Talk:Greenwich Mean Time

Archive of Time Cube discussion

Article had become clogged with off- (GMT)-topic material about the overall time zone UTC+00:00
I've been bold and done a big cleanup of off-topic material that is really about UTC+00:00 and belongs in that article, not this one. I guess the problem arises because older editors still use the term GMT with world-wide application?

UTC+00:00 is the overview topic, GMT is one of the aspects of it but in addition has historical significance that other local designations (like Mountain Time) do not. The extent of generic material that belonged in the UTC+00:00 article waa beginning to swamp the specific stuff. --John Maynard Friedman (talk) 17:39, 8 October 2021 (UTC)

Unfortunately you built your bold cleanup on a bad edit. The edit revised the article to indicate that "Greenwich Mean Time" these days always means the time zone UTC and it is always incorrect to use it to refer to UT1. But the sources indicate its meaning is ambiguous. For example the article says "GMT is defined today as UTC+00:00 and no longer by the transit of the sun. Thus the term GMT cannot be used for purposes that require precision."

But what Hilton and McCarthy actually state is "In the United Kingdom, Greenwich Mean Time has been identified with the civil time or Coordinated Universal Time, UTC (§ 6.8.3.1). This connection, however, has never been formalized, so using GMT to refer to UTC should be done with care. For navigation, however, Green Mean Time has meant UT1 (§ 6.8.3). Thus, GMT has two meanings that can differ by as much as [0.9 s], and the term GMT should not be used for precise purposes. [Wikipedia does not permit precisely reproducing the typography used for 0.9 s.]"

It's also silly to claim that there is a precise meaning (purportedly UTC) and at the same time say it can it can't be used for purposes that require precision.

Jc3s5h (talk) 19:42, 8 October 2021 (UTC)


 * Yes, I saw that and tried previously to resolve it. It was I who added the footnote (a) The glossary of HM Nautical Almanac Office does not contain a definition of GMT. I too found sources that say that GMT can't be used without qualification and others saying that it is used routinely in navigation without reservations. I tried a few weeks back to resolve but failed and largely let it stand. If you can crack it, you have my support and applause.
 * But I still believe that the transfer of UTC+00:00 content made sense and it does so even more now in that this article is no longer cluttered with that content, leaving nowhere left to hide for the erroneous explanations. --John Maynard Friedman (talk) 20:51, 8 October 2021 (UTC)

"GMT is now a time zone, not a time reference": Bold, revert, discuss
boldly deleted, and I reverted per BRD, the second part of this statement: English speakers often use GMT as a synonym for Coordinated Universal Time (UTC): in modern usage, this is incorrect – GMT is now a time zone, not a time reference. The reason given in the edit note is "that is not a wp:RS".

There are two citations: the first (TimeAndDate.com) is certainly a commercial undertaking but it is not listed at Reliable sources/Perennial sources, in fact it has never even been discussed there. Unless a source is self-evidently unreliable, individual editors should seek consensus there before declaring it such. The second is HM Nautical Almanac Office, which is as reliable as they come, though arguably it only cites the first half of the sentence.

There is a case to reword the phrase slightly to say GMT is now one of the names for the time zone UTC±00:00, not a time reference but let's resolve the principle first before getting into this detail.

If there is a more convincing justification for the bold edit, let's see it. --John Maynard Friedman (talk) 16:56, 27 October 2021 (UTC


 * @JMF, I'm a notorious pedant. As far as I'm concerned personally, GMT doesn't exist any more. I find it's use on BBC World Service intensely irritating; "Four Greenwich Mean Time", as they usually say, is at best idiosyncratic. It seems nativist, nationalistic, and jingoistic.


 * I'm perfectly happy with your revert. Sometimes I make an edit to provoke discussion. Usually no discussion ensues, but it seems to have this time!


