (talk) 22:35, 28 July 2009 (UTC)

The claims you wanted supported are indeed supported, either by a citation at the end of the paragraph, or earlier in the article. --Jc3s5h (talk) 00:19, 29 July 2009 (UTC)

Pre-1960 Standards

"WWV time signal's frequency was set to a simple offset from the TAI frequency: initially an offset of 1.0 × 10-8, so that WWV ticked exactly one second for every 1.00000001 s of TAI."

I'm not sure that is accurate. First, there was nothing simple about what WWV did in the late 1950s. 20 millisecond jumps were added, and the clock rate was changed daily to match USNO's measurements of universal time. I'v seen it quoted that they used an offset of 100 parts per 1.0E-10, but I find no evidence of that in their publications. The frequency offsets fluctuated around 0.0, and in 1959 they rose to around 25 - 35 parts per 1.0E-10. In 1959, the british MSF service used a frequency offset of 170.

Perhaps an average shift of 100 for the US frequency standard comes from combining the 25-35 shifts in 1959 with the switch from UT Hz to ET Hz cesium frequency standard (9,192,631,838 Hz 9,192,631,770 Hz), which amounts to about 74 parts in 1.0E-10. But I'm not at all sure I am comparing the right things (USFS, WWV, UT vs. ET, etc).

Maybe someone can provide a reference that clarifies this? DonPMitchell (talk) 18:09, 7 August 2009 (UTC)

OK, I see the -100.0E-10 offset implicit in a graph in a January, 1960 IRE paper from the NBS. (National Standard of Time and Frequency in the United State, Proc. IRE, January 1960, p 106). That frequency offset reflects the hypothetical UT oscillator around 1956. DonPMitchell (talk) 20:40, 7 August 2009 (UTC)

Does this help? --- The paper "Time Scales" by Louis Essen in 1968 (Metrologia, vol.4, pp.161-165) says on p.162 that Essen and Parry measured the UT2 mean solar second of their time at 9,192,631,830 cycles (taking a year for the calibration), and then (in collaboration with Markowitz and Hall --- who tracked ET using a moon camera at the USNO) measured the ET second over three years at 9,192,631,770. Terry0051 (talk) 10:05, 8 August 2009 (UTC)

Thanks, yes I've read that paper. I'm not sure that is why they were running their frequency standard at 100.0E-10 below the ET rate. It may have just been done to help them track the UT rate with fewer corrections. The British standards lab was running 170.0E-10 below ET than, probably for the same reason. These frequency shifts are explicitly talked about in reports from the 1960s, but not mentioned in the monthly bulletins in the 1950s. DonPMitchell (talk) 23:51, 8 August 2009 (UTC)

Dubious history

The history section seems to need a bit of clearing up. There is a statement UTC was officially initiated at the start of 1961 (but the name Coordinated Universal Time was not adopted by the International Astronomical Union until 1967). This is said to be supported by Nelson & McCarthy 1995, 15. But nothing of 1995 by Nelson & McCarthy is cited, either in the 'bibliography' or in the misnamed 'notes' (which actually are inline citations to references, or anyhow references with at most a few traces of notes about them).

On the other hand, a source that is cited, Nelson, McCarthy et al 2001 (Metrologia 2001, vol.38, pp.509-529) positively says on page 509 "Since 1972, when UTC was introduced, there have been twenty-two leap seconds, all of which have been positive."

(Any clear-up action needs to be done bearing in mind that epoch dates in matters of astronomy and time-scales have sometimes been assigned retrospectively. I don't know if that happened here, it might be a possibility, I haven't had time yet to look at the details.)

Terry0051 (talk) 23:32, 7 August 2009 (UTC)

Sorry, I wrote the date of the reference wrong. The other source by Nelson and McCarthy (with no et al.) states "Name Coordinated Universal Time (UTC) adopted by IAU in 1967". --Jc3s5h (talk) 23:58, 7 August 2009 (UTC)

Also, Nelson, McCarthy et al. in 2001 wrote "The name 'Coordinated Universal Time (UTC)' was approved by a resolution of IAU Commissions 4 and 31 at the 13th General Assembly in 1967". (Page 515.) --Jc3s5h (talk) 00:12, 8 August 2009 (UTC)

Thanks for that. But it does still leave some weirdness, because the two articles (with overlapping authorship) seem to flat-out contradict each other. On WP:RS grounds, maybe the peer-reviewed article in Metrologia saying "1972" should be preferred over the non-reviewed presentation saying "1961", but I wouldn't want to be too picky about that, because I strongly suspect you are right.

What one needs, I guess, in a situation like this, is additional sources (and after all, working scientists are often not too worried about historical matters anyway).

At least part of the answer seems to be in the paper on "Time Scales" by Louis Essen in Metrologia vol.4 (1968). Essen states on page 162 "UTC. This scale is now used by many time services ..." (and he then describes its features, substantially different from what they are now, involving frequency offsets and step adjustments of 0.1 second at a time.) Essen's article was received May 1968, thus completely disproving the "1972 start" statement by Nelson and McCarthy in Metrologia (2001).

