Setting the date of the console or PC that the game is running on back to December 13th, 2004 (the day the game's developer, Rocksteady, was founded) and talking to Calender Man in the court house basement triggers special dialogue from him where he claims that "the end is near" and when it comes, he'll be there. This is likely a reference to the final game of the series, Batman: Arkham Knight, which was in development at the time, as well as it being the final entry in the Arkham game series.

This easter egg wasn't found by fans until 3 years after the game's release, when Rocksteady hinted at it's existence.
Contributed by Boyobmas