in the cleric page on the SRD it says
Clerics meditate or pray for their spells. Each cleric must choose a time at which she must spend 1 hour each day in quiet contemplation or supplication to regain her daily allotment of spells.
It doesn’t say how much of a buffer zone they have. For example say my meditation time is midnight but I get stuck in an encounter around that time. How long before and after that is the “safe zone”. Most classes require a long rest before they can regain their class features (at least to my knowledge they do). So only needing 1 hour would be game-breaking if there wasn’t a time restriction in place. On the other hand, if the time restriction is too picky you could be locked out of your spells for a prolonged period of time due to bad luck with encounters or just being in transit at the wrong time, which would be underpowered.
So that begs the question, how close do you have to be to the time you choose to meditate in order to get your daily spells.