 FACT: SPOILER: The timer which sets off the explosives is clearly  shown to be a digital one, not a mechanical one. Consequently it  would either carry on keeping perfect time if it had battery backup  or else it would reset itself to a dummy time and become disarmed  when the power came back on. It would not simply resume where it had  left off, having "lost" the intervening time while the power was  off, and therefore it would not be possible for it to go off at a  later time than intended.