1> An US Citizen born on the east coast and living on the west coast. So does he/she become 21 - 3 hours earlier on the West Coast than a person born and living on the West Coast ?
2> What about a naturalized immigrant born in Japan ?
3> What about a person born on the last day of Feb in a leap year ? When does he/she become 21 ?
4> (Lame) But say a person born on the last day of a non-leap year has his/her 21 b’day on a leap year. So when does he/she turn 21 exactly ?
Not a law expert, but I believe that US laws generally disregard the exact time and say that you turn 21 at 12:00 AM on your birthday (i.e. the earliest possible time on the date of your birthday.)
If you’re concerned about buying alcohol, the establishment will consider you 21 at midnight on the date of birth shown on your ID. Arguments like, “I was born in Fiji” won’t work in your favor.
Haj
Oh, and if you were born on Feb 29th, they will use March 1st for you.
Haj
I’ve noticed a lot of convenience stores either have a calendar or a digital clock that states “You must have been born on this date of earlier in order to buy alcohol” (or tobacco in some cases)
I had a friend who turned 21 on a Sunday, so we went to a bar that was open til 2:00am after 12:00 Saturday night. They wouldn’t let her in. At that time (1983) in Missouri they couldn’t open on Sundays so the hours between 12:00 as 02:00 were considered “still Saturday” so by their rules she was still just 20.
My story is exactly the opposite of kferr’s. We took my brother out the night before his 21st birthday, ending up in a bar around 12:30 am. The guy checking IDs looked at my brother’s license, looked at his watch and let him in.
I got into a bar on my 21st birthday at 12:00 midnight, which was on a Sunday. And this is in a very conservative area that still has blue laws 
However some bars can get around serving alcohol on Sunday, as long as they serve enough food to call themselves a restaurant.