Why did I win the New Hope hat for asking, answering or voting on December 18, 2015? I mean that's tomorrow, right? Today is the 17th of December, as much as I can see on my desktop computer's clock, and I am sure neither me nor the desktop is drunk. Similar things have happened to more users as well. So what's going on? Are we travelling ahead of time?
From Aarthi's answer on Meta Stack Exchange from a previous Winter Bash:
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.
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!
Following the dupe chain we get to an answer by balpha
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.