VGFacts

Julian Gregory Day, aka Calendar Man, is notorious for performing crimes on holidays. When visited on specific days of the year (or if the date us changed on a console or PC), and spoken to, he reminisces about crimes he performed on the day.
If he is visited on December 13, 2004 he says "Do you remember my early work? Flawed, but it showed promise. The end of days is coming. I was there at your beginning and I'll be there at your end."
This is an Easter egg referring to a possibility of several things:
In December 2004, Sefton Hill and Jamie Walker founded Rocksteady Studios. This message foreshadows the success of the franchise and then saying they'll be there until the end of the franchise.
Also, this could possibly be a statement about Calendar Man's possible foreshadowed involvement in an upcoming Arkham game and that he may be a big part of the plot.

Sources:
http://arkhamcity.wikia.com/wiki/Calendar_Man
http://m.youtube.com/watch?v=temF1OFCIPg

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Edit: after doing more research, it may be more likely Calendar Man's involvement in a future title. Warner Bros. took over the series with Arkham Origins. I have not been able to find out the depth of Sefton Hill's involvement, if any, after being a director for Arkham Asylum and directing and writing Arkham City.

Another edit/add-in: I just found out Sefton Hill is back as director and a writer for Arkham Knight. Back to square one with it being about him being there at the beginning and in the end.
If anyone has anything else, that would be appreciated.

Hahaha, how did I miss that?!