This seems like the sort of thing he'd do as a joke. Heck, it seems like the sort of thing I'd do as a joke.

Seriously? Okay, education time! This is the so called Space-Time Relativity, Einstein's greatest discovery. Basically and in short, time (and space) isn't universal but Observer-dependent, which means for you time is different than for me, so relative, for example if you are on a space station and you observe the explosion of a fire cracker and an alarm clock going off in a point of space and time for me these events may be different, like it happened in a different order. I hope you understand it, cause explaining the joke is not funny!