Talk:Greenwich Mean Time

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Archive of Time Cube discussion

is 'GMT' EXACTLY the same C.U.T zero-hour?[edit]

CorvetteZ51 (talk) 11:21, 30 June 2015 (UTC)

There is no precise definition for GMT because the scientific community, which would provide an exact definition, no longer uses the term. Thus it isn't EXACTLY the same as anything. Also, the correct abbreviation for Coordinated Universal Time is "UTC" in all languages. It isn't clear what the original poster means by "zero-hour". Jc3s5h (talk) 18:21, 30 June 2015 (UTC)

Common useage is that GMT is exactly the same as UTC[edit]

First off, I agree that GMT should be obsoleted and replaced with UTC. It is slowly being replaced in common use but GMT is still widely used to mean UTC in a great many laws in the UK, European Union and other countries. However, the old name remains in common use and there are a huge number of systems which require sub millisecond accuracy yet still identify as using a time based on GMT. It doesn't seem logical that governments would legislate to use GMT if it would drift by up to 0.9 seconds when accurate time is vital to many systems. It's clear that the intention and practical use is that GMT is a synonym for UTC.

From 1972 UTC and UT1 were allowed to diverge by up to 0.9 seconds and at this time GMT had to follow either UTC or UT1. No one ever formally decided either way and it has never been formalised or tested in court so the de facto standard is that GMT = UTC. For people (including the UK National Physical Laboratory) to try and make a point by doggedly persisting to assert that GMT = UT1 is just bizzare and unhelpful. Indeed, the pre atomic era definition of GMT as the mean solar time at the Greenwhich Meridian is useless because 0 degrees longitude in the WGS84 datum has shifted about 100 meters from the Greenwhich Meridian line. If GMT was indeed the time at the Greenwich Meridian, then it would now either need to drift from UT1 or the old Greenwich meridian line would have to be moved. Neither of these things happened becuase people who need it this time just use UT1.

Let's just live and let GMT live on as an archaic but common name for the winter timezone in the UK and a synonym for UTC.101.98.248.252 (talk) 03:17, 3 April 2016 (UTC)

I fully agree, but how do we convince our anonymous IP editor from London and Newcastle who has some legalistic argument? Dbfirs 07:27, 3 April 2016 (UTC)
In the phrase "Let [us] just live and let", who is "us"? If you mean Wikipedia, Wikipedia should document the ambiguity. If you mean the world outside Wikipedia, consider this case: some system records time at the sub-second level, does not force users to acknowledge that the timescale kept by the system is the one and only timescale for all purposes related to the system, calls the times it records "GMT", and has important legal consequences. Perhaps the system is used to accept the filing of tax returns, or for bid submission, which have deadlines. Inevitably someone is going to miss a deadline by a fraction of a second, and sue to force the system operators to recognize the plaintiff's transaction as having met the deadline, because the system was actually keeping UTC but GMT is really UT1 and the transaction was on time according to UT1. (I don't know how such a lawsuit would turn out, but I feel confident a suit of this kind will eventually be filed.) Jc3s5h (talk) 10:15, 3 April 2016 (UTC)
The point about "Greenwich Mean Time" is that it identifies the longitude on which it is based. The term "UT1" does not. The claim that GMT is, or is believed to be, the same as UTC is absurd. Nobody who has been through secondary school could believe that a "mean" timescale could include arbitrarily inserted leap seconds. Once upon a time GMT was the mean solar time at a specific place in the Royal Park at Greenwich. Nowadays it's calculated from observation of objects in deep space. It still relates to the mean solar time at Greenwich. Greenwich is a big place, and tying it down to a specific place in the Royal Borough is unnecessary. A degree of longitude represents four minutes in time, and at the latitude of Greenwich one mile equals five seconds. So a yard is about 0.003 seconds.
Nobody can adjust a timepiece to such a degree of accuracy, although Dbfirs claims to be able to differentiate between GMT and UTC. There will never be a case on the lines described by Jc3s5h because computer timings are not that accurate - it is well known that computers cannot accommodate leap seconds. If the time is corrected by a "leap smear" the difference between GMT and UTC is never going to become an issue. The British people have decided that they are not going to let their timescale be hijacked by faceless bureaucrats in Paris. In a few weeks they may decide to turn their back on Europe altogether.
Anyone who wants to make GMT equal to UTC has an uphill task. They would have to convince the British people, their elected representatives, the House of Lords and the NPL. Being legalistic is the only way to handle a dispute. From a legal point of view, GMT was set equal to UT1 in the middle of the last century and there is no desire on the part of either astronomers or the government to change that. Supporters of change did get the idea discussed in parliament but the decision was to retain the status quo.
UTC is disseminated by radio signals but it's not a standard for anything. If you look at the digital clocks on any railway station you will see that the seconds are not aligned. Furthermore, they're designed to run to GMT rather than UTC. That's why when a leap second is broadcast the seconds change from 23:59:59 to 00:00:00 as normal. 77.98.244.158 (talk) 14:35, 3 April 2016 (UTC)
A few facts to correct here.

