However, keep in mind that there is *no way* to store the time of a future event in a way that won't someday break unexpectedly. It just physically can't be done.

Your approach assumes that we know what timezone the doctor's office will be in when the event happens. However, unless you know the exact lat/lon of that office — and maybe not even then — that's not something you can rely on.

Countries sometimes split themselves up. Provinces get annexed. Border towns may end up on the other side due to a treaty, even in times of peace. Multi-timezone countries may change which parts belong to which timezone. A town may get occupied, and the answer to the question of "what time is it" may depend on the loyalties of the person you ask.

Unless the doctor's office is physically located in Berlin, Germany, there is no guarantee that europe/berlin will always be its correct timezone. Even then, you may get the east/west Berlin split and one side deciding to abandon DST.

When an event happened in the past, we know exactly when it happened, and we can express that timestamp as "number of seconds after some reference point." When an event is planned for the future, we usually plan it for a specific hh:mm in a specific location, but we don't know when that is actually going to be.

Everything in the future is provisional and uncertain. The doctor could die, humanity could get obliterated, the database could go offline and you lose all your appointments.

Should we all add precise GPS coordinates to our salon appointments in case that neighborhood is seized by commandos from Newfoundland who really really want us all on GMT? I’m personally not sure it’s worth the effort.