Showed up to a doctors appointment during their lunch break once because iCal supports the time format "TZ is whatever is local, trust me" and the device I used to add an appointment was set to UTC, not UTC+1.
My doctors calendar app vendor was pretty happy I found the root cause of their very occasional mystery appointment drift too.
If I invite you (who lives in Spain, let’s say) to fly over and enjoy my New Year’s party (in New Zealand) it definitely is not going to occur at “midnight local time wherever you are” - Trust me.
(Actually there’s a famous missing person’s case related to a New Year’s party in New Zealand … I don’t think that timezones were part of what went wrong, but I can’t be sure.)
My doctors calendar app vendor was pretty happy I found the root cause of their very occasional mystery appointment drift too.