This particular practical joke has probably lead to at least seven different time traveler conspiracies floating around the Internet, but it's a silly throwaway gag and one that allows for much inventiveness and creativity from the person in question. You can pick a time period, the way you react, and play as it loud and as long as you want. Mature? Probably not. But if you have a real hankering for a harmless prank to make your day a bit better, and a dumb, dry joke to pull on the unsuspecting, then this is a sure bet. Apply your outfit - futuristic sunglasses and board shots circa Back to the Future, or a full houndstooth suit and monocle as if you've just surfed in from the late nineteenth century - head to a public place and prepare to ask someone the time. They might even think it's weird - after all, you might have a watch and should have a phone. Then ask them, earnestly, 'no, I meant... what's the year?'. If they tell you, feign excitement and wonder on a H.G. Wells level and run around proclaiming 'it worked!'. Yes, it's silly, and your target will probably realised that this is a practical joke, but you'll be grinning ear to ear and you'll hopefully find your day markedly improved.