What seems to have happened is that 1972 marked, not the beginning of UTC, but the beginning of a substantially different basis of UTC, with steps of a whole second and use of a constant frequency without offsets. I think there are references from about the time of transition that talk in terms of the "new UTC", so that may be some explanation of why Nelson and McCarthy could say it started in 1972. I'll see if I can dig these out when I get a chance.

Terry0051 (talk) 09:44, 8 August 2009 (UTC)

Nelson et al (Metrologia 2001) gives a detailed history of the evolution of coordinated universal time: Broadcasting a UT-like time signal, governed by atomic clocks was first done in the US and UK in the late 1950s. On Jan 1, 1960, those two countries began to coordinate their broadcasts and frequency offsets and time steps, and other countries soon joined. Nelson calls this the "Original UTC system" A UTC standard was adopted by CCIR (Redommendation 374) in 1963, using 100 ms time step adjustments. Nelson calls that the "Revised UTC System". The name "Coordinated Universal Time (UTC)" was approved by the IAU in 1967. These early forms used frequency shifts and time step adjustments to map atomic time to universal time, but frequency shifts were unpopular for technical reasons involving the equipment that generates pulse trains. In Jan 1, 1972, a new UTC system began using a constant frequency the same as TAI and 1 second adjustments. Nelson calls this the "Present UTC system". DonPMitchell (talk) 01:21, 9 August 2009 (UTC)

1961 was when Britain and America turned over the coordination of UT time signals to BIH. So you could argue that begins the offical international system. The 1960 standards was just an arrangement controlled by USNO and RGO and other labs in the US and UK. DonPMitchell (talk) 01:24, 9 August 2009 (UTC)

No form of the term "UTC" exists in CCIR Recommendations prior to 460-1 in 1974. UTC could not have been standardized by the CCIR in 1961 because their Plenary Assemblies were in 1959 (for Rec. 319) and 1963 (for Rec. 374). The earliest documented occurrence of "UTC" or "TUC" is in the first BIH Bulletin Horaire of year 1964.Steven L Allen (talk) 21:56, 7 November 2011 (UTC) CCIR Rec. 374 from 1963 does not use the term UTC, but rather UT2. Nothing in the IAU proceedings from 1967 can be construed as approving of the name UTC, but merely as recognizing that the time service bureaus used the term UTC as the name of the time scale they were constructing in order to satisfy the CCIR Recs that specified broadcasts to track UT2. The first IAU approval of UTC was in 1973.Steven L Allen (talk) 23:54, 9 November 2011 (UTC)

I have placed Citation needed for the claim of a 1961 origin in the CCIR. The CCIR (then) and the ITU-R (now) exist to hold meetings, produce documents at those meetings, and produce documents about those meetings. If there is no document from the CCIR/ITU-R then there was no "official" action.Steven L Allen (talk) 07:27, 5 January 2013 (UTC)

Use of UTC vs. use of name "UTC"

I undid a recent revision because it claims that "formally:, UTC replaced GMT as the basis of civil time, "although informally, GMT continues to be widely used."

This is not correct. Outside of astronomical observatories and a few other special situations, almost all clocks are set to UTC, or UTC adjusted by a time zone offset and possibly daylight savings time. (That is, almost all clocks are set as best the person doing the setting is able, and if one traces the setting path from one clock to another, one will almost always come to a clock that is based on UTC.) So UTC is almost always used. The formality or informality lies in what it is called. If the time scale is offset by a time zone or daylight savings time, neither of the names "UTC" or "GMT" are applicable. If actually referring to the UTC scale, it is formally named "UTC" but often the informal name "GMT" is used instead.

Of course "GMT" is also the name of a time scale based on the mean solar time at the Royal Observatory, Greenwich, but no scientific body maintains a time scale named "GMT" so there is no rigorous definition for the term. Jc3s5h (talk) 18:11, 26 August 2010 (UTC)

I will look at the passage to see if it can be improved.Jc3s5h (talk) 18:11, 26 August 2010 (UTC)

I recently learned that the Danish national metrology agency interprets Danish law, which bases the legal time of Denmark on the Greenwich meridian, so strictly as to exclude UTC, because UTC is not GMT. Also, The Nautical Almanac published jointly by the Nautical Almanac Offices of the US and UK still defines UT as GMT, explicitly stating that it differs from UTC by up to 0.9 seconds.

"The time argument on the daily pages of this Almanac is 12 + Greenwich Hour Angle of the mean sun and is here denoted by UT, although it is also known as GMT. This scale may differ from the broadcast time signals (UTC) by an amount which, if ignored, will introduce an error of up to 0'.2 in longitude determined from astronomical observations. (The difference arises because the time argument depends on the variable rate of rotation of the Earth while broadcast time signals are now based on an atomic time-scale.) Step adjustments of exactly one second are made to the time signals as required (normally at 24 on December 31 and June 30) so that the difference between the time signals and UT, as used in this Almanac, may not exceed 0.9." The Nautical Almanac, 1989–2011, page 254.

