Batman: Arkham City is full of all kinds of easter eggs, but it seems like not all of them have been discovered.

Batman Arkham Videos revealed the secret behind a mysterious video titled “Arkham City Secret?” that appeared last week, featuring a character called Calendar Man which was doing a piece of dialog players have never heard before. It turns out that, if you set your PC date to December 13th, 2004, which is when the game’s developer, Rocksteady, was founded, you will trigger Calendar Man’s full dialogue.

It’s awesome that players still find easter eggs in the Arkham Series. You can watch the video and hear what Calendar Man has to say below: