Our brothers-in-tech at Gizmodo were stoked to rent movies from iTunes but less excited about the 24-hour expiration clock iTunes places on the rentals. After a bit of time in the think tank, they discovered you can trick iTunes into extending a movie’s rental period for years with a little Marty McFly finesse. In essence the method is very simple: Just set your system clock ahead a year or two before you download a rental, download it, start playing the video in order to start the…