> Appointments become past events. Sounds like an operational nightmare to convert all of them right as they become past events.

That is fair. In some models there's a natural separation between a scheduled-at time and a happened-at time (e.g., in one I worked on scheduled_events was a separate table from past_events), but I accept that's certainly not all, or even most. I shouldn't have made such a blanket statement. I do believe the statement is good advice in cases where your data does not mix both future and past times.

> Sometimes those costs are worth it, but certainly not for every use case.

This is also fair. My experience with time zones has been the more correct I can make my time models, the better I sleep at night, but of course you're right engineering involves tradeoffs. I think it's important to at least know the most correct way so that you are making educated decisions in your tradeoffs.

Also, FWIW I'm not the person you originally replied to, although the Ukraine example was mine.