Talk:UTC offset

Current times

I'm placing this table of "current times" on the talk page, pending clarification from Tim Starling. It's only good through 12/31/2006 anyway.

ERROR. By US Law, the rules changed at the end of 2007-02-28, presumably at midnight local time. Note that in an international medium dates need to be given in an international frorm. Eschew FFF. 82.163.24.100 (talk) 15:06, 14 July 2008 (UTC)[reply]

On second thought, it seems okay. I'm going to incorporated it, unless Tim finds that it slows down the server. --Uncle Ed 16:47, 21 April 2006 (UTC)[reply]

Can this be extended to cover all the worlds time zones? Not everybody lives in the USA.--Tivedshambo (talk) 06:54, 25 June 2006 (UTC)[reply]

Accuracy?

The "This page was served on (current time) UTC doesn't seem to be accurate. Actually, it doesn't even seem to be changing when I refresh the page. I've cleared my cache and cookies, also. I think it either should be fixed or removed. 76.245.44.228 (talk) 04:59, 19 March 2008 (UTC)[reply]

Looks like the wikipedia page updates in batches. This table should be replaced with something that doesn't cleverly try to update the time. Other non-american time zones can be added at that point. DavidRF (talk) 20:37, 19 March 2008 (UTC)[reply]

Zone & Offset

Page has "Many time zones employ two time offsets, one for standard time and one for daylight saving time.". That's literally true, but not well put. Summer Time is not a zone-wide matter. Note that, if there were to be an additional place in a temperate part of the UTC+10:30 zone, then there would probably be a zone with three offsets. 82.163.24.100 (talk) 15:32, 14 July 2008 (UTC)[reply]

Greenwich

I have replaced "in Greenwich, England" with "(GMT, approximately)". By UK Law, we still in principle use GMT-based time in the UK, not UTC-based - so our UTC-based time signals are not legal. More importantly, the civil time in Greenwich is advanced one hour in Summer.

The term GMT really means GM solar T, and was introduced before Summer Time was instigated.

82.163.24.100 (talk) 21:37, 17 October 2008 (UTC)[reply]

Confusion

I've seen clearer instructions on government forms. (In the army, they had to give us classes on how to fill them out. I deleted the confusing parts, namely the entire body of the article.

In the next few days, I'll dredge it out of history and make it clear, if no one else has the time before then. --Uncle Ed (talk) 03:40, 26 February 2012 (UTC)[reply]

Mea culpa, I just realized that I myself created this article 5 years ago! (And it still sucks.) Well, I guess there's no one else to blame, so I better fix it. --Uncle Ed (talk) 21:40, 2 May 2012 (UTC)[reply]

Something that's surprisingly difficult to find on the Internet is a definitive and authoritative statement of which date/time accompanies the UTC offset: is the UTC date/time with which the UTC offset is combined to get the local time, or is the local date/time with which the UTC offset is combined to get UTC time? Almost always, the answer must be inferred, which means the reader might infer wrongly. This article comes the closest by saying "place and date" but, again, the reader must infer.
To illustrate (USA abbreviations, to preserve alignment in this font):
2021-07-30 15:00:00 (UTC) -07:00 = 08:00:00 (CA) and
2021-07-30 15:00:00 (UTC) -04:00 = 11:00:00 (NY)?
or
2021-07-30 08:00 (CA) -07:00 = 08:00 - (-07:00) = 15:00 (UTC) and
2021-07-30 11:00 (NY) -04:00 = 11:00 - (-04:00) = 15:00 (UTC)?
The latter is more useful but the natural instinct for a date/time you find on the sidewalk is:
2021-07-30 08:00 (CA) -07:00 = 01:00 (UTC) and
2021-07-30 11:00 (NY) -04:00 = 07:00 (UTC), which is wrong (and beside the point).
Pbyhistorian (talk) 16:12, 30 July 2021 (UTC)[reply]

Because the offsets are with respect to UTC+00:00, London/Dublin/Lisbon/Accra. So if it is 11:00 in London (winter time) then it is

11:00-8:00=03:00

in LA and

11:00+8:00=19:00

in Singapore. The difficult part is how to explain the reverse: I am in NY, the local time is 11:00 (before DST), how do I calculate the UTC equivalent? My offset is -4:00, whaddyamean I godda add 4:00 already??

Mission Impossible time: your mission, if you choose to accept it, is to find a clear explanation from a reliable source and paraphrase it. There's not a lot to be gained my doing it, as someone who lives in UTC+00 and so can't see what is so difficult about it. --John Maynard Friedman (talk) 18:56, 30 July 2021 (UTC)[reply]

I found a Note submitted in 1997 to the W3C Consortium by Reuters Limited, perhaps best known for their timely coverage of worldwide news. Their Note "defines a profile of ISO 8601, the International Standard for the representation of dates and times". This Profile is a selection of just a few ISO 8601 data/time formats that prevent ambiguity by specifying a four-digit year but, near the end of the Note, they say "Times are expressed in local time, together with a time zone offset in hours and minutes."
I'll try to sneak that detail in without hijacking the focus on UTC offset.
Pbyhistorian (talk) 15:06, 2 August 2021 (UTC)[reply]

Recent updates

@Pbyhistorian:, have I reduced (or increased!) the confusion any with my updates? Do you think it would be useful to expand the table to say, for example, "if it is noon in Accra then it is 02:00 in Honolulu and 21:30 in Darwin"? Or, with a lot more effort, "if it is noon in Honolulu then it is 22:00 in Accra and 07:30 next day in Darwin" (difficult to see how to lay this out). --John Maynard Friedman (talk) 15:37, 31 July 2021 (UTC)[reply]

I think the table is a good idea. I borrowed the general idea to illustrate UTC offset math up top, but using the equator to remove daylight savings time would never have occurred to me.

