For all purposes you turn the next age at midnight. It's not that bars "don't care," it's that legally you age on the day of your birth, not at the minute or even second of your birth. Sure, if you want to get extremely technical about it, you aren't technically 21 years of age until the microsecond you were born. But that's irrelevant to everything, if you were born at 11:59:59.999999998 PM 12/29/1997 you would be eligible to get your license in a lot of states right now, even though you wouldn't technically be 16 years of age for another few hours.