The relationship can't be changed retroactively, or at least not in a practical de facto way. For example "what UTC was at the moment when clocks in Elbonia sh

The relationship can't be changed retroactively, or at least not in a practical de facto way. For example "what UTC was at the moment when clocks in Elbonia showed 5PM on December 1st 2024" is a fact that has already happened and won't un-happen just because the Elbonian government passes a law claiming otherwise. What you do need to worry about is the relationship being after an event has been planned and before the event occurs.