Nobody is going to adjust your birthdate when you move abroad.
If you move east, you’ll be able to drink a little earlier than if you move west.
Nobody is adjusting Christmas Eve against the time zone of Bethlehem.
There are many situations where a date is just a number in a calendar and not a specific time on planet earth.
But that’s just a convention, right? The day you are born is still dependent on the time zone. If you are born in the US at 11pm EST then someone born at that same moment in the UK has a different birthday.
Dates have boundaries. These boundaries are dependent on time zone. We can talk about dates irrespective of time zone but day periods cannot be understood without reference to time zone.
Yes, "just."
Dates are "just" a convention.