The stated rule is that they expire when they say they expire. But MANY on this board have provided anecdotal evidence of being allowed to do otherwise.
From what I've read over the past year or so, MOST people haven't had a problem with same-day FPs that are past their time window. HOWEVER, there have been a handful of reports of being turned away. It all depends if the CM decides to enforce the rule or not, so you're really gambling if you try to use an "old" one. It's probably only a small gamble, but a gamble nonetheless.
You will undoubtedly read MANy posts on this subject as people read your OP. Just remember -- it's not GUARANTEED that "old" ones will work.