Owning my own data, part 1: Integrating a self-hosted calendar solution

But timezones are super byzantine. Especially when the rules update. More especially when the rules update on short notice.

If you scheduled an event at 5 pm local time at some place on some day and the rules change, the event's UTC (and TAI) would change. But you may want to confirm with the user, because maybe they're calling into an event somewhere else and they entered their local time but the event is really scheduled in the other time.

You might also have fun when multiple clients are using the same event and the clients and the server don't all agree on timezone definitions. iCalendar suggests that the timezone definition be included in the files, but my experience is definitions are often missing and most things ignore the definitions if present; you've got to include a timezone name the software is going to process as you want or you won't have a good time (or use UTC and hope)

If the event was scheduled in the DST window, the time may no longer exist, or may be ambiguous. iCalendar has rules for that, but they might not actually meet your user's needs (otoh, who is scheduleding things at 2-3am on a Sunday ... probably not a lot of people)

What I do know for sure is that it‘s feasible for a calendar server to service both JMAP for Calendars and CalDAV. All our work in that regard is open-source in the Cyrus server [1] and is used at Fastmail.

Apart from the exchange protocol one also needs to convert between iCalendar and the data structures used in JMAP. For this, I am currently working on an IETF RFC [2]. Anyone also interested in implementing conversion between iCalendar and JSCalendar, please contact me! We have a test suite for interoperation tests.

[1] https://github.com/cyrusimap/cyrus-imapd [2] https://datatracker.ietf.org/doc/draft-ietf-calext-jscalenda...

However it doesn't seem particularly difficult to fix. My error was suggesting using TAI for storage. I guess that works only for events in the past. So the timezone that the event belongs to is what should have been used for storage, you should forget leap seconds exist because this is a human centric calendar so who cares, and you should trust the datetime library to just do the right thing when converting timestamps.

Is there some other issue I'm missing? Because so long as all the timezone complexity is stuffed into the datetime library (and thus NotMyProblem™) it seems like the really difficult part is already solved for you.

Isn't that scenario unsalvageable regardless? It presumably means one or more of the devices are displaying the wrong time to the user. Or at least will be upon arrival in the timezone of the event.

For example (intentionally extreme) suppose the library used by my calendar thinks EST is -4, the OS on my device thinks EST is -6, and everyone else thinks EST is -5. What possible program logic could prevent disaster in this scenario?

It's a really interesting edge case to point out, I'm just not clear what a sensible algorithm for handling it would look like. I strongly suspect that assuming that can never happen is the right answer. Everyone agrees the event is scheduled for 9 am EST. If outdated client software translates that to PST incorrectly or displays the incorrect time on the clock then the affected user is going to have issues and I don't see what the programmer can realistically do about it.

> ... DST window, the time may no longer exist, or may be ambiguous.

Assuming the timestamp disambiguates the daylight timezone from the standard timezone this should be handled gracefully by the datetime library since the stale timestamp is merely "out of bounds" for that timezone now that the laws changed. I'm failing to think of a scenario where it becomes ambiguous (although reasoning through this is once again reinforcing my view that we should have ditched DST long ago).

However this did cause me to realize that there's an additional edge case for far future events where the timezone is modified multiple times. If the event timestamp was never updated in the interim you land in an ambiguous situation and have no way to detect it.

I'm not sure that particular edge case is worth worrying about, but it seems like tracking a second timestamp indicating the last time that the event was processed by the datetime library would resolve it. The library should hopefully have all the necessary historical information on hand to update a long since stale timestamp (I haven't tested this though). Anyway if it doesn't I don't see that there's anything you can do about it.