Previous Arkham games rewarded players for booting the game up on certain days, and Arkham Knight retains this clever tradition. By futzing with their PC's clock, one player has already stumbled upon a great surprise.
If you don't want to have this easter egg spoiled for you, walk away now!
Batman Arkham Videos is dedicated to... well, exactly that. So it's not a huge surprise one of the largest YouTube channels dissecting Rocksteady's games stumbled upon this already.
Chances are there's a bunch more of these hidden in Arkham Knight too.
In Arkham City, there was an achievement for visiting with and talking to the villain Calendar Man on a bunch of very specific dates.