1. UTC is indeed the standard time used throughout the world and this is the time which is diseminated from all the accurate time sources (including the NPL in the UK) it's wrong to say "it's not a standard for anything".

2. GMT was originally the mean solar time measured at the Greenwhich meridian which was 0 degrees latitude. The Greenwich meridian is now about 100 meters away from 0 degrees latitude (in the WG84 datum which is what UT1 is based on) and GMT has not been redefined so there is no way GMT can be reckoned to be equal to UT1 anymore. GMT is now some offset from both UT1 and UTC which isn't clearly defined and no one would probably be bothered calculating because it's of no practical use.

3. Your contention that clocks in the UK run GMT is an interesting one and I would be interested to know where they get that time from because as far as I know it would be technically difficult to calculate what the actual value of the solar time at the Greenwich meridian is in order to determine GMT. My contention is that the clocks you saw at the railway station were just poorly synchronised rather than being deliberately set to GMT. I have never seen a time source anywhere that purported to diseminate GMT but if you can give an example then I'd be interested to consider it.

All that said, the name "Greenwich Mean Time" that we all know and love is widely used throughout the world and that isn't going to change any time soon. The point I am making is that for all practical purposes GMT is actually the same as UTC. I have worked with a large number of systems which require highly accurate timings and are synchronised to UTC time sources with sub millisecond accuracy yet they present their time as GMT. Quite simply there is no actual source for GMT available and it is very unlikely that anyone would bother to go to the trouble of adding the DUT1 offset to UTC to get UT1 so they could display this as GMT. The actual practice is that clocks are just synchronised to UTC and systems present it as GMT.

It may sound pedantic to be arguing over a difference of a few hundred milliseconds but I can assure you that there are numerous systems in the world to which sub second accuracy is critical to their proper operation. 101.98.248.252 (talk) 10:58, 4 April 2016 (UTC)

Thank you for your corrections here and elsewhere. Dbfirs 11:13, 4 April 2016 (UTC)
  • Point 1 - When I said "UTC is not a standard for anything", fairly obviously I was referring to Britain. When the government eventually switches off the analogue radio signal the average Briton will have no access to UTC. It doesn't matter what is disseminated from Anthorn if nobody is listening, and even if they are, since timepieces are calibrated to run to mean solar time and it's impossible to split GMT and UTC when setting them they won't show UTC anyway. Dbfirs claims he is so dextrous that he can do this, but he's the only one in recorded history with that power - everyone else has to make do with stopwatches. Alternatively, he could invest in a chronometer, and then he's guaranteed accurate time (GMT of course).
  • Point 2 - I think you will find that the fact that there is a calibrated brass strip in a park in Greenwich in no way inhibits the international astronomical community from decreeing that Greenwich Mean Time shall be based on another location in that pleasant suburb - it's hardly an offence under the Trades Descriptions Act now, is it? GMT doesn't need to be redefined because when UT1 came in it was made equal to GMT. If you don't believe me check with the National Physical Laboratory.
  • Point 3 - If it's "technically difficult to calculate what the actual value of the solar time at the Greenwich meridian is" how did those mariners with their chronometers on the other side of the world manage it? I agree wholeheartedly that for all practical purposes GMT is actually the same as UTC. By the same token, GMT is actually the same as UT1 and UT1 is actually the same as UTC. Perm any two from three. Can you give me a screenshot of a system that uses UTC and calls it GMT? Now that would be a contravention of the Trades Descriptions Act. I advise you to report the matter to your local Trading Standards and ask them to investigate. The first thing they will ask you is "How does this system handle leap seconds?" I'm not aware of any system that can handle leap seconds. One thing you're right about - nobody would set their timepiece by adding an offset to UTC to get GMT. Since it runs on mean solar time they don't need to.