— Joe Kress (talk) 08:56, 29 August 2010 (UTC)

I think the article was meant to mean '"although informally, the term GMT continues to be widely used." Would changing the text to that solve the problem? Martin Hogbin (talk) 17:58, 29 August 2010 (UTC)

Before I reverted the change the paragraph in question read:

Time zones around the world can be expressed as positive or negative offsets from UTC as in this list; formally, UTC replaced GMT as the basis for the main reference time scale or civil time in various regions on 1 January 1972, although informally, GMT continues to be widely used.

I think the paragraph above the one that was changed adequately covers the informal nature of "GMT". The paragraph in question is about time zones. Except in the British Isles during winter, neither "GMT" nor "UTC" are used in time zone names. The zone time is almost always an integer number of half hours different from UTC; that is the sense in which zone time is based on UTC. Since neither of the terms "UTC" or "GMT" are used when presenting zone time to the public, there is no reason to discuss casual use of "GMT" in this paragraph. Jc3s5h (talk) 19:20, 29 August 2010 (UTC)

I agree, a section on time zones is not the right place to mention GMT. The subject is discussed above. Martin Hogbin (talk) 00:00, 30 August 2010 (UTC)

Necessity of making the opening comprehensible to non-scientists

Without in any way losing the absolute scientific description of UTC - there is a need that the opening be understandable by ordinary people arriving at this page seeking to know what UTC is. If the opening line is a purely scientific definition - with no regard for the lay-person - then the article fails in the purpose of elucidating readers. As long as the scientific information is prominent - then the simplified descriptive is a benefit to the article and the purpose of Misplaced Pages. Davidpatrick (talk) 11:09, 1 November 2010 (UTC)

Coordinated

Should "coordinated" be spelt "co-ordinated"?Osborne 14:38, 17 November 2010 (UTC)

It should be spelt "Coordinated" because it is part of the proper name of a time scale.

Jc3s5h (talk) 16:14, 17 November 2010 (UTC)

UTC is 5 hours ahead of local time on the east coast of the United States during winter, but 4 hours ahead during summer

This implies that UTC changes. —Preceding unsigned comment added by 81.140.57.113 (talk) 12:11, 15 December 2010 (UTC)

The paragraph begins "UTC does not change with a change of seasons...." I'm sorry, but sometimes a sentence cannot be properly understood without reading the whole paragraph. If, in light of the whole paragraph, you still think the paragraph implies it is UTC and not civil time that is being adjusted, please explain your reasoning. Jc3s5h (talk) 13:30, 15 December 2010 (UTC)

Citation format

This article uses a mixture of different citation formats. The first inline citation I find uses the {{Cite xxx}} family of templates. Thus, I intend to

• change all citations in "References" to that family of templates
• change any full citations in "Notes" to short citations and move the full citation to "References"

Are there any comments or objections? Jc3s5h (talk) 15:17, 3 July 2011 (UTC)

I have substantially completed this change, although I have to investigate a couple of cases where clicking on the short footnote does not leap to the full citation; the {{Svn}} template is used for the footnotes. Jc3s5h (talk) 15:16, 18 July 2011 (UTC)

Too many solar time articles

Please see Misplaced Pages talk:WikiProject Time#Too many solar time articles Jc3s5h (talk) 17:38, 2 August 2011 (UTC)

Universel Temps Coordonné?

The sources supporting the claim that "Universel Temps Coordonné" is unofficial French for Coordinated Universal Time are weak. The first is an online forum, the other is some instructions for an online chat service. Both appear to have been written by people fluent in English, and no evidence is presented that the authors are knowledgeable about actual French usage. Jc3s5h (talk) 04:18, 5 August 2011 (UTC)

There has been some discussion on this topic in the past, but if the references are weak I suggest you remove the offending statement. Martin Hogbin (talk) 08:50, 5 August 2011 (UTC)

Now, this has been changed to Temps Universel Coordonné. But how is this shortened to UTC?? —Kri (talk) 14:49, 24 April 2014 (UTC)

This is explained in the "Abbreviation" section of the article. Jc3s5h (talk) 16:10, 24 April 2014 (UTC)

Legal time

Question, should we add a list of countries/regions/etc. where UTC is in law described as the basis for time-keeping? I would find this to be highly interesting. Also considering that the European Union Commission's directive 2000/84/EG (http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32000L0084:EN:HTML) states that EU summertime uses GMT. But a later directive UTC (http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:C:2011:083:0006:01:en:HTML). Another issue regarding EU law is that different translations uses different terms (e.g. GMT, UTC and even others). Wronguy (talk) 17:57, October 5 2011 (UTC)

It would be interesting, but difficult. The European Union illustrates some of the problems, where it isn't very clear to a non-lawyer when individual country laws would control and when EU laws would control. Jc3s5h (talk) 19:53, 5 October 2011 (UTC)

GA Review

This review is transcluded from Talk:Coordinated Universal Time/GA1. The edit link for this section can be used to add comments to the review.

Reviewer: Looie496 (talk) 17:01, 15 October 2011 (UTC)

I will review. Pyrotec (talk) 19:57, 9 August 2011 (UTC)

Sorry for the delay. I will review from today.

Initial comments

On the basis of a very quick read, perhaps the article will make GA-status time round. However the WP:Lead appears to be quite poor and the Uses section is somewhat under-referenced.

