I hope that whoever first postulated that "time flies when you're having fun" won a Nobel Prize or two for that idea. Probably one for Physics and one for Philosophy, though I believe that they call it "Literature" just to keep everyone honest. Try and imagine a moment in which you were having fun, and then contrast it with one in which you were not. Maybe this is a very special kind of relativity, and we should be congratulating Albert Einstein for yet another bit of oppressive cleverness.
There are all kinds of tests for this theorem. Anytime you find yourself in a line, you know that there are seconds wedged into each minute spent there, watching the front that line expand as your position remains unchanged. This feeling is exacerbated by the relative unpleasantness of the line in which you find yourself. The line at the bank is somewhat interminable, but that same line in the Department of Motor Vehicles is tantamount to eternity. At least at the bank there is the possibility of getting cash money at the end. Or maybe the place will get robbed. Even criminals avoid the DMV.
I know that certain days stretch on for weeks, while others are gone in a heartbeat. I understand that this is all due to my own perception. When I have a day filled with happy kindergartners who hang on my every word, the day is over before it started. If I am confronted by a room full of surly fourth graders who lost their recess, I know that I will be in for a long one. And so it goes, until we find a way to even it out. I only hope that reading this took you only a few seconds.