Comment by rawling
10 hours ago
Has anyone proposed versioning timezones? Or is this such an edge case it would be overkill? (Either specify your future instant in UTC if you mean to stick to that, or specify it in a timezone and accept that it could change before it happens, or if you need something else get it in a contract and don't trust the computer!)
There is actually a simple heuristic you can use: if a point in time should be sticky to a calendar (e.g. calendar app or appointments which need to be synchronized between multiple humans or parties for a given context/location/region), store a datetime _without_ a timezone and make the timezone configurable for the user/infer it from the user. If you want a point in time which will not "physically" change, store a datetime _with_ a timezone, always, preferably UTC (e.g. logging, timers, measuring the occurrence of events).
The trouble is that you can't really make safe assumptions about whether to use the "sticky" paradigm or the "point-in-time" paradigm.
Sticky really only makes sense in two scenarios:
1. when all participants are assumed to be in the same geographic/political time zone for the foreseeable future (in which case the only advantage over point-in-time timestamps is future political changes to that region's time zone, like DST changes), or
2. when there's some privileged participant such that everyone else can assume events follow that participant's time zone (e.g. a company headquarters that moves very rarely, or an individual's personal wakeup alarms which can probably be assumed to follow their current location's time zone as they travel).
If you have a group of friends who like to stay in touch with regular group calls, and all/most of them are digital nomads who change their time zone of residence multiple times per year, you probably don't want the sticky paradigm.
In practice, if you have participants from different timezones, you agree on one "reference" timezone, which can be also UTC, as in your nomads example. You can consider UTC as just another calendar you can stick to (but you should just not hardcode this in software for this appointment use-case). You actually need a reference to be able to plan an event in the first place. Otherwise you don't know which time you can propose. You would need to propose a "fixed" time for every timezone which participates, but that doesn't work, because these times might not refer to the same physical instant, so the meeting would not be (fully) synchronized. So instead, you take some time from some timezone and translate that to all the other timezones. You can even see that in online games, where some of them have an official "server time", which is globally the same, helping players to meet at the same time instant. UTC itself is another examples of this, used e.g. for global navigation or air traffic control.
That "only advantage" is doing a lot to dismiss the actual use case of, say, everyone keeping appointments on their calendar when the legislature passes laws around time zone changes.
What’s the difference between these two other than how the client would convert to display to a user?
The 2 different types of datetimes encode different information and is not purely a display/presentation issue, it's also storage issue.
I happen to call them "scientific datetime" vs "cultural/political datetime". However, the software dev industry has not converged on a standard vocabulary to delineate the 2 types which is unfortunate because that means programmers are unaware that the difference exists. Concepts are more top-of-mind when there are good names to label them.
If a programmer doesn't understand how the 2 datetimes behave differently, they will create software bugs as I've outlined before: https://news.ycombinator.com/item?id=39418897
There's the meme of "store UTC everywhere" (maybe perceived as correct because of superficial similarity to "use UTF-8 everywhere") ... but storing datetimes as UTC is only unambiguous for historical events such as timestamps of activity stored in server logs.
But future datetimes can have ambiguous edge cases which causes the split into 2 different types.
2 replies →
It's not only about display. If you store a user's appointment only as a UTC timestamp, you actually can't know the hour of the day (and the day itself to be precise) on which this appointment should happen, for a given calendar (probably the user's calendar, in a specific non-UTC timezone). You would have to guess by using the calendar's/user's timezone and compute some offset with UTC. But what if the user changes timezones or the timezone itself changes its value? Store without a timezone, and you know the exact hour and day the user intended. One is pointing to a day and hour in a calendar, the other is pointing at a point on the line of a linear timeline.
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tzdb is versioned, but I can't recall if the individual timezones carry a version though...
One way to manage is to store the datetimes with a timezone identifier and the offset, and when you load a new tzdb, go through and validate that the calculated offset matches the stored offset... for those events where they don't match, you have an exciting challenge of figuring out if the event should stay with the time zone or stay with the offset; both answers may be right ... ideally you inform the user(s) about what you've done and allow them to fix things software has messed up.
It’s really not clear what you’re asking.
The point of using zoned events is to match the life and expectation of people living in the real world e.g. if there is a meeting in Perth at 10AM, and the Perth timezone gets shifted, the meeting is still occurring at 10AM perth time. If it’s broadcast then every other time is what changes (or not).
If people want to fix their meeting internationally they can already do that by setting their meeting time in UTC.
In reply to
> future zoned datetime carry outright uncertainty as to their actual location on the timeline (as the zone’s offset can be updated at any point and any number of times until the event has elapsed)
I wondered if it would be worth applying a (optional?) version to a timezone when stored, so you could distinguish between "whenever it is this time in Perth" vs "when I currently think this time will be in Perth, though if Perth changes its mind on how it offsets time, I want to keep what time I currently think that will be".
But you're right, that doesn't really add anything over storing it as UTC.
The fix is storing the datetime without a timezone, so the hour stays stable, no matter what the timezone of Perth does, plus optionally and separately the location (Perth), which could be stored as the timezone id of the location. But you could also use coordinates. Anything which can be mapped to its timezone later works. Now the time will never change for the original location (sic), and you can always compute the time for any timezone in the world, because in the instance of computation, you take the current valid timezone of the location, and you apply that to the stable datetime. Of course a current computation of that might have a different value in the future. But that doesn't matter, since the time for the original location (sic) never changes, and the value for different timezones is supposed to change.
> if there is a meeting in Perth at 10AM, and the Perth timezone gets shifted, the meeting is still occurring at 10AM perth time.
The thing about this is that if Perth's time zone ever changes so erratically or with such little notice that participants need to be notified that the point-in-time of an upcoming meeting has changed, it's no longer clear whether the participants would actually want the zoned "Perth at 10am" time to be canonical.
If athletes were flying in from around the world for an international competition tomorrow at 10am, and Australia decided to increase Perth's UTC offset by 1 hour as of today, would we expect all athletes, organizers, fans, etc. to show up and do everything 1 hour earlier (by solar time)?
> The thing about this is that if Perth's time zone ever changes so erratically or with such little notice that participants need to be notified that the point-in-time of an upcoming meeting has changed
"erratically" and "with little notice" are pretty relative when it comes to timezone shifts. DST decisions have been made with as little as a week lead time, Samoa dropping an entire day off of its calendar was done with under a year lead time (noises started about 9 months prior, the act was assented 6 months prior).
> would we expect all athletes, organizers, fans, etc. to show up and do everything 1 hour earlier (by solar time)?
Usually yes.