I've seen some obscure and hidden Easter Eggs in video games before, but this one may take the cake. Batman: Arkham City came out October 18, 2011 and it seems that three years later folks have just found this Calendar Man Easter egg. If you set your PC date to December 13, 2004 and go visit the villain, you get the below scene. What's so special about that date? It was the day Rocksteady, the company that makes the game, was founded.
The video was posted by Batman Arkham Video two days after a mysterious YouTube channel posted a brief clip of the scene with the title "Arkham City Secret?" Some folks figure it was Rocksteady pointing fans toward the egg… Makes you wonder how many more have yet to be discovered.