I wonder about local solar time, though. Remembering my (ancient) astronomy and astrolabes and sextants, solar time should be absolute; local time is more often a thing of convenience. Pbyhistorian (talk) 16:29, 2 August 2021 (UTC)[reply]

"Local time" is possibly a little ambiguous, because of the confusion with civil time which is national standard time ± DST, but I've seen "local solar time" many times (and especially local solar noon), so I'm confident enough to let that phrasing stand. --John Maynard Friedman (talk) 17:09, 2 August 2021 (UTC)[reply]

Solar time is a completely different concept and IMHO should be removed from this article. A UTC offset describes the difference (at a given point in time) between UTC and local time which is a form of civil time.

Those last two articles linked are also in need of updates - the local time article should reference the civil time article.

There's also some wording in this article that says "... which may not be the current civil time ..." which just adds to the confusion. (It is indeed the offset from civil time, just not necessarily the "current" civil time.) mattjohnsonpint (talk) 16:58, 14 December 2022 (UTC)[reply]

Local time is not a "form of civil time", it is civil time and has been so for about 150 years when railway timetables enforced national standard time.

Yes we know but is there a better term? Take for example Kashgar in far western China. The civil time == the local time == the "time zone in use" is China Standard Time, UTC+08:00, same as Beijing. But if China would operate multiple time zones like Australia, Canada, Europe and the US do, Kashgar's time zone might be UTC+05:30 (same as New Delhi). By any definition, the local solar time is most definitely not the civil time. The same issue arises at Santiago de Compostela (8°32′40″W): its "natural" time zone is UTC-01:00 but its civil time is UTC+-01:00, same as Berlin Could we say "approximate solar time" (with an explanatory footnote)? Anyway, feel free to propose a new wording. --𝕁𝕄𝔽 (talk) 18:31, 14 December 2022 (UTC)[reply]

Solar noon is the moment in the day when the Sun is at its highest point in the sky. Similarly, solar time describes the hour angle of the Sun in the sky, which varies by longitude throughout the year irrespective of UTC time zones and offsets. Instead of "solar time", I suggest any of the following: civil time, local time, standard time, clock time, or official time. If we cannot agree on a replacement for "solar time", perhaps we can agree spell it out as something like "the time in the given time zone" or "the time on clocks at a particular location"? Maptv (talk) 04:24, 3 December 2024 (UTC)[reply]

Yes, there is certainly a problem with the term "solar time" for the reasons you explain. The problem is how better to reflect By convention, every inhabited place in the world has a UTC offset that is a multiple of 15 minutes but the majority of offsets are stated in whole hours. There are many cases where the national standard time uses a time zone that differs from the UTC offset appropriate to its longitude. Coming back to Kashgar (75° E), its UTC offset is +5:00 (75/15 at one hour per 15°). Its civil time is UTC+08:00 because it on China Standard Time, so we can't use that. Per the article local time, current common usage says that it is the same as civil time, so we can't use that either. "Clock time" and "official time" are synonyms for civil time too. "The time in the given time zone" is certainly not the UTC offset at Kashgar, it is out by three hours (twelve UTC offsets); "the time on clocks at a particular location" is another synonym. I recognise that "solar time" is not perfect but I believe that error is rather less than the 15 minutes specified for each UTC offset. The floor remains open to proposals, though!--𝕁𝕄𝔽 (talk) 14:36, 3 December 2024 (UTC)[reply]

The UTC offset article we are discussing states that China applies a single standard time offset of UTC+08:00 (eight hours ahead of Coordinated Universal Time). The Kashgar Prefecture article gives it’s time zone as UTC+8:00. The Time zone article defines a time zone as an area which observes a uniform standard time. Further, Each time zone is defined by a standard offset from Coordinated Universal Time (UTC).

The "standard offset" for Kashgar Prefecture, and indeed all of China, is +8:00. The UTC+8:00 article provides a time offset from UTC of +08:00 and uses the term "physical" UTC+05:00 to describe a UTC offset that is based on the exact longitude of 75° E. As the longitude for Kashgar Prefecture ranges approximately from 75.84° E to 76.155°, its "physical" UTC offset ranges approximately from +5.056 to +5.077, but the "standard offset" is +8:00 everywhere in Kashgar Prefecture.

