A appointment with your dentist at 2pm Pacific Time in December 2026 has changed Unix timestamps in British Columbia. The dentist doesn't care about the Unix timestamp, she cares about the wall clock local time when you arrive for the appointment.
We can all make pretty-good guesses about when that condition will be fulfilled in N seconds, but until it actually finally happens, the true occasion could land somewhere else.
"Three months later at 2PM where I live" is not so different than "when the thrush knocks during the setting light of Durin's Day." You can guess that it's N seconds from now, but you might be wrong.
I don't understand this. The consumer books in his/ her local time stamp i.e. 12 PM pacific. Gets stored as Epoch milliseconds (and is passed around as a data structure i.e. Date struct with UTC as the timezone) and the providers sees the time stamp 3 PM EST or 2 PM CST depending on it's timezone at runtime (interface the provider it works with).
I don't understand why a specific timezone has to be "authoritative" here. What am I missing.
This is the exact reason people store time in local time zones.
Also remember the date/time where DST switching occurs is entirely timezone-specific, and it's not necessarily the same pattern every year (as demonstrated with British Columbia).
It's the scheduling changes that disrupt DST switch (or something else) need adjusting to. But this is usually planned in advance and everyone would (or at least should...) update their tzdata.
The amount of issues you'll have due to those (comparatively rare) changes cannot even begin to compare to the amount issues you'll have with datetime stored in timezones.
I don't really see how that is a good approach in any capacity.
So you set the time to 10pm UTC to match 2pm British Columbia. But because the timezone for BC has changed that 10pm now matches 3pm in British Columbia
So the Dentist is expecting you at 2pm and you come along at 3pm
If the consumer booked something for 12PM Dec 1 2026 PST, and this booking was made before BC's changes were announced, and the provider saved this using their local time (say EST), then they would have saved it as 3PM EST. But now with BC's changes, when the consumer shows up at 12PM their time, that will be 2PM EST, an hour before the provider was expecting.
Was the consumer correct for showing up at the booked time according to their wall clock, or is the provider correct with their saved the time that was an hour later? The answer probably depends, but whichever you choose is the authoritative time.
Between the time of booking and the time of the appointment, the government changed what timezone there will be at the time of the appointment. If you calculated the Unix timestamp at the time of booking using the old laws, by the time of the appointment the Unix timestamp would be off by an hour.
No matter which of these you choose, there's a possibility that someone says: "Hey! That's wrong! I never agreed to that":
1. The meeting shall be in precisely N seconds, no exceptions. (UTC, or something very close to it.)
2. The meeting shall be when participant A's wall-clock shows X.
3. The meeting shall be when participant B's wall-clock shows Y.
At the present instant, you might have predicted values so that X Y and N all land on the same spot, but the prediction is not reliable and the equality will collapse if anybody's government makes timezone changes. Or if their country is taken over by another. Or splits due to civil war.