I note that all this argument avoids confronting the elephant in the room - numerous official statements by the British government that the civil time is and will continue to be Greenwich Mean Time in the winter and British Summer Time in the summer. So which is the reliable source - government statements verified by Hansard or unsupported opinion of anonymous Wikipedia editors in America, Holland and New Zealand? 77.98.244.158 (talk) 20:10, 4 April 2016 (UTC)

You've said all this before as a sockpuppet of a banned user. We don't wish to go over it all again. Dbfirs 20:17, 4 April 2016 (UTC)
When you've lost the argument pull out the WP:NPA, eh? 77.98.244.158 (talk) 20:21, 4 April 2016 (UTC)
Do you deny being that editor? ( ... and you started the personal attack ) Dbfirs 20:36, 4 April 2016 (UTC)
  • Point 1 - If you want to set your time to GMT (as opposed to UTC) what do you use as a time source? I don't think there are any accurate time sources which actually diseminate GMT but I'm happy to be corrected on this. The time sources I used for precision timing (eg sub millisecond) for broadcasting, stock trading, telecommunications and other precision measurement systems in the UK were from Rugby (now Anthorn), GPS or NTP. All of these report UTC, although people routinely refer to the time setting as GMT. My contention is that every clock which is accurately synchronised to a time source is displaying a time based on UTC, even if people colloquially call it GMT.
  • Point 2 - I've never disputed that the NPL say GMT = UT1 nor do I dispute that GMT is the legal time in the UK. The point I am trying to make is that the modern practical usage of the term GMT, is in fact UTC because there is no accurate source for GMT time any more. It is possible to obtain the current offset between UTC and mean solar time and set your clock using that but the vast majority of people have no inclination to do so.
  • Point 3 - Allow me to offer some practical examples of people using the term GMT when they are actually using UTC. Consider the BBC, they refer to GMT editorially yet the time they use internally and diseminate on analog radio is UTC. Microsoft Windows XP also uses the term GMT even though the internal clock synchronises to within a millisecond of UTC using the Network Time Protocol (NTP). There are numerous other examples, Hotmail, Google, Microsoft, Linux have all used the name GMT interchangably with UTC. If you have a Hotmail account you can observe this by creating a calendar entry and have a look at the timezones offered, one of them is "(UTC) Greenwich Mean Time". Similar thing happens in Google calendar which offers a time zone as "GMT+00:00 London" (in winter) or any of a number of other offsets for other time zones based on GMT.

Feel free to counter with a practical example of a system that presents UT1 accurately and calls it GMT. I understand that most people don't consciously care about sub second accuracy but it is absolutely vital for commerce, telecommunications, broadcasting, banking and navigation so the accuracy of the time displayed is an important point.

As for leap seconds the question "How does this system handle leap seconds?" is answered in detail in Leap_second#Insertion_of_leap_seconds. 101.98.248.252 (talk) 09:51, 5 April 2016 (UTC)

Let's take this stage by stage. We now have consensus that GMT = UT1 = civil time in the United Kingdom. I'll work something up on that basis. 77.98.244.158 (talk) 12:30, 5 April 2016 (UTC)
Is that a consensus of one, or are you counting your many London IP addresses? Dbfirs 12:41, 5 April 2016 (UTC)
I'm evaluating the consensus from the current discussion.

In favour:

  • 101.98.248.252
  • myself

Against:

  • none.