Time zone databases can give us the UTC offset for a given latitude and longitude. In the top right of Kashgar Prefecture article there is a link to information on its coordinates: 39.47°N 75.99°E. At the linked page, there is a link to an API call for the given coordinates: http://new.earthtools.org/timezone/39.47/75.99. In the API response, the offset is 8 and the isotime ends in +0800, matches the localtime exactly, and is exactly 8 hours ahead of the utctime. The ISO 8601 article states that The offset is applied to UTC to get the civil time in the designated time zone.

UTC offsets are exact, i.e. there is no error in UTC offsets. The 15 minute multiples in UTC offsets are convenient increments, not the bounds of tolerance for error.

The solar time calculation is complex and error-prone, because the equation of time cannot perfectly model the Earth's orbit. In contrast, the UTC offset calculation is simple and error-free: we take UTC, as measured by atomic clocks, and add the given offset. An offset based solely on longitude rounded to however many degrees, e.g. 15°, is perfectly valid, but so is the offset I get from an API call to a time zone database. I propose that we avoid all of this complexity and keep it simple like in the List of UTC offsets article, which defines a UTC offset as the difference in hours and minutes from Coordinated Universal Time (UTC).

In the UTC offset, I suggest that we change and local solar time, at a particular place</, as measured by atomic clocks, > to "and the time at a particular place", specify the local solar time (which may not be the current civil time, whether it is standard time or daylight saving time) to "specify the time at a given location" or "specify a given time", and There are many cases where the national standard time uses a time zone that differs from the UTC offset appropriate to its longitude. to "There are many cases where the national standard time uses a UTC offset that differs from the UTC offset defined solely by its longitude."

Essentially, my suggestion is to circumvent the use of any of the terms we have discussed, "solar time", "civil time", etc., in favor of something we can all agree on, whatever that may be, e.g. "a given time". Of the articles I linked to above, I believe the UTC offset article is the only one that incorrectly uses "solar time". It would be great to change this article so that it matches every other article regarding offsets and time zones. Maptv (talk) 20:54, 3 December 2024 (UTC)[reply]

Here are my suggestions in list form:

In the UTC offset article, I suggest that we change

- and local solar time, at a particular place to "and the time at a particular place",

- "specify the local solar time (which may not be the current civil time, whether it is standard time or daylight saving time) to "specify the time at a given location" or simply "specify a given time", and

- "There are many cases where the national standard time uses a time zone that differs from the UTC offset appropriate to its longitude" to "There are many cases where the national standard time uses a UTC offset that differs from the UTC offset defined solely by its longitude" Maptv (talk) 21:02, 3 December 2024 (UTC)[reply]

Ah, yes, I see what you mean and so I've been misinterpreting the sentence I quoted. I should have taken a simple example: Iceland, capital Reykjavík, 22° W. The "UTC offset defined solely by its longitude" is UTC-02:00. Your version clarifies it completely: the national standard time uses an offset (+00:00) that differs from it. I support your proposed wording. Unless anybody else can see a problem in the next 24 hours, you should go ahead and make the change. WP:BRD will still remain open thereafter. --𝕁𝕄𝔽 (talk) 23:55, 3 December 2024 (UTC)[reply]

I like both of the examples you used but the latest indeed seems simpler.

I made the changes I listed earlier.

Thank you 𝕁𝕄𝔽 for taking the time to help a newbie like me! Maptv (talk) 03:01, 5 December 2024 (UTC)[reply]

14:00:00+02

I'm a bit worried by your addition of Subtracting the UTC offset from the local time gives the time in UTC so 14:00:00+02 is 12:00:00 UTC, but in the South Sandwich Islands, 10:00:00-02 is 12:00:00 UTC (because subtracting a negative UTC offset results in addition). for two reasons. First, I don't think it belongs in the lead and second, I haven't seen the style 10:00:00-02 used anywhere? Do you have a citation? Everything I have come across uses the style 10:00 UTC-02:00. See for example UTC-02:00, which says UTC−02:00 is an identifier for a time offset from UTC of −02:00. --John Maynard Friedman (talk) 17:09, 2 August 2021 (UTC)[reply]

I've cleaned it up; should be OK now. Pbyhistorian (talk) 23:14, 2 August 2021 (UTC)[reply]

Thanks, that's better.

Would you double check the calculations in my table, please? --John Maynard Friedman (talk) 23:35, 2 August 2021 (UTC)[reply]

Yes, those are the results I get too. Cheers! Pbyhistorian (talk) 00:38, 3 August 2021 (UTC)[reply]

Relation of offsets to time zones

A time zone is a place. A time offset is a number of hours and minutes. I can't believe that people are getting area confused with quantity, but that's life for you.

Half the reference information I see on the Internet uses the UTC offset to identify time zones. Is this pushback against the US-led NATO designations? Zulu time is "zero" offset from Greenwich Mean Time, and various letters of the alphabet are used for the other Nautical time zones. — Preceding unsigned comment added by Ed Poor (talk • contribs) 16:02, 27 May 2012 (UTC)[reply]

Merge proposal

There is a proposal to merge this article into Coordinated Universal Time and make this a simple redirect to that article.

Please contribute to the discussion at Talk:Coordinated Universal Time#Merge from UTC offset, not here. 𝕁𝕄𝔽 (talk) 00:50, 6 December 2024 (UTC)[reply]