Once you have downloaded you might have to use horizon to open, rehash & reasign as i couldnt get it to open on modio for some reason.
Once id's have changed disconnect your xbox from xbox live and change the date to 25/12/12. Load up the game and you should spawn in front of a door, go through it then around the corridor and down the stairs and walk up to calander man and listen to his story to get the acheivement.
Thought i would upload to save people time from changing dates 12 times.
Hope it helps