Talk:Greenwich Mean Time

Archive of Time Cube discussion

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. — 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 @John Maynard Friedman: 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)

Observance of DST in the infobox

I have temporarily hidden the contradictory "Observance of DST" section from the infobox, as it previously mentioned "DST is observed throughout this time zone." while the article mentions "Greenwich Mean Time is used as standard time all year round in the following countries and areas:" below. I am not sure what the best description for DST in the infobox would be:

• Revert to |display observance=Yes and add |dst use=some for "DST is observed in some of this time zone.", because some areas use DST and some areas don't;
• Revert to |display observance=Yes and add |dst use=some with |dst initials=, |dst offset=, to include the UTC offset UTC+01:00 during DST (but not two "current times" during DST);
• Keep |display observance=no, to hide the observance text.

I wonder what you all would prefer? — Peterwhy (talk) 03:34, 10 November 2025 (UTC)

Nautical time

Would one or two of the watchers of this topic care to take a look at the article Nautical time? Is it salvageable? Much of it is (or was until I removed it) based on the false premise that GMT is different to UTC+00:00 and the difference from UT1 is significant on the ocean wave. And no doubt it was all made obsolete by GPS. 𝕁𝕄𝔽 (talk) 10:31, 9 February 2026 (UTC)

I'm not sure of your 'false premise' comment. GMT is defined in UK statutes, but not very well. It is variously considered UT1, the solar mean time (midnight based) at the Greenwich meridian, and informally for the local time zone name - UTC+00:00.

See this discussion in the House Of Lords, where clarification is sought. https://api.parliament.uk/historic-hansard/lords/1996/nov/27/united-kingdom-legal-time-standard

GMT, as a mean time, does not get leap seconds, while UTC does, keeping it with a second of GMT/UT1 as a solar time.

That 'GMT' has been adopted as a label for UTC+00:00 does not change the definition from the UK law makers. GMT is the legal time standard in the UK, but that is nowhere defined as UTC+00:00, even though the BBC time signal does use UTC, not GMT which is too poorly defined for modern usage.

We need two sections really - one for the historical and UK legal GMT time standard, and one for the now customary synonym for UTC+00:00. GPS time carries n approximate diff from UTC, but the GPS time standard is of course independent. Somewhen (talk) 12:57, 9 February 2026 (UTC)

ok, de facto, GMT = UTC. De jure is anybody's guess. Practically, less than a second difference between the metrics is trivial at the scale of ocean-going shipping. False precision doesn't even begin to describe it.

(Or had you in mind a revision to this (GMT) article to reflect the vagueness)? 𝕁𝕄𝔽 (talk) 20:31, 9 February 2026 (UTC)

UTC is maintained by metrologists to provide the high precision time scale needed by modern society. Network time via NTP provodes computers time within milliseconds, not a second, of atomic clock baded UTC. Not GMT, which is a solar time.

It's only false precision for those interacting via a wrist watch - scientists, networked computers, and others need UTC instead of the poorly defined and inconstant GMT.

I'd suggest additional text to clarify the important technical meanings, in addition to the more vague social ones. Somewhen (talk) 22:49, 9 February 2026 (UTC)

GPS reception is not reliable, due to solar storms and war zones. A 2016 NPR story describes the US Navy re-instituting training for celestial navigation. Jc3s5h (talk) 19:17, 9 February 2026 (UTC)

yes, I certainly recognise that as true. GPS jamming is certainly used as a weapon of war so use of a sextant is essential naval training. But in normal navigation, the merchant navy uses GPS to determine their position and time zone. 𝕁𝕄𝔽 (talk) 20:24, 9 February 2026 (UTC)