If you though that you have 100% completely found all the hidden secrets of Batman: Arkham City or just found everything then you are going to be in for a shock. It looks like there is a new Easter Egg from Calendar Man that pretty much everyone has missed up until RockSteady Games began posting hints and clues to something missed. It only took three years now but it looks like there is a 13th conversation from the villain outside of what was required to get the Trophy/Achievement for talking to the character over the course of the year.
For those who may not know, during Batman: Arkham City on specific real world dates the villain Calendar Man said something new around the events or holiday involved for the date. This was an attempt to keep players coming back every month for at least a year after Batman: Arkham City launched. This was of course bypassed once people found that by changing the clock on the computer, PS3, or Xbox 360 to the date in question and could get the little extra to play out. All those hunters out there know what I am talking about.
It looks like RockSteady left a few clues here that have popped up the following conversation in Batman: Arkham City.
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 will be there at your end.
This seems to be in reference to Batman: Arkham Knight as far as "the end" part of the quote. Especially when you look at that you can only get this by setting you system's clock to 12/13/2004. That would be the year that RockSteady was founded for those questioning. Here's a video sowing you just that.
Batman: Arkham City — Calendar Man's Secret Message
It is a bit cool and shocking to learn that RockSteady had the franchise's end already planned in Batman: Arkham City but I guess it makes sense. They are also lucky that the user who found this didn't accidentally stumble upon this before Batman: Arkham Knight was announced because it could have ruined the fun announcement and everything. Even though we did have Batman: Arkham Origins in the mix since then. It is a nice little insert that took too long to find and I hope the keep it up with their future titles.