A lot of this could be solved by inferring the time zone by location of the appointment. If you know the event occurs in a specific place at a specific time, you almost certainly want it to be the standard time zone of that place. If that place uses DST, I couldn’t think of any reason to ignore it and opt for standard time regardless.

Bonus: geopolitics change country lines, and that affects time zones as well. When the time zone at the appointment location changes due to border conflicts, you can get the new time zone without having to change anything.