There's an Xbox 360 Achievement (and PlayStation 3 Trophy) in Batman: Arkham City called "Storyteller". The description: "Have 12 murderous dates with Calendar Man." To earn it, you have to talk to the incarcerated villain on 12 holidays, from New Year's to Christmas.
Your Xbox 360 or PlayStation 3 needs to be running the game on the 12 holidays -- or at least it needs to think it is.
It could take you a year to unlock this, but the folks at Wiki Game Guides believe they've found a major shortcut. Take the system offline. Change the system clock 12 times. They show their method in the video here. One caveat: we don't know if putting your system back online will somehow invalidate the Achievement. (If anyone tries, let us know!)
Check Wiki Game Guides for more on the Achievement, including the list of dates you'll need to use for the Achievement, honestly or otherwise.
I love the idea of an Achievement that, in the spirit of Nintendo's village-simulator Animal Crossing, is programmed to reward players who play the game at set times of the year. When I found Calendar Man in the lower level of Arkham City's courthouse, I noticed that he wanted me to visit him on Halloween. At first I thought that time would simply move forward in the game. The Animal Crossing approach is more delightful.
As helpful as the tip for this Achievement is, I'm not going to use this advice. I don't want to cheat. If you do and the game catches you, let's hope there's no Batman version of Mr. Resetti, the Animal Crossing character who scolds you if he catches you resetting your game clock.
(Side note: Arkham City already plays like Assassin's Creed. Now we learn that it's got an Achievement in the spirit of Animal Crossing. What's next? The discovery of some Ace Combat-style Bat-jet?)
Story Teller Achievement - Batman: Arkham City [Wiki Game Guides]