I'm now going to work my way through the article starting at the Uses section and considering the WP:Lead last. This is likely to take at least one day, possibly more. Pyrotec (talk) 20:05, 30 August 2011 (UTC)

• Uses -

• The first two paragraphs are unreferenced.
• The claim in the third paragraph, i.e. "International broadcasters such as the BBC World Service also use UTC ..." is unreferenced.
• The claim in the fourth paragraph about Zulu time is unreferenced.
• The claim in the final paragraph about the international space station is unreferenced.
• Most of these paragraphs are short, very short, sometimes a single sentence, so they should be merged.

• The first paragraph is probably OK as it stands. The second paragraph is a single sentence about about global commerce: I see no reason why it can be merged with the fifth paragraph which is about global transportation.

• Definition and relationship to other standards -

• This has much more in-line citations but also suffers from mostly single sentence paragraphs, one two-sentence paragraph and a three-sentence paragraph. The first three paragraphs could be merged into one.
• There is no need to wikilink both occurrences of Universal Time is a single section, this is WP:Overlinking.

Pyrotec (talk) 18:00, 4 September 2011 (UTC)

• Notation -

• Looks OK. I'll accept Notation as a section title, but is it really notation that is being discussed (its more like abbreviations)?

• Mechanism -

• Poor grammar: I'm not convinced that the second paragraph is a paragraph, i.e. "Thus, in the UTC time scale, the second and all smaller time units (millisecond, microsecond etc.) are of constant duration, but the minute and all larger time units (hour, day, week etc.) are of variable duration." It seems to a single sentence that has escaped from the first paragraph.
• The second proper paragraph, i.e. the 86,400 second-day paragraph is unreferenced. I'm happy to accept that 24 hr x 60 min x 60 sec = 86,400 seconds, but all the claims and statements about 59 second and 61 second minutes need a citation, i.e. compliance with WP:Verifiability.

....Stopping for now. To be continued. Pyrotec (talk) 18:54, 4 September 2011 (UTC)

• • Time zones -

• Looking at my old school atlas, Reykjavik is listed as 21° 53' W so it is not obvious that "is always on UTC time and does not use daylight saving". There are two claims here that need a citation (possibly only one citation is needed).

• • Daylight saving -

• The given example about East Coast time needs a citation.

• History -

• The first, fourth, fifth and sixth paragraphs are unreferenced.

• Rationale -

• This is mostly unreferenced, but as it is mostly explanation, and the one statement is referenced, I'll accept this as it is.

• Future -

• There is a {{who}} flag that needs to be addressed.
• The penultimate paragraph states: "....An ITU study group was to have voted on this possibility during 2008, possibly leading to official approval by the World Radiocommunication Conference in 2012 and the subsequent cessation of leap seconds.", as we are now 75% of the way through 2011 I would have expected an update as to whether the 2008 vote took place, and whether approval next year (in 2012) is possible?

• WP:Lead -

• This part of the article has to functions: to introduce the topic cover by the article and summarise the main points. What we have is four paragraphs: one comprised of two sentences and three paragraphs of one sentence. Perhaps they are bullet points with out the points.
• For a short article such as this, I would have expected one or two (proper) paragraphs introducing the topic (yes it possibly does) and summarising the main points. Its not really my job to state what the main points are: perhaps they are all covered, in which case perhaps a more detail is needed e.g. Atomic time has a link, but no mention of a Caesium clock, GMT is mentioned by not Zulu time.

At this point, I would regard the article as non-compliant in respect of prose/grammar and WP:Verifiability. The article is well referenced in parts and some of the existing references might provide adequate verification for the statements discussed above that are not currently verifiable. I'm therefore going to put this review On Hold to allow corrective actions to take place. None have taken place in the last five days. I've been distracted elsewhere on wikipedia, so I know that distractions do occur. Pyrotec (talk) 19:23, 9 September 2011 (UTC)

I believe I have addressed this to some degree, although I'm sure some editors will feel the writing can be further improved. Jc3s5h (talk) 16:56, 21 September 2011 (UTC)

Status check

This review seems to be inactive at the moment, and it's been open for two months. Do we have a decision? Do you need some help? WhatamIdoing (talk) 15:17, 11 October 2011 (UTC)

I observe that Pyrotec has not made any Misplaced Pages contributions since 15 September. Jc3s5h (talk) 16:34, 11 October 2011 (UTC)

It appears that s/he announced a wikibreak on the user page. No e-mail, either. What would you like to do? WhatamIdoing (talk) 22:00, 13 October 2011 (UTC)

I think it would be best to have another editor do the review. Pyrotec has made a real contribution by finding a number of issues, and it couldn't hurt to have someone else look at it from a different point of view. Jc3s5h (talk) 22:32, 13 October 2011 (UTC)

Picking up review

I'm not quite sure what has been going on here, but I'll pick up this review. Contrary to the previous reviewer, the first thing I would like to address is the lead, which is seriously lacking. I would like to see the lead give brief answers to these questions:

• How is UTC information provided to clients?
• Where is the clock located?
• How are UTC times determined? The lead currently says that UTC is based on TAI, but the reader should not need to go to the TAI article for a basic answer.
• How precisely are UTC times specified?
• What is the rationale for deciding when to add a leap second? In other words, who makes this decision and what is the basis for the decision?
• In what way does UTC differ conceptually from TAI and UT1?
• Where did the acronym UTC come from?
• Who decided to make UTC a standard, and when did this happen?
• Will UTC be the standard forever?

