I thought SE measured time in UTC, but I just got notified that I had earned the "A New Hope" hat whose criteria are described as "ask, answer, or vote on December 18th" – as I write this UTC is 15:48 on Thursday, December 17, 2015.
In case you haven't seen it yet, balpha addressed this on the network meta. From there:
Leeway on date-based hats
Unless the requirement mentions a specific timezone, hats that are date-based are often awarded for 14 hours before and 12 hours after the specified date in UTC, to accomodate people everywhere in the world. This means you can earn such a hat even though for you it's not even that day yet.
Example: The hat Living in a Box is awarded for voting to close a question on December 26th. To earn this hat, you have to cast a close vote between 12/25 at 10:00 UTC and 12/27 at 12:00 UTC.
In other words, you got the hat because you voted or posted when it was December 18th somewhere in the world.