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Is there an explanation for the time zones in the user details list, specifically what seems to be unnecessary duplicates? Examples:

  • (GMT-05:00) Central Daylight Time (America/Indiana/Knox)
  • (GMT-05:00) Central Daylight Time (America/Indiana/Tell_City)
  • (GMT-05:00) Central Daylight Time (America/Chicago)

These all seem to be in the same time zone (Central), and all observe DST, so I'm not sure why they all need to appear in the list. Wouldn't one suffice? The only thing I can think of is that maybe they want the user to be able to pick their nearest city.

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  • SIDs reflect the TZ identifiers from the tz database, which includes all of these and more. You can read something around this starting on wikipedia. Having the multiple values for different cities or areas enable changes to DST policy to be localized better.
    – Phil W
    Jun 12, 2023 at 14:34

2 Answers 2

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Wikipedia suggests that Indiana is split between eastern and central time, and that their observance of DST didn't take effect until 2006 (reference).

As some of the time standardization changes happened after the unix epoch (1970-01-01), it's important to capture that for accurate datetime handling.

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    Stuff like this is why date/time libraries are a lot more complex than most people realize, and I don't know if I could be paid enough to agree to maintain one (much less write one from scratch).
    – Derek F
    Jun 12, 2023 at 14:41
  • I hadn't considered the historical point of view, which makes sense, and was the type of explanation I was seeking! In the examples I posted in the original question, going forward, I would assume all three could be considered "the same" (assuming they don't change for some reason) since they all are CST and observed CDT, but from the historical point of view, I can see where it matters. And I've dealt with time zones enough to know I hate them from a development perspective!
    – Jeff
    Jun 13, 2023 at 15:17
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They're not duplicates even though it appears that way. There's various reasons for each "duplicate" version of a time zone.

For example, not all regions within the geographical boundaries of a time zone observe DST. Or if they did, some regions may have adopted DST on different historical dates (as in Derek's example). Or some regions turn their clocks on different days of the year. It's important to be aware of these variances, especially if you're in an organization where it might matter.

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  • I knew that some regions within regions do/don't observe DST, but didn't think of that. I believe (going from memory here) that some Native American communities don't, while their surrounding non-Native communities do. Another thing I hadn't considered was the historical view. I suspect, in the examples I gave, it's for historical reasons since some are in Indiana, and they haven't always observed DST.
    – Jeff
    Jun 13, 2023 at 15:24

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