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October 25

Time zones and DST

Does daylight saving time change time zones? Is it so that Finland's "time zone" is UTC+3 in the summer, and Finland and Moscow are in same time zone in the summer, as they are both on UTC+3 then? Or does it just change local times? Tomorrow DST ends in Finland, and does Finland switch time zones then? --40bus (talk) 13:46, 25 October 2025 (UTC)[reply]

Don't you live there? ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:27, 25 October 2025 (UTC)[reply]

From the Time and Date website: Eastern European Summer Time (EEST) is 3 hours ahead of Coordinated Universal Time (UTC). This time zone is a Daylight Saving Time time zone and is used in: Europe, Asia, Africa. Furthermore, European countries using EEST in the summer and EET in the winter: [...] Finland, and Some time zones exist that have the same offset as EEST, but can be found under a different name: AST – Arabia Standard Time, C – Charlie Time Zone, EAT – Eastern Africa Time, FET – Further-Eastern European Time, IDT – Israel Daylight Time, MSK – Moscow Standard Time, SYOT – Syowa Time, TRT – Turkey Time. So I conclude yes and no: yes it changes time zones, no you are not at any point in the same time zone as Moscow because technically EEST is the time zone, along with seven others that have the same +3 offset. (Charlie Time Zone is for aviation and military use. There is also Mike Time Zone, Oscar Time Zone and Victor Time Zone. But no Juliett Time Zone.) Card Zero (talk) 16:34, 25 October 2025 (UTC)[reply]

This has nothing to do with science. Probably not worth moving it, but future questions like this should probably go on the Humanities or Miscellaneous refdesk. --Trovatore (talk) 19:30, 25 October 2025 (UTC) [reply]

It depends on how the concept of time zone is defined. A "standard time" can be defined as a rule, formula or algorithm for mapping UTC time designations to local time designations. A time zone is then an area using the same standard time. If the algorithm used for Finland doesn't change, neither does its time zone. ‑‑Lambiam 19:31, 25 October 2025 (UTC)[reply]

Time zones are purely political. If the political authority in Finland says it changes, then it changes. That's why the question has nothing whatsoever to do with science. Politics and science are disjoint sets. --Trovatore (talk) 19:36, 25 October 2025 (UTC)[reply]

In America, at least, we might change times twice a year, but we don't change time zones. And you're right, this is not a science question. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:33, 25 October 2025 (UTC)[reply]

Where I live we currently use Pacific Daylight Time but in a few (?) weeks we'll switch to Pacific Standard Time. Whether that's a different "zone" or not seems like a language question. However one of my peeves is people who say PST when they mean PDT. --Trovatore (talk) 22:41, 25 October 2025 (UTC)[reply]

Just one week, actually. And either way, you're in the Pacific time zone. About PDT, etc., they used to say it as PDST. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:55, 25 October 2025 (UTC)[reply]

If the algorithm mapping UTC to local time is allowed to use the calendar date, PST and PDT are the same standard time and the time zone does not change twice a year on the Pacific coast. If the definition of standard time does not allow for such a date-based change of regime, PST and PDT are different standards and summery Sacramento is not in the same time zone as Sacramento in winter. ‑‑Lambiam 08:00, 26 October 2025 (UTC)[reply]

This is actually a nice, real-world instance of the grue and bleen problem. —scs (talk) 13:17, 26 October 2025 (UTC)[reply]

But there are subtleties of usage in any case. Me, I would say that 7:00 PST ≡ 8:00 PDT, at any time of year. (Right now, in Boston where I live, it's 8:33 EST, even though the clocks all say 9:33.) I would say that "the algorithm mapping UTC to local time" is allowed to use the calendar date, and that what it chooses between is UTC-8/PST and UTC-7/PDT. —scs (talk) 13:32, 26 October 2025 (UTC)[reply]

We have articles on Time zone and Daylight saving time. As you can see from the preceding answers, definitions and usage can vary, but I would say that a time zone is a legally-defined geographic region in which the local time has a specific, uniform offset from UTC, perhaps including an extra DST adjustment for part of the year. So yes, it's a local time thing. No, DST doesn't change the time zone; DST is part of the definition of a time zone. —scs (talk) 23:17, 25 October 2025 (UTC)[reply]

Hmm, I would say it's a separate parameter from the time zone. For example in the Mountain Time Zone, DST is observed in ~~most~~ many places, but not in most of Arizona or in Mexico or the Yukon Territory. So in that case it's really important to distinguish between MST and MDT, as they may both be relevant on the same day. --Trovatore (talk) 02:36, 26 October 2025 (UTC)[reply]

My thinking is colored, I suppose, by the philosophy of the tz database, which I spend a fair amount of time working with. In the tzdb's nomenclature, Arizona (which, you're right, does not observe DST) is actually in a different zone than the rest of the states in what we might informally call the "Mountain Time Zone". The rest of those states are in zone Mountain-zone-that-observes-DST, while Arizona is Mountain-zone-that-does-not-observe-DST. Those aren't the real names, of course; internally the tzdb names its zones after cities, so those two zones have the identifiers America/Denver and America/Phoenix, respectively. But if your computer adjusts for DST automatically (as most these days do), then you can't just tell it that you want it to use the "Mountain Time Zone"; you have to be a bit more specific in specifying where you're located. (Different operating systems perform this selection differently.) —scs (talk) 13:14, 26 October 2025 (UTC)[reply]

I would imagine how to explain if something happened between 00:00 and 00:59 UTC in the UK because the time went back at 02:00 BST to 01:00 GMT. How would you announce an RTC for example which happened on the last Sunday of October (and last Sunday of March) between 00:00 and 02:00?

I have heard that there were plans to abolish DST or to permanently be at UTC+1 in the UK, so that we wouldn't be changing our clocks twice a year. That's why I woke up at 8 instead of 9 seeing as I experienced a (minor) jetlag.

This is one reason why Wikipedia uses UTC. US, CA and AU have multiple time zones. And that's before to mention that not everyone editing Wikipedia lives in a majority English-speaking country. JuniperChill (talk) 17:33, 27 October 2025 (UTC)[reply]