 * MrDemeanour (talk) 04:15, 30 October 2021 (UTC)


 * I wouldn't say Buckle is an unreliable source, but among the wide variety of sources expressing differing opinions on this, she is less reliable than several others. The glossary for the Astronomical Almanac Online does not support the claim that GMT is a time zone. Indeed, one could argue that the only time zone that officially uses GMT, the British Isles, doesn't seem to have a well known name, but observes two different time scales in different parts of the year: GMT in winter and British Summer Time in summer.
 * In any case, GMT is sort of a time zone, but it also is a name for UT, UT1 and UTC which persists in international use despite the wishes of time authorities. (Yes, I know UT1 and UTC are different. But people sometimes call both of them GMT.) Jc3s5h (talk) 11:37, 28 October 2021 (UTC)
 * Whilst you are correct about persistent 'custom and practice', it is surely correct to say that GMT is not a standard reference time (even though used as such, indeed the article specifically mentions seafarers' continued use). HM Nautical Almanac Office says UT ... loosely, mean solar time on the Greenwich meridian (previously referred to as Greenwich Mean Time). (my emphasis), which I don't think would infringe wp:SYNTH to read as saying that GMT it not a formal time reference any longer.
 * As for the time zone aspect, Royal Museums Greenwich says Greenwich Mean Time is also the name of the time zone used by some countries in Africa and Western Europe, including in Iceland all year round. So would you be content if the wording were changed to use the construction GMT is now one of the names for the time zone UTC±00:00, not a time reference and the RMG citation added as a third source? Perhaps not too strangely, I can't find any RS that takes this flag waving seriously. --John Maynard Friedman (talk) 15:20, 28 October 2021 (UTC)
 * As far as I can tell, the community of time scientists and time authorities don't use the term "standard reference time", at least not in any uniform way. I would call both UT1 and UTC "time scales" and I would call specific sources, such as the GPS satellite constellation, radio station CHU, and radio station WWV, implementations of the UTC time scale. GMT is many things, which is the problem. Jc3s5h (talk) 19:34, 28 October 2021 (UTC)
 * About time zones: in my mind, a time zone is a package that includes the offset from UTC, whether or not summer time is observed, what dates the winter/summer change occurs, what law or regulation established the time zone, what the authority that established the time zone calls it, and what the inhabitants of the zone call it. Do Icelanders call their time zone GMT? How about those governments in Africa? Jc3s5h (talk) 19:39, 28 October 2021 (UTC)
 * Yes, it is difficult to resolve this without bogged down in the nature of time as an abstract concept. The full RMG text reads:
 * What does GMT stand for?
 * GMT stands for Greenwich Mean Time, the local clock time at Greenwich. From 1884 until 1972, GMT was the international standard of civil time. Though it has now been replaced by Coordinated Universal Time (UTC), GMT is still the legal time in Britain in the winter, used by the Met Office, Royal Navy and BBC World Service. Greenwich Mean Time is also the name of the time zone used by some countries in Africa and Western Europe, including in Iceland all year round.
 * so it seems to me that, rather than the questionable "standard reference time", we can use their "international standard of civil time" and we have a highly reliable source to cite in support. Would that work? --John Maynard Friedman (talk) 21:37, 28 October 2021 (UTC)
 * RMG may be a reliable source in general, but this passage is rather loosely worded. For example, the lead sentence, the first sentence says GMT is the local clock time at Greenwich. That's only true in winter, not summer. The phrase "international standard of civil time" suggests the whole world used GMT directly as local time. But of course they didn't; those responsible for disseminating time added or subtracted the appropriate offset to GMT to find the local time. Perhaps "international basis of civil time"? Jc3s5h (talk) 16:21, 29 October 2021 (UTC)
 * Yes, if the RMG text were on Wikipedia, it would drown in a sea of 'citation needed' and 'clarify' tags. But it is appropriate for its intended audience: just a pity that HM Nautical Almanac Office wasn't being so coy. I'll change it to "international basis of civil time". --John Maynard Friedman (talk) 17:31, 29 October 2021 (UTC)

