The answer is that no age related verification-problem in real world is so sensitive that a day plus or minus would make any difference. So they skip the entire clusterfuck of a problem of timezones by just ignoring them.

I grew up in a country where you could legally buy beer with 16 and and hard alcohol with 18. So if the answer to "when should someone be allowed to choose to drink alcohol" has a variance of multiple years between countries who cares about a day or two.