Totally did not notice that. Pretty cool too. There was an easter egg found late last year in Arkham City. By setting the date to December something 2004, Calendar Man had new dialogue. Said something like the end of days is coming. I was there at your beginning, and I'll be there at your end. And he's there. Now that's cool.

http://kotaku.com/batman-arkham-knights-true-ending-has-a-cool-easter-eg-1715248781

Screw it, image is fucked. I'll link to the site.