I just read the reply to Bill4Rest that suggested posting the details of your idea to yourself and I offer the following, picked up many years ago...A guy posted in to a book maker his bets for a forthcomming race meeting and since the book maker required that bets be placed prior to race day the accepted evidence of this was, as mentioned in the posting, the post date on the envelope. Ha! Ha! The punter did very well on this deal. He posted a letter to himself with the area around the stamp cut-out and put the stamp on a enclosed envelope. Result: A post-stamped envelope with a date prior to the race date. This is one way to bet on a sure thing and - I bet- the authorities are onto this one these days.