What's Countdown?  Referred to as the Countdown to Final Crisis, it's a comic book limited series published by DC way back in May, 2007.  It followed the conclusion of the last issue of another comic sereis called 52. The series was written primarily by Paul Dini with a rotating team of writers and artists and a continuous supply of lackey interns who are more than anxious to work for free.  Countdown consists of 52 issues published weekly for one year, with your standard annoying crossovers, with the series being numbered in reverse order. The series is told in parallel narratives, which is pretty cool for the A.D.D. people with short attention. So, judging by this 100% cotton The Joker Countdown T-Shirt, it looks like the Joker spent some time being crazy. That sounds about right.