[treve]

Common mistake that I don't think the author got right either, is that if you want something to happen at a future local time time, say 2PM in São Paulo at some date next year, you don't use UTC + a timezone, you use local time + timezone.

Timezones are not immutable, and timezone database updates happen every year. Only if you track dates that happen in the past UTC makes sense.

In some cases/jurisdictions they change DST quite late so you'll only know with certainty what UTC time corresponds with what local time a few weeks in advance.

[Terr_]

Yeah, I often like to emphasize that many "scheduled events" are not simple numbers, but pattern-matching conditions, a contract of triggering-rules for something an unknowable number of elapsed seconds from "now."

To be specific, it's "whenever the desk-calendars and wall-clocks in that jurisdiction will say X because the local government says so."

The government of Bizarro São Paulo could decide to pause the clocks at 1:00PM, wait for one sunset and one sunrise, then set their clocks to 12:00PM, and then advance it by one "minute" every time a rooster crows until it reaches 12:10pm at which point they skip straight to 3:00pm.

When is your schedule your 2PM wall-clock meeting? Probably around the tenth rooster-crow. If you had reason to distrust the clock-culture of Bizarro São Paulo, you shoulda made it a different timezone. :P

[Veserv]

To elaborate further on your point, the key takeaway is that you can not, in general, predict what second a future date will occur because dates are societal and civil constructs meant for coordinating human activity.

Counterintuitively, this is not true for the past and present. A specific second is unique. As long as you know the second you can derive the exact date (in the past) it occurred in whatever civil time standard you are using. The past and present should always be stored in TAI. The future should usually be stored as “pattern-matching conditions” (though you can use TAI if you are recording a elapsed time/seconds until the event).

[esafak]

Can off-the-shelf libraries translate from any Bizarro time to UTC?

[marcosdumay]

Not in advance. Or better, they can translate it in advance, but they will translate to an incorrect value.

[esafak]

So what do you do, record both the local time and UTC?

[toast0]

For future events, you need to store at minimum the local time; of course, that's not super useful to use, so you may also want to store the UTC as determined by the best current information when it was saved.

Whenever you get new rules, you need to look at at least all future events and see if their UTC offset changed and then decide what to do about it. Often the right thing to do is just adjust the UTC offset based on the new rules; but sometimes you might want to ask the user, or maybe just notify the user: hey, we got new DST rules, please confirm these modified events: [list]. Users don't always input times accurately[1], so a heads up lets them review the modified dates and update the events that were modified in a way that doesn't reflect their actual intent.

You might also want to look at past events and maybe also notify users that the rules changed, and you're sorry about any inconvenience because you didn't get the rules or didn't process them until after the event.

[1] If I'm planning to watch a sporting event on TV, I'm most likely to input it into my calendar in my local time, but the event is most likely scheduled in local time at the event. But not necessarily. IMHO, the closer to the time change the event is, the more important it is to get user feedback.

[Terr_]

IMO the way to avoid insanity is explicit modeling, the pain comes from where different code was written with different beliefs about what the value "really means."

For example, if there's a lunchtime event in Bizarro São Paulo you record "Noon in Bizarro Brazil Time" or even ("Noon in whatever timezone that office building will exist in") because it really does depend on the rhythm of the city. In contrast, if it's an international video-conference, you might record it as a time in UTC or the timezone/city of the most-critical participants.

In both cases, users of the system should be able to see the "contract" along with the predicted moment in a probably-useful local representation. (I might not be in Bizarro São Paulo while I'm looking at the event, but I might know I'll be flying there later...)

If you're afraid a Bizarro-govt is going to do something weird at the last second and cause all sorts of customer complaints for missed events, you might code a system that keeps track of when predicted-times (in something safer like UTC or TAI) suddenly change for events in a way that might catch users by surprise, and then send out notices.

[scatters]

But also "whatever timezone that office building will exist in" may not actually be the same as the timezone that is actually used by the occupants of that office building. There are numerous communities which unofficially observe a different timezone to the one officially assigned to their locality.

[Veserv]

You record a context sensitive date/condition based on the user intent (i.e. birthday travels with the user, in person meeting based on meeting location, international call based on local time or UTC, explicit timezone keeps the timezone, etc.). How you map user intent to recorded date is problem specific.

When the event happens you record the TAI. You may also wish to record the context sensitive date for auditing purposes, but it is not strictly necessary. You can rederive the occurred date if the event occurred in the past.

[midasuni]

https://mm.icann.org/pipermail/tz/2020-October/029376.html

For example - 5 days notice for a DST change.

[throw-ru-938]

You use local time + location, because tzdata timezones can and do get split.

[struant]

Local + Timezone is still going to be wrong if the people are viewing/planning this scheduled event are in different timezones and the timezone DB makes changes to either the viewers' or creator's timezone.

In my opinion, it is better to just store UTC when you can't possibly make the date right in advance for all users in all timezones anyway.

And if everyone does this, hopefully it discourages government from changing the timezone DB frivolously and without spending a lot of money broadcasting the changes to anyone that may be affected.

[tshaddox]

I think this case is sufficiently complicated that we can't simply declare that the author got this wrong and that you should always use local time + timezone. What if that local time zone changes its DST rules to completely skip over that 2pm? Can we really confidently say that the user's intention when scheduling that event was that the event should be skipped if such a time zone change occurs between now and then?