I just got the And I Feel Fine hat which states "post or vote on December 21".
Does SE have a time machine now?
So, the official answer is: we over-awarded the hat so that people in Auckland / Tokyo / Shanghai / Vanuatu wouldn't have to wait until their midday to receive it.
You're all mostly programmers, so you should be reasonably familiar with how much of a headache timezones can be sometimes. Since Stack Exchange is run on UTC, we have to leave a +14/-12 hour buffer so that the nations around the International Date Line have just as much opportunity to grab hats as people along the 0 longitude.
This behavior will also occur for the other hats that are date-sensitive. This included L'Chaim, and will include Tis the Season and the New Year's Eve Hat / Auld Lang Syne.
Basically: Don't panic! It's a feature, not a bug!
Nah, The Mayans weren't prepared for the confuddling time zones. So hopefully the end of the world will be delayed by a few millenia while they are mired in bureaucracy trying to untangle tricky issues like:
StackOverflow is preparing for the DOOMSDAY, so distributing the hats right away before we face a scary day tommorow...Chill out guys users with maya hat are safe ~Cheers~