MZK is correct...For example, we are staying for a single night in
August. We are paying for our room only; we're getting our tickets online through the Universal website, using their "5 days for the price of 2" deal.
Our room keys (which are really plastic cards) will be our FOTL pass, but the tickets we're buying separately will get us into the parks.
As for a "use by" date, I don't think there's a limit. But for that, you might want to give them a call just to confirm....
Enjoy your trip!