Unsigned comment added at 14:02, April 5, 2016‎ by 77.98.244.158

Well I am against, because GMT is no longer precisely defined and is used in different ways in different contexts. I'll allow other editors to add their votes if they wish, but many have expressed their views in this and previous sections. Dbfirs 13:18, 5 April 2016 (UTC)
Yes, I agree 100% that GMT is not precisely defined. The NPL calls it UT1, the The point I'm making is that in every day usage the de facto meaning of GMT is UTC. So far no one has come up with an example of anyone using GMT for day to day business and actually having their time accurately synchronised to a timesource other than UTC. 101.98.248.252 (talk) 11:53, 7 April 2016 (UTC)
That's what I was trying to convince our London, Newcastle and Nottingham IP editor who is now blocked. He wouldn't believe that I used to own a watch accurate enough to set to UTC rather than UT1. (That was 40 years ago when there was a more regular difference. I still own the watch, but it now shows the correct time only once a day, and it ceased to be sufficiently accurate after I dropped it.) Do you think the current version of the article accurately reflects modern usage of "GMT"? Dbfirs 13:44, 7 April 2016 (UTC)
I agree that the overwhelming majority of modern utterances and text strings that mention GMT are referring to a time scale that is, or is periodically corrected to, UTC. But this article is not just about setting modern timekeeping devices. This article could potentially be read by people who are trying to interpret old documents, or who are performing celestial navigation and are reading instructions that refer to GMT as an intermediate result. So this article can't treat GMT as a synonym for UTC. Jc3s5h (talk) 14:05, 7 April 2016 (UTC)
The first paragraph makes this clear, doesn't it? Our moving IP friend was trying to restrict GMT to UT1 only. (What a lot of time we've wasted over a difference of less than a second!) Dbfirs 14:26, 7 April 2016 (UTC)
It's claimed that some electronic devices get atomic time signals but none of them can handle leap seconds. On specific points, if you want an accurate GMT timing listen to Big Ben or get the time from clocks in public places. When people refer to Greenwich Mean Time they're talking about what the time actually is, not what some vibrating atom thinks it is. The NPL say GMT = UT1 and they're the most reliable source you could possibly have. See my observation on this at Talk:Time from NPL and the explanation at Big Ben of how the standard is set.
109.98 makes some claims about how time is measured within the BBC. How authoritative is his/her statement? Does (sh)e work for them? If (s)he's right then their clocks are different from the ones used by every other enterprise in the country. Since it is impossible for any piece of computer software to accurately present UTC (for the simple reason that no software yet devised can handle leap seconds) it's more likely than not that those companies who say their systems present GMT are actually telling the truth.
Every system that presents UT1 accurately calls it GMT because that's what it is. Nobody presents UT1 (i.e. GMT) and calls it UTC because it's not. There is one source of UTC - the NPL - and they go to great lengths to explain what the difference between the timescales is. At the end of the day we're just anonymous Wikipedia editors with no particular expertise, so why don't we just follow the reliable sources and stop the speculation? 217.44.223.15 (talk) 19:18, 28 April 2016 (UTC)
The BBC is fully aware that UTC is not GMT. They, like every member of the British public, know that GMT is based on the rotation of the Earth, and UTC is based on the vibration of atoms, which necessitates an occasional leap second when the atoms get out of sync with the sun.[1] So please, all you Americans, Dutch and New Zealanders leave the editing of this article to the British, who know what they are talking about. As an accurate encyclopaedia, we should not even mention that some people (wrongly) think that GMT = UTC, as it only serves to confuse our readers.

217.44.223.15 (talk) 17:56, 29 April 2016 (UTC)

References

  1. ^ BBC (6 February 2004). "Pip pip". Retrieved 29 April 2016. 

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Discrepancies between legal GMT and geographical GMT[edit]

Why is Spain legal changes so thoroughly described? Any country keeps a time zone as a result of a political decission. I wonder why there is no description on how France got to UTC+1, which is quite interesting.

The so called Presidental Order is thoroughly and poorly described. By March 1940 the decree just enforced a daylight summer time, a commonplace during economic shortages that was also observed during the Spanish Civil War (1936-39); hence the provision for its future phase out (in autumn). Even more, that decree followed similar decisions enforced in United Kingdom and France in Feb 1940. That was a bit earlier from the common practice. Governments were in a hurry as a result of the ongoing world war. All of these decisions came just before the German invasion of France.

The time zone shift silently came when clocks remained in UTC+1 during winter, again as a result of the economic shortages. Noticed, that clocks in United Kingdom also remained in UTC+1 during the winters of WWII. Also clocks in Spain had earlier experienced winter time in UTC+1. It was during the Spanish Civil War (1936-39) in the Republican zone, in the very last winter of the war: 1938-39.

This bracket "(most likely enacted to be in synchrony with Germany and Italy, with which the Franco regime was unofficially allied)" is an hoax and should be dropped out. Clocks were advanced as a result of economic shortages. Advancing clocks in United Kingdom, France and Spain results in German and Italian Mean Time. It is not any kind of synchrony. It is just geography.

Also, for this purpose it should be mentioned that Germany also moved clocks forward by that time. So there was no such synchrony.

Corsica should also be listed in the regions of France which are in physical UTC+1 with small parts of Provence, Alsace and Lorraine.

Etaoin Shdrlu (talk) 06:06, 13 November 2016 (UTC)

Semi-protected edit request on 20 December 2016[edit]

The african map is incorrect. Benin uses West Africa Time /Central European Time 195.6.224.137 (talk) 09:54, 20 December 2016 (UTC)

Not done: please provide reliable sources that support the change you want to be made. JTP (talkcontribs) 14:43, 20 December 2016 (UTC)

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