Time, to me, is the canonical example of "things that people think should be easy, aren't". It has everything - complex, constantly changing "business" rules, exceptions generated essentially randomly by a shadowy cabal[1] nobody's ever heard of, "impossible" situations like the 11 days that never happened and other technically arbitrary calendar edits[2], multiple silly base conversions, really weird rules for picking certain dates[3], lots of opportunities for making fencepost errors, and lots of other things.

Which leads me to my rule with time programming: Never fail to use a solid library, unless you're unfortunate enough to be writing one.

This[4] is a great, necessary but not sufficient book if you have to do that.

[1] https://en.wikipedia.org/wiki/International_Earth_Rotation_a...

[2] For example, https://en.wikipedia.org/wiki/Swedish_calendar

[3] https://en.wikipedia.org/wiki/Easter#Computations

[4] https://www.amazon.com/Calendrical-Calculations-Nachum-Dersh...

> expected local timezone

I'm not sure what this means. Either way, to store an appointment for a certain user, you need the local timezone of that user and the local timestamp. UTC version of that timestamp is optional (maybe for easier calculations in the database), but you must have the local timestamp to convert correctly.

Local TZ => UTC is a formula that is not constant. You cannot store the output of this formula and expect to reverse and get back the original input precisely because of the changing formula.

What doesn't sound right? You might be saying the same thing. To store this appointment correctly, you store the timezone of the appointment (let's say EST) and the timestamp within that timezone (4pm). So the final version is 16:00 EST. You can store a UTC version as well, but you need the local timestamp and timezone first.

If you only store the timezone with a UTC timestamp, when you convert back after DST shifts you'll end up with 3pm or 5pm (depending on the shift), not the expected 4pm for the actual meeting. UTC in this case is not the anchor, the local timestamp the user set for the meeting is.