As Vera pointed out I just bought some Priority stamps at the PO the other day and the ones I bought from the self-serve machine have the date of purchase on them and the postal worker said you can use them for as long as the postage amount is correct, the date of purchase means nothing.
The worker actually looked at me like I was totally crazy to think that I had to use it that day...:rolleyes:
Even the postal worker only had printed postage for me to buy they didnt even have the actaul Priority stamp.
Drop it in the mailbox and don't look back;)
HTH