As you say, timezones are relevant if you need to covert an instant to a date, or vice versa.

But you can store, operate, and query on dates without the foggiest clue about instants or timezones.

But if you have a date, and a timezone...how do you do anything useful with those 2 things? If you add 8 hours to a date due to PST, what do you get?

UTC isn’t a time zone, its a specification for how many seconds are in a day. In UTC, there are 84000 seconds on most days, but the IERS may announce a “leap second” which makes some particular day either 84001 seconds or 83999 seconds (historically always the former).

That was already clear

> How are time zones irrelevant?

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.

> Nobody is going to adjust your birthdate when you move abroad.

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.

You are mixing two things up into one conversation. You are adding time into the conversation, in which case yes you need timezones, but if you don't add time into the conversation and just have dates, then you don't have timezones.

> But that’s just a convention, right?

Yes, "just."

Dates are "just" a convention.