Absolute time.
Time remains constant.
I.e. On Wednesday I am in london I set a lunch appointment for Thursday at 13:30 without selecting a time zone.
On Thursday I am in LA and my appointment notifies me at 13:30.

With your calendar I get notified 05:30 in the morning as the appointment was made in London and the phone is now in LA wiht an 8hour time différence...
I know where I am. I travel.
For those people who never go anywhere or who collaborate they just leave the calendar as is .

Please, it could just be a switch in time zones... ??