One year a friend/roommate of mine (who worked nights) made a date for
the next afternoon to go out with his new girlfriend who he was trying
desperately to impress. Just before his alarm was set to go off we
turned it off and set every time keeping device in the house (including
his pocket watch and the microwave) forward 4 hours...

poke, poke "Hey dude, weren't you supposed to do something today?"

He was on the phone apologizing to his very confused girlfriend for
about five minutes before she figured out what he was babbling about
and told him what time it was. We nearly wet ourselves at the look on
his face...

This works especially well because the victim is just waking up, but
only if they are supposed to get up in the afternoon or very late at
night, so they can't tell the difference easily from the light coming
through the windows.