We have used our fastpasses much later in the day. I think WDW allows this, because, if you get a FP for something, then head to another area of the park to ride, see a parade or perhaps eat, you may not be able to make it back in the exact time frame to ride.
EE has a different word at the bottom of their fastpasses that changes each day, kind of like what Soundgod was describing at DL. DH got fastpasses one time, put them in his wallet. When we went to ride EE with our fastpass, he gave the CM another FP he had in his wallet from several days earlier that we didn’t use. The CM explained to us how it worked. So then, DH digs thru his wallet and finds the ‘real’ FP, and we got on the ride!
I am not sure if other rides are like this one, if you could use a fast pass from one day on another day. I think they are all dated, but if the CM was even awake, it would be easy to see they are from another day.