The magician asks a spectator to mentally select an hour of the day.  He hands them a blank deck of Bicycle cards, closes his eyes, and asks them to count off and pocket their secret "hour" in cards.

When they're done he takes the deck and lays out a "clock", as well as a prediction envelope in plain sight that cannot be changed.  The spectator now announces their number which is counted on the card "clock".  That card is layed in the middle.  When the prediction is opened it's a "match", both blank!  Not impressed?  Turn the cards over... The spectators card and the prediction are the only two with red backs!  All the rest are blue!