Most or all of this information is in the article, of course, but it should all be addressed in the lead. Looie496 (talk) 17:01, 15 October 2011 (UTC)

That's too much for the lead, in my view. Some of these questions cannot possibly be answered (will UTC be the standard forever?). Others have multiple interpretations and answers (How is UTC information provided to clients?) Some can't really be answered in their present form (where is the clock located?) Let me give some off-the-cuff answers to show the complexity:

1. How is UTC information provided to clients? Pure UTC with no source designated is not provided in real time; instead, a list of how far off each of 68 laboratories were is issued retrospectively in Circular T.
2. Where is the clock located? There is no one clock that provides UTC. There are quite a few laboratories around the world that are included in the computation of UTC (listed in Circular T), and there are many more clocks that provide approximations of UTC.
3. How are UTC times determined? The lead currently says that UTC is based on TAI, but the reader should not need to go to the TAI article for a basic answer. The requirement is to provide leap seconds to keep |UT1 - UTC| < 0.9 s.
4. How precisely are UTC times specified? UTC is computed, not specified. The largest difference between the retrospectively computed UTC and a time laboratory in the latest Bulletin C that I can pick out by eye is for Budapest, -56436.6 ns.
5. What is the rationale for deciding when to add a leap second? In other :words, who makes this decision and what is the basis for the decision? The criterion is stated in the answer to question 3. The decision appears to be made by the International Earth Rotation AND Reference Systems Service and is contained in Bulletin C.
6. In what way does UTC differ conceptually from TAI and UT1? It is a compromise between them, having seconds of uniform length (as measured by atomic clocks) like TAI, but, because of leap seconds, staying close to mean solar time (as implemented by UT1).
7. Where did the acronym UTC come from? It was chosen by the CCIR (now named ITU-R) in 1967, according to McCarthy and Seidelmann. That organization also stated the names were Coordinated Universal Time in English and Temps Universel Coordonné in French. (2009, p. 227)
8. Who decided to make UTC a standard, and when did this happen? See previous answer. Also, various other organizations and governments have explicitly or implicitly endorsed UTC.
9. Will UTC be the standard forever? There is a big fight about this going on in the precision time community.

Do you think the following draft might serve as a starting point for creating a more informative lead?:

Coordinated Universal Time (abbreviated UTC) is the primary time standard by which the world regulates clocks and time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose. If only limited precision is needed, clients can obtain the current UTC time from a number of official internet UTC servers . For nanosecond precision, clients can obtain the time from radio or satellite signals. Time zones around the world are expressed as positive or negative offsets from UTC, as in this list.

Coordinated Universal Time is closely related to International Atomic Time (TAI), a time standard calculated using a weighted average of signals from over 200 atomic clocks located in over 70 national laboratories around the world. The only difference between the two is that UTC is occasionally adjusted by adding a leap second in order to keep it within one second of Universal Time, which is defined by the Earth's rotation. In the 49 years up to 2010, a total of 24 leap seconds have been added.

The UTC standard was officially initiated in 1961 by the International Radio Consultative Committee, at the request of five national laboratories. The system was changed several times over the following years, until it reached its final form until 1972. A number of proposals have been made to replace it with a new system, but no consensus has yet been reached to do so.

Looie496 (talk) 22:21, 15 October 2011 (UTC)

That looks promising. I intend to break it down into a list of claims, and see what claims are presented as introductory information in the current article and one or two books I have. Then I'll suggest additions or deletions. Jc3s5h (talk) 22:28, 15 October 2011 (UTC)

That isn't quite the right way to think about it. In a Misplaced Pages article, lots of readers only look at the lead, so it should give them as comprehensive a picture as it can without going into too much detail. In books and printed articles the introduction serves a different purpose -- a Misplaced Pages lead is more like the abstract of a paper than the introduction of a paper. Looie496 (talk) 22:43, 15 October 2011 (UTC)

Can I suggest, 'Coordinated Universal Time is based on International Atomic Time' because that is the way things work. The actual clocks determine TIA and UTC is calculated from this. Martin Hogbin (talk) 11:59, 16 October 2011 (UTC)

I will put a draft lead section derived from Looie496's version below. Jc3s5h (talk) 15:21, 16 October 2011 (UTC)

Proposed lead section

Coordinated Universal Time (abbreviated UTC) is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose. If only limited precision is needed, clients can obtain the current UTC time from a number of official internet UTC servers . For sub-microsecond precision, clients can obtain the time from satellite signals. Time zones around the world are expressed as positive or negative offsets from UTC, as in this list.

Coordinated Universal Time is based on International Atomic Time (TAI), a time standard calculated using a weighted average of signals from atomic clocks located in nearly 70 national laboratories around the world.(International Bureau of Weights and Measures 2011) harv error: no target: CITEREFInternational_Bureau_of_Weights_and_Measures2011 (help) The only difference between the two is that UTC is occasionally adjusted by adding a leap second in order to keep it within one second of UT1, which is defined by the Earth's rotation. In the 50 years up to and including 2011, a total of 34 leap seconds have been added.