A few things I'm sure about
 * UTC is the basis for international civil time.
 * UK law considers GMT to be the lawful time in the UK in winter, and provides financial support to various technical organs of government that disseminate GMT that matches, as closely as possible with modern science, GMT that matches UTC.
 * Various people around the world may refer to GMT when intending UTC, or UT1, or while being ignorant of the distinction. Jc3s5h (talk) 23:35, 29 October 2021 (UTC)
 * I admit to being surprised by your reversion: I thought I had reflected our discussion accurately. What did I write that was "factually incorrect"? Can you provide an alternative text?
 * I agree with your three points of certainty, but I'm not sure what to do with them given that we may only reflect reliable sources (which is what I thought I had done). And of course, reflect the world as it is and not what it 'should' be. [Actually I don't think we cite any relevant UK law?] --John Maynard Friedman (talk) 00:28, 30 October 2021 (UTC),
 * Since there is no clear definition of what GMT is, we can't say it isn't the basis of international civil time; for those who use GMT as a synonym for UTC, it is. Jc3s5h (talk) 00:47, 30 October 2021 (UTC)
 * Technically true in detail, but in practice irrelevant in its effect on the huge majority of people. Per WP:VNT, we have a reliable source that uses the words I wrote, and we should reflect that. --John Maynard Friedman (talk) 10:15, 30 October 2021 (UTC)
 * I can't find a source that says GMT is not a /whatever/. Jc3s5h (talk) 17:31, 30 October 2021 (UTC)
 * Not exactly to your specification but
 * "Although GMT and UTC share the same current time in practice, there is a basic difference between the two:" ... "GMT is a time zone and UTC is a time standard." - TimeAndDate.com
 * "... (previously referred to as Greenwich Mean Time)" - HMNAO
 * "has now been replaced by Coordinated Universal Time (UTC)" - RMG
 * "On 1 January 1972, GMT as the international civil time standard was superseded by Coordinated Universal Time (UTC)," - already in this article, but no citation (!)
 * All say that it has been supplanted by UTC as the basis for civil time. (Except in the UK of course.) So true none of them says what it is not (probably because it was, until 50 years ago). So what? Or, more practically, can the sentence be rephrased to avoid saying something we know not to be true (even if widely believed). --John Maynard Friedman (talk) 20:19, 30 October 2021 (UTC)


 * To my mind, the biggest problem with referring to GMT as a standard, is that you can't use it as a standard. For example, at some times it was calculated from noon, at others from midnight. The definition of GMT has changed many times; so if you find a citation a historical time in GMT, you can't accurately determine what time is being referred to, without also stating which version of GMT you mean.
 * If you want to determine the interval between two historical times given as simply "GMT", good luck with that. You'll just have to guess.
 * I think that makes GMT a perfect example of "not a standard".
 * MrDemeanour (talk) 15:50, 3 November 2021 (UTC)
 * The difference between GMT+x and UTC+x is that first is done using a different meridian than the other and also that UTC has insane delta T calculations for leap seconds. Why does it matter when it was calculated from? They also did not have atomic clock in every GNSS sattelite back then, now they do! The fact of the matter GMT is UTC+0 now, while GMT+0 IS NOT UTC+0, they are some time apart. Valery Zapolodov (talk) 21:48, 17 December 2021 (UTC)

To-the-second accuracy
Per the request at the 2nd reversion of my edit:
 * 20:18, 4 November 2021‎ John Maynard Friedman talk contribs‎ 18,587 bytes −7‎ Undid revision 1053528528 by Chalst (talk) rv good faith but please argue it on the talk page. More specifically, if the source doesn't say "to the second", we can't use it as a citation. So let the original stand until you can produce a new citation that does. Tag: Undo
 * 14:02, 4 November 2021‎ Chalst talk contribs‎ 18,594 bytes +7‎ Reverted revision 1053527773 by Jc3s5h (talk) - thanks for catching the typo. I don't think fidelity to the source is more important if we can uncontroversially be more accurate with being verbose. If you are really bothered about the distinction, why not add a quote in a note? Tag: Reverted
 * 13:56, 4 November 2021‎ Jc3s5h talk contribs‎ 18,587 bytes −2‎ Undid revision 1053526091 by Chalst (talk). Ungrammatical, and further from language in source. Tag: Undo

