I recently had a similar experience on vacation. I entered all of the shows and events I was attending in Vegas into my iOS calendar while home in NY. Upon arriving all my events were 3 hours off, and I had to manually correct each one.
A cursory look through Calendar didn’t turn up any way to enter time zone independent events.
But Google Calendar also had the time for the departure and reurn flights blocked out (including departure airport/arrival airport both ways), yet it was too dumb to notice I was setting an appt for 8PM for a Las Vegas location - but Google Calendar decided to put an Eastern USA timezone on the appt.
That's how you know the Google Calendar devs/PMs don't travel often. And also none of the same Google devs/PMS cared enough nor high-level Google execs themselves use Google Calendar - cuz if they had the same thing happen to them, you'd know that it would have been fixed before the next performance review.
A cursory look through Calendar didn’t turn up any way to enter time zone independent events.