It really is no surprise that Batman: Arkham Knight has at least one interesting "Easter Egg" that is all based on the current date of the system you are playing on. I mean, Rocksteady Games made it a trophy in their last title in the franchise so it would be safe to assume that something would have to be added in for Arkham Knight right? Even if Calendar Man isn't one of the big bad villains in this game. If you couldn't piece it together yet that is what this is all about as the first major Easter Egg based on system date for Batman: Arkham Knight has been found and it is a bit spoilery if you have not played though most of the game just yet.
Before you move on do note the big spoiler alert here for Batman: Arkham Knight. I'll say it again. SPOILER ALERT for Batman: Arkham Knight as the following video does reveal a small bit of the game's side story and characters because it does require the player to complete a specific Most Wanted mission to activate. Take the time and think if you want to spoil any bit of Arkham Knight before you scroll down, read, and watch the attached video. Just know that there is something hidden in Batman: Arkham Knight if you have completed all of the Most Wanted missions and set you system date to October 31st.
Scroll down if you don't care about Batman: Arkham Knight spoilers.
Okay. If you are still with us then you truly want to see what we are talking about. This whole thing revolves around the villain Man-Bat and you having already caught him in Batman: Arkham Knight. It looks like if you set your system's date, be it PC or console, to Halloween and ascend a specific tower the Man-Bat will give you a bit of a jump scare. Not only a jump scare but if you go back to the GCPD you will see that Man-Bat has broken free from the enclosure he was in. It looks like not everyone is permanently taken down in Batman: Arkham Knight when all is said and done.
Here have a look at the whole scene in Batman: Arkham Knight.
Batman: Arkham Knight — Easter Egg - Man-Bat's Halloween Escape
I am a bit curious to see if the escape only lasts one day and then magically Man-Bat is secured again or if he will always be flying around the streets in Batman: Arkham Knight after the Halloween escape. Presumably as the video has you set the date to Halloween of this year it was something that was supposed to take place for those who don't alter their system's date so moving forward on November 1st that would place the time-stamp always ahead of the event. It would be kind of cool to see that in Batman: Arkham Knight but I have a feeling that it is only this one instance. I guess we'll see as more hidden gems are sure to pop up for Batman: Arkham Knight as the dates change.