...it's clear this needs to be sorted out on talk. It's the previous sentence that established the error margin of GMT as being around one second; for non-digital applications this actually is precise, hence my edit. I can see the case for my edit as being SYNTH, which is why I suggested adding a note that actually quotes the relevant source. However, I don't have access to that article: could someone provide the relevant passage? With this, assuming the content doesn't run completely against my expectations, I would be able to propose a revised sentence together with clarifying note that I hope would satisfy everyone. &mdash; Charles Stewart (talk) 20:25, 4 November 2021 (UTC)
 * My first thought is that if a statement is supported by a source, one should read the source before editing the statement. If one does not have the source and feels the statement is inadequate, one should obtain a better source before editing the statement.
 * The quotation requested reads, in relevant part,

"In the United Kingdom, Greenwich Mean Time has been identified with the civil time or Coordinated Universal Time (§ 6.8.3.1). This connection, however, has never been formalized, so using GMT to refer to UTC should be done with care. For navigation, however, Greenwich Mean Time has meant UT1 (§ 6.8.3). Thus GMT has two meanings that can differ by as much as 0.9 s, and the term GMT should not be used for precise purposes."


 * Although not mentioned by Hilton & McCarthy, a reason to avoid the "to-the-second" phrase is that there are serious international discussions in progress about eliminating leap seconds in civil time, so that the difference between UTC and UT1 would grow. This would leave the article more vulnerable to becoming out of date. Jc3s5h (talk) 15:51, 5 November 2021 (UTC)


 * Eliminating leap-seconds in civil time is a laudable goal. But my understanding is that the ITU has been discussing (for more than a decade) eliminating leap-seconds from UTC.


 * That is *not* desireable - we'd then end up with two versions of UTC - before the change, and after the change. That would render UTC as confusing as GMT. When you change a standard, you should also choose a new name.


 * MrDemeanour (talk) 11:08, 7 November 2021 (UTC)
 * I'm afraid that this is wandering into wp:NOTFORUM territory and too far off-topic for this article. But the issue has been discussed at talk:leap second.--John Maynard Friedman (talk) 11:23, 7 November 2021 (UTC)
 * GMT referes to UT1? Seriously? Valery Zapolodov (talk) 22:09, 17 December 2021 (UTC)

GMT as solar time
In the first sentence, GMT is defined as "mean solar time". However, GMT is based on UTC which is in turn based on the International Atomic Time that is only loosely kept in sync with solar time using the addition of leap seconds.

I think it should be better to start the article with the explanation that there are multiple (historical) definitions of GMT and then continue to explain them further, because I think that in its current form it's confusing.

Since it seems to be a bit of a controversial topic, I'd like to discuss it on this talk page first. Pinging as a fellow editor who does a lot work on this article recently. Please let me know what you think. PhotographyEdits (talk) 15:37, 18 December 2021 (UTC)
 * It is a real minefield because some readers have difficulty accepting that it no longer has the status it once had and because "the popular press" tends to use it a synonym for UT. So to find a way to write it that is not seen as pettifogging detail is not going to be easy. How do we say that the actual mean solar time at Greenwich could differ from GMT as reported by the BBC (and NPL), by up to 0.9s? On past experience, hacking that opening sentence is a surefire way to get bogged down. So yes, starting from a different beginning along the lines you propose might well solve the problem. I suggest you post a draft here for discussion. --John Maynard Friedman (talk) 16:19, 18 December 2021 (UTC)

Why is the Crimean peninsula green. Ffs.
Put it back to the internationally recognised EET not the illegal occupiers. 2A00:23C6:8493:5401:7C4B:EFD1:B19B:846A (talk) 20:47, 4 November 2022 (UTC)
 * This map is inherited from Wikicommons (see https://commons.wikimedia.org/wiki/File:Time_Zones_of_Europe.svg) so it appears every European time zone article. I have requested that it be corrected. --𝕁𝕄𝔽 (talk) 21:40, 4 November 2022 (UTC)
 * As of 20:10 GMT last night, the disruptive edit to the file on Commons has been reverted and the image on Commons is now showing Crimea correctly. However, as of now, Wikipedia hasn't caught up. If it is still wrong this evening, I will chase. --𝕁𝕄𝔽 (talk) 10:53, 5 November 2022 (UTC)
 * Now showing correctly. If you are still getting the old version, use WP:BYPASS to clear your browser cache. --𝕁𝕄𝔽 (talk) 17:57, 5 November 2022 (UTC)