The UTC standard was officially standardized in 1961 by the International Radio Consultative Committee, after having been initiated by several national time laboratories. The system was changed several times over the following years, until leap seconds were adopted in 1972. A number of proposals have been made to replace it with a new system, which would eliminate leap seconds, but no consensus has yet been reached to do so.

• International Bureau of Weights and Measures (10 October 2011). "Circular T". {{cite web}}: Invalid |ref=harv (help)

Jc3s5h (talk) 15:21, 16 October 2011 (UTC)

As I see it, my role here is to make suggestions, and perhaps to copy-edit, but not to add substantive content to the article, because if I did I would not be a neutral reviewer. So you should edit the article in any way you think is appropriate, and if I think there is a problem with it I'll say so. In this particular case, the version above looks fine to me. Looie496 (talk) 15:31, 16 October 2011 (UTC)

I have made changes substantially as I described above; I hope the review process can continue. Jc3s5h (talk) 12:33, 22 October 2011 (UTC)

Picking up review again, sorry for delay

• Uses: Is the bit about amateur radio important enough for this article? If so, a ref should be added to support the statement.

I think the use by amateur radio operators is relevant because they illustrate that UTC is used by a widely-distributed group of people with a wide range of educational background; not just by physicists, astronomers, and telecommunications engineers. It will also raise in readers mind the possibility that any geographically dispersed group might use UTC for scheduling. I added a citation, and reworded the article to mention scheduling, rather than logging. A general readership would be rightly uninterested in amateur radio logs. Jc3s5h (talk) 17:03, 24 October 2011 (UTC)

• Mechanism: I think this section should probably mention that the schedule of addition of leap seconds can't be predicted because of tectonic events that cause small but significant changes in the earth's rate of rotation.
• "UTC is derived from International Atomic Time..." This paragraph is very hard to understand. Can I suggest that you leave out the material about what happened before 1972, and simply describe how it works now? I don't think that alone will solve the problem, but at least it should help.
• "As with TAI, UTC is only known with the highest precision in retrospect." I don't understand what this sentence means.
• "Because of time dilation,..." This paragraph doesn't seem to belong in this section, since it describes how UTC is conveyed, not how it is calculated.
• Daylight savings: This doesn't really justify a separate section -- I think it could just be a paragraph added to the previous section.
• History "The signal frequency was changed less often." I don't understand this sentence.
• "In 1958, data was published linking the frequency for the caesium transition, newly established, with the ephemeris second." You should either explain what this means and why it is relevant, or delete it.

• It will take a little time to look into all these points, but I can answer one right off. "As with TAI, UTC is only known with the highest precision in retrospect" means that there are many atomic clocks being compared; each is an approximation to TAI or UTC. These comparisons are carried out by the International Bureau of Weights and Measures and published in Circular T, which is issued at one month intervals. So if one wanted the most accurate value of UTC possible, one would obtain the best real-time transmission from a time laboratory as is availible in one's locality, and record events according to that time scale. Then one would wait about a month and adjust the value by the amount in Circular T to get the best possible estimate of the time of the event.

For example, if the best time source available to someone was UTC(MKEH) in Budapest, and on that time scale an event occurred at 00:00:00.000 000 000 0 on Sept 30, 2011, one would see that UTC was 56436.6 ns slower than UTC(MKEH), so a better estimate of the time of the event would be 00:00:00.000 056 436 6 Jc3s5h (talk) 20:10, 28 October 2011 (UTC)

• I think "Daylight saving" should remain as a separate section, because it is one of the key points a person learning to convert between zone time and UTC must keep in mind. Jc3s5h (talk) 15:30, 29 October 2011 (UTC)

• A colloquium on the future of UTC was held, and the presentations have just become available. I intend to consider these papers before making further edits. Jc3s5h (talk) 19:50, 3 November 2011 (UTC)

Closing review

Due to the lack of progress I am going to fail this GA review. I think the article still needs significant work -- feel free to renominate it when it is ready. Looie496 (talk) 18:33, 14 November 2011 (UTC)

Citation in paren. seems wrong

It's a small thing but I've traced it as far as I can, have gotten in trouble before on sci. notations.

In the Rationale section, 2nd & 3rd paragraphs, there are two McCarthy/Seidelmann cites, 1st p. 54, 2nd p. 230. For some reason the 2nd shows up all in paren.; looked off to me.

I got that the 1st used {sfn...}, the second {harv...} formats; and here; but, sorry, have to leave it at that. Hopefully helpful somehow; or please explain for the uninf'd -- I'll check back. Thanks. Swliv (talk) 12:01, 11 April 2012 (UTC)

The sfn template has built-in <ref> elements, but if the citation is too complicated for that template, one must use the more flexible harvnb template. That one is like sfn in that there are no parentheses around it (or as the brits like to say, no brackets). But by accident the harv template was used, which does have parentheses. I fixed it. Jc3s5h (talk) 13:33, 11 April 2012 (UTC)

I can't imagine understanding it right now; maybe someday; maybe some other nimbler brain will learn from you; and it's fixed meantime; best Wiki tradition! Thank you. And someday I will learn what those brits call a "

Abbreviation?

