I was recently told that a retired schoolteacher in the area once had a backwards-turning analogue clock in his room, simply to confuse and upset his students. This made me want one, so I hacked a dollar-store clock to go backwards.
The process was surprisingly easy - all I had to do was open the back of the clock and, without touching any of the gears, remove the electromagnet, flip it upside-down, and put it back in. Now the clock goes backwards! If set properly, you can even read the time by using the following rule:
[real hour] : [real minute] = (12 - [hour on clock]) : (60 - [minute on clock])