[jrochkind1]

It does not assume that -- implementing libraries need to -- and do, in practice right now -- implement the rules for jurisdictions change the rules for offsets.

That's part of why they are tied to a certain city -- time zone rules are unlikely to bisect a city, although if they did I guess they'd have to deprecate it as a timezone name and use something else! Not sure if this has ever happened.

All of this is kept track of in the IANA Time Zone database, and labels like `Europe/Paris` are from keys into that database, not arbitrary. https://www.iana.org/time-zones

Sometimes when jurisdictions do weird stuff like changing their rules for (say) when Daylight Savings starts with no notice (effective tomorrow!), the libraries can take a bit of time to catch up and be correct again (and have the new version be distributed to all users).

But keeping track of (say) that America/New York on March 25 2024 is UTC-4 but March 2025 1990 is UTC-5 hours (they changed when Daylight Savings started in between those years) is absolutely something current (eg OS) time libraries do.

As well as keeping track of the gregorian correction in (depending on country) October 1582 (skipping over 10 days in the calendar!), when calculating historical intervals. They really do this, already!

That's why you say "Europe/Paris" or "America/New York" instead of "UTC-5", to let the library figure out the rules for offsets at that location on the time specified.

I assume Temporal will do the same. JS environments are usually running on OS's that will already provide this service, the browser or other execution environment won't have to implement it from scratch. Although I think moment.js did it from scratch, and distributes a timezone database with moment.js packages.

[burntsushi]

> As well as keeping track of the gregorian correction in (depending on country) October 1582 (skipping over 10 days in the calendar!), when calculating historical intervals. They really do this, already!

Which libraries do this? Libraries usually implement proleptic calendars, including Temporal[1], which specifically do not account for shifts like this. And indeed, the Temporal docs even call out this specific example.

(I agree with the rest of your comment!)

[1]: https://tc39.es/proposal-temporal/docs/calendars.html#handli...

[jrochkind1]

Ah, I guess not as many as I thought!

Ruby DateTime does it, I hadn't realized it was unusual, if it is!

Here it is particularly called out in documentation with example taking account that April 23rd 1616 in England was not the same day as April 23rd 1616 in Italy, and DateTime knows that! https://ruby-doc.org/stdlib-2.6.1/libdoc/date/rdoc/DateTime....

(That class however is using weird "Date::ENGLAND" constants for locale instead of ISO timezone locales, which is a legacy mistake!)

(I work in archiving cultural history, so probably deal with pre-19th century dates a lot more than the average coder)

update: I was curious what would happen if I gave DateTime a non-existent date...

DateTime.iso8601('1752-09-05', Date::ENGLAND) => invalid date (Date::Error)

it knows! (England skipped some days in September 1752 as part of Gregorian adjustment)

[burntsushi]

Ah yeah, those look like opt-in calendars to me I think. Definitely other datetime libraries do that. They just also require opt-in AFAIK.

[jrochkind1]

Ruby date/time handling is confusing, with too many overlapping classes, developed at different times with odd interfaces and/or semantic edge cases (not unusual on other platforms either), but I believe the things people use as default/recommended standard these days on ruby actually do support gregorian/julian changeover!

But I guess ruby is unusual there! Or I'm wrong.

But good to know most have it as opt-in, anyway!

[burntsushi]

I would need to see the APIs and examples to be able to evaluate it honestly.

(FWIW, I'm the author of a datetime library in Rust called Jiff.)

[mason55]

> That's part of why they are tied to a certain city -- time zone rules are unlikely to bisect a city, although if they did I guess they'd have to deprecate it as a timezone name and use something else! Not sure if this has ever happened.

It's actually easier to create this problem than by bisecting a city, and the easier way is even more complex than bisecting a city.

You obviously can't put every hamlet, town and village into tzdb, for a lot of reasons. So, if you're trying to represent a time in a place that isn't in tzdb, you have to pick the nearest location that is in tzdb. And it's quite possible that between when you enter your time and when that time comes to pass, the location you were specifying for changes it's rules in a way that's different from the original place you chose.

If you bisect a city, you could create two new names, so that if you encountered the old name you'd know that something needed to be reconciled. But if you chose the nearest place and then your rules changed, you'd have no way to know automatically that it needed to be revisited.

For example, parts of Chile decided not to do DST any more. To support this, a new timezone, America/Punta_Arenas, was added to tzdb. Before this, if you were in Punta Arenas, you would just put all your times as America/Santiago. And now you have no way of knowing if those times are really supposed to be Santiago or if they were Punta Arenas and Santiago was just the best you could do at the time.

Location-based tz's are the best we can do right now but even still they have intractable problems when things change.

[justingrantjg]

Yep. That said, for end users it's a fairly good story because consumer OSs have gotten very good at automatically adjusting clients' time zone based on geo-location. Just like when you fly to another country and the first thing that happens when you connect your laptop to the internet is you're offered to change your time zone to the local time zone. I assume that the same thing happens when you're in a place whose IANA time zone changes like your America/Punta_Arenas case or many others like it.

[jrochkind1]

On re-read, I understand the other aspects of what you mean better. True!

For dates in the past it isn't much of a problem. `America/[city in chile]` in the past (created before the change, refering to times before the change) still has a specific point-in-time meaning even when things change.

The problem is dates records created in the past but referring to times in the future. Which could now be ambiguous or wrong... and this is the first time I'm thinking about it, I'm not sure how easy it is to detect, I guess it should be detectable which dates may be ambiguous/wrong if you know the date of their creation (before the change was known), but it would take caring to write guards about it and having access to databases with sufficient info.

[mason55]

> For dates in the past it isn't much of a problem. `America/[city in chile]` in the past (created before the change, refering to times before the change) still has a specific point-in-time meaning even when things change.

Right - date-times in the past are always easy (at least until you have to take relativity into account). An event happened at some instant in the universe and you just need an agreed upon representation of that instant. UTC works fine for this - record the UTC-based instant at which the event happened and you can always translate it into any other representation without losing information. Recording it in your local timezone is fine too, as long as you also record the UTC offset or timezone along with the instant.

> I guess it should be detectable which dates may be ambiguous/wrong if you know the date of their creation (before the change was known)

Yeah - in theory, when a timezone is added, you could probably link it to timezones that users of the new timezone might have previously used. And then any future times that were saved using that timezone, you ask someone if they are still correct or if the timezone needs to be adjusted to the new one

For example, if a new timezone was added for southeast Colorado, you might ask someone about all times scheduled in both the Denver & Phoenix timezones, because you don't know which one people might have picked.

It gets complicated though because you need to keep track of which entries have been double checked and which ones haven't, and you need to keep track of the version of tzdb that you reconciled against, because there could be another change in the future.

[jrochkind1]

Right, I mean that if in 2030 for some odd reason half of Punta Arenas does DST and half does not, then 'America/Punta_Arenas' would not work as a timezone designator anymore! Obviously unlikely, I probably should not have mentioned it.