"UTC" is not an abbreviation; it's technically an initialism. The word 'initialism' isn't used by many people though... would correcting this only serve to confuse visitors?

As per Abbreviation, "In strict analysis, abbreviations should not be confused with contractions or acronyms (including initialisms), with which they share some semantic and phonetic functions, though all three are connoted by the term "abbreviation" in loose parlance."

Jediknightbob (talk) 02:09, 14 October 2013 (UTC)

I believe that the term initialism would confuse many readers, as it is a rarely used term, especially by the lay public.Wzrd1 (talk) 02:28, 14 October 2013 (UTC)

Why not just call it an acronym? Although, technically, an acronym is also a type of abbreviation ;) Wolfbeast (talk) 10:53, 25 April 2014 (UTC)

I understand that when spoken, the letters of an acronym are considered to form a word; the individual letters are not spoken one at a time. Since UTC is spoken yew tee cee, it is not an acronym. Jc3s5h (talk) 12:51, 25 April 2014 (UTC)

Misplaced Pages use

I always understood that WP and the Wikimedia Foundation strongly prefer the use of UTC over GMT in their own applications. Yet, here's an error message I got today:

• If you report this error to the Wikimedia System Administrators, please include the details below.
• ....
• ....
• Error: 503, Service Unavailable at Sun, 23 Feb 2014 19:24:17 GMT. (my bolding).

What gives? -- Jack of Oz 22:58, 23 February 2014 (UTC)

Suggestion: split out the explanation of variable earth time

I'd suggest that the entire explanation of the earth's rotation and how it impacts measured/agreed upon time in detail be split out into a separate Misplaced Pages article. It has nothing to do with UTC as a time zone/definition concept and makes this article too complicated to read for people looking for a definition and explanation of UTC. Wolfbeast (talk) 10:49, 25 April 2014 (UTC)

Variable earth rotation is the reason UTC must exist; if earth rotation were not variable, UTC, TT, UT1, and TAI would all collapse into a single time scale. Since it is the fundamantal reason UTC exists, it must not be omitted. Jc3s5h (talk) 12:54, 25 April 2014 (UTC)

Out of date?

" In the 41 years up to and including 2012, a total of 25 leap seconds have been added; the most recent was added on 30 June 2012." Is this still up to date, and if not can someone who knows where to find the info update it please? Richard75 (talk) 16:04, 11 June 2014 (UTC)

It's still up-to-date. I put the current offset in a separate section and referred to it from the lead so that when it changes it won't be necessary to update several parts of the article. I also provided the source. Jc3s5h (talk) 16:42, 11 June 2014 (UTC)

Good idea. Martin Hogbin (talk) 09:30, 8 February 2015 (UTC)

parsed to idiocy

on seeing an eclipse expressed in UTC I came here to discover what that even means and scanning the article I can NOT tell what freaking time is meant ... and so you have written this and parsed meaning to idiocy so that you can NOT tell what time is meant by expressions in UTC ...it appears to mean about Greenwich time corrected by approx. an hour ....re write all of this article into English thanks 47.18.43.166 (talk) 17:08, 19 March 2015 (UTC)real time sr

I'm trying to address this but I'm running into headwinds. This is all non - controversial. It is well - known that many countries use daylight saving time and the 0.9 second offset referred to in the lead is discussed in more detail later. As for the change in nomenclature, I'll add a reference.

References

1. Whitaker's Almanack 1977 (London 1976) p. 141.

Information introduced to lead

These changes introduce incorrect information.

• "Coordinated Universal Time (Template:Lang-fr), abbreviated as UTC, is the primary civil time standard by which the world regulates clocks and time. It is always, within 0.9 seconds, mean solar time at 0° longitude;"

No. As the cited author Bernard Guinot writes in the abstract of his paper, "The International Conference held in 1884 at Washington defined a universal time as the mean solar time at the Greenwich meridian (GMT). Now, the Universal Time, version UT1, is strictly defined as proportional to the angle of rotation of the Earth in space. In this evolution, the departure of UT1 from GMT does not exceed one or two seconds." The 0.9 potential difference between UT1 and UTC is in addition to the one or two seconds stated by Guinot.

The change to the article also stated "Greenwich Mean Time corresponds to UT1" which is clearly contrary to what Guinot stated.

Dennis McCarthy also indicates there are at least two possible modern meanings for GMT:

The nomenclature ‘GMT’ continues to cause confusion to this day because of its use in the United Kingdom as the name attached to the civil time and its common navigational usage to mean UT1, discussed below. With all of these identities it is not advisable to use the term GMT as a timescale for precise purposes without carefully defining its meaning. .

