I honestly have no idea who came up with this first. I've told it to at least two people, both of whom responded "No, I told YOU that idea first" so you can imagine my confusion. Still, I'm writing it up first, so there.
Okay, here's the gag. You get some vaguely/slightly futuristic-looking clothes. Make it plausible, somewhat based on current trends, you're probably aiming for maybe ten years in the future. You can most likely make do with an interesting combination of whatever clothes you currently own. Ooh! Or make a fake tour T-shirt for a band that doesn't exist and mark it "Wild Tour 2008" or something. whatever. The point is to make it look plausible that you might come from the future.
Then just run out into the street, select somebody at random and shout at them, "What's the date today?! Quickly, tell me!"
When they respond, you shout, "What YEAR, man, what YEAR is this?!"
And when they respond again you go, "Noooo! They've sent me back too far!" or alternatively "I'm too late! It's all going to happen again!"
Then you run away again.