To get the current time, just look at the number of lives you have to get the hour, and checking out the number of aliens will get you the minute. If you don't feel like sitting there and counting aliens, the last digits of the high score will also give you the info you're looking for.
I really love that he used an old portable TV in this mod. Something newer would have taken away from the old-school feel of the clock.
Space invaders clock [via make]