Jc3s5h (talk) 17:28, 16 April 2015 (UTC)

What is the "angle of rotation of the Earth in space?" Is this an hour angle or something different? 156.61.250.250 (talk) 18:19, 16 April 2015 (UTC)

Universal Time article says "all of these versions of UT are based on Earth's rotation relative to distant celestial objects". It's always been this way. The sidereal time is calculated and then an adjustment made (knowing delta T) to get UT. 156.61.250.250 (talk) 18:44, 16 April 2015 (UTC)

In the context of the current definition of UT1 , the angle of rotation of the Earth in space is the Earth Rotation Angle. The glossary of the Astronomical Almanac defines that as:

Earth Rotation Angle (ERA):

the angle, θ, measured along the equator of the Celestial Intermediate Pole (CIP) between the direction of the Celestial Intermediate Origin (CIO) and the Terrestrial Intermediate Origin (TIO). It is a linear function of UT1; its time derivative is the Earth's angular velocity.

This introduces some other new 21st century nomenclature. The Celestial Intermediate Origin replaces the March equinox (in the sense of a direction in space, not as an event). It is part of a system (ICRF) that uses Very-long-baseline interferometry to measure radio sources outside the Milky Way galaxy. Because they are so far away, they seem to be motionless, so make a better reference framework than the bright stars that used to be used. The ICRF is designed to be aligned, as best they could, to the north pole and March Equinox at a certain point in time, J2000.0, in January 2000. The Terrestrial Intermediate Origin is 0° longitude in the International Terrestrial Reference Frame (ITRF), which replaces the meridian through the Airy transit circle which was used back when Greenwich Mean Time was first introduced. Zero degrees longitude in the ITRF system is very close to the system used by GPS receivers, and is around 100 meters away from the Airy transit circle.

So if we use the nomenclature informally, the angle of rotation of the Earth in space is the angle between the 0° meridian and where where the March equinox used to be in January 2000.

Also note that calculating UT1 is not quite as simple as converting the earth rotation angle from degrees to hours; it also has to be multiplied by a constant to allow for the average position of the Sun. Without the multiplication, Earth Rotation Angle is very similar to sidereal time. Jc3s5h (talk) 18:52, 16 April 2015 (UTC)

156.61.250.205 wrote "The sidereal time is calculated and then an adjustment made (knowing delta T) to get UT." Not anymore. The Earth Rotation Angle is calculated and multiplied by a constant to get UT1. The difference is that the Earth Rotation Angle is referenced to radio sources outside the Milky Way, and those radio sources have virtually no change in direction over long periods of time. Sidereal time is referenced to the equator and the March equinox, which is subject to precession and other changes. Jc3s5h (talk) 18:57, 16 April 2015 (UTC)

There's nothing new here. The Terrestrial Intermediate Origin is just a fancy name for what used to be called the ephemeris meridian. There was always a constant added to allow for the sun to move from the ephemeris meridian to the Greenwich meridian. The Earth Rotation Angle is also referenced to the March equinox, under its fancy name of Celestial Intermediate Origin.

According to the Astronomical Almanac:

UT universal time; counted from 0h at midnight: unit is mean solar day

prior to 1925 was reckoned, for astronomical purposes, from Greenwich mean noon (12h UT).

universal time (UT): a measure of time that conforms, within a close approximation, to the mean diurnal motion of the Sun and serves as the basis of all civil time - keeping. UT is formally defined by a mathematical formula as a function of sidereal time. Thus UT is determined from observations of the diurnal motions of the stars. The time - scale determined directly from such observations is designated UT0; it is slightly dependent on the place of observation. UT1 is derived by removing from UT0 the effect of the variation of the observer's meridian due to polar motion. When the designation UT is used in this volume, UT1 is implied.

Dennis McCarthy confirms this. There are inaccuracies within the tables - for example a decrement of 1.34 seconds in delta T has been applied to reconcile the moon's computed position to observation. None of this warrants an observation that GMT has differed from UT1 by two seconds. Guinot does not say this has ever happened. Unless there is within his paper a statement that an event has been timed by both GMT and UT1 and a discrepancy has been observed we can go with the Astronomical Almanac and McCarthy. 156.61.250.250 (talk) 08:45, 17 April 2015 (UTC)

Since I provided above a link to the latest edition of the glossary of the Astronomical Almanac, and since the purported quote from 156.61.250.250 contains a significantly definition of Universal Time, I consider this to be a deliberate false statement and trolling. 156.61.250.250 this is your last warning! Any editing of the article based on this outdated information will cause me to seek dispute resolution from administrators.

The current definition of Universal Time in the Astronomical Almanac Online is

Universal Time (UT):

a generic reference to one of several time scales that approximate the mean diurnal motion of the Sun; loosely, mean solar time on the Greenwich meridian (previously referred to as Greenwich Mean Time). In current usage, UT refers either to a time scale called UT1 or to Coordinated Universal Time (UTC); in this volume, UT always refers to UT1. UT1 is formally defined by a mathematical expression that relates it to sidereal time. Thus, UT1 is observationally determined by the apparent diurnal motions of celestial bodies, and is affected by irregularities in the Earth's rate of rotation. UTC is an atomic time scale but is maintained within 0s.9 of UT1 by the introduction of 1-second steps when necessary. (See leap second.)

Jc3s5h (talk) 12:39, 17 April 2015 (UTC)

• Former good article nominees
• C-Class Time articles
• High-importance Time articles
• Misplaced Pages articles that use British English
• Selected anniversaries (December 2004)