Fastpasses are only valid for the day they are issued. It is true you can sometimes trick CMs into taking them on a different day, just like you could color photocopy them and trick CMs who are not paying close attention. People alos sell them on Ebay. All of the latter are breaking the rules as they clearly say what day they are good for.
As for the OP, I have gotten a Space Mtn FP at 8AM and used it after 11PM at night. It is DLR policy (and WDW policy FWIW) to accept FPs any time during or after the FP window on the day issued.
The funny experience some of us have had (and I have had twice) is talking a FP from say 9AM back for use at say 7PM in the evening and having a CM reject it because "it is too early" and you cannot use a FP before its window. When I point out the FP is for 9AM and not 9PM (as they thought) then they say "Oh, then you are OK. Go ahead".