12:00:00 AM is Midnight. It's a common misconception that the new day begins exactly Midnight. So he is correct stating that it will be released on 12/21/2013 12:00:00 AM, assuming he is isn't one second too late, cause that would be 12/22/2013 12:00:01 AM.
This isn't true. In every environment where there's an official stance on this, (military, for example) 12:00:00 AM midnight is the start of the next day. 12:00:00 AM is just as much tomorrow as 12:00:01, it's the second between 11:59:59 and 12:00:00 that makes the difference.
In casual day-to-day use, however, people treat midnight as if it belongs to the day it follows all the time, so it's common enough that no one should be surprised this dev did so.
That said, it's best to avoid saying anything is happening at midnight, to avoid confusion. That's why airlines never have midnight flights, they have 11:59PM and 12:01AM flights, and a midnight showing of a movie is typically at 11:59PM.
+1