Originally Posted by
Mathor
I don't really understand the debate, the time-turners are all destroyed by the time Voldermort comes back. Dumbledore tells Hermione to use time travel as a way of "giving her permission to use it". Hermione had kept it a secret in order to keep a promise that she had made to her Professor not to tell anyone of it (Since they are to be used only by the ministry). Dumbledore mentions that she use it in a way of allowing her permission, as well as conveniently spoiling a secret of Hermione's.
EDIT: now to speak for Science fiction. I do not think it is possible for anyone to go back that far. This is not because the time-turner does not allow it to be done, but because as people have eluded to, a person traveling through time has to go through the events up until the present without being seen. The reason they cannot be seen is evident in that if they were to be seen it would destroy the fabric of time. Unlike Terminator or a typical time-traveling device, there is no way to go back to the present, it has to be "re-lived". It seems plausible that the reason the ministry does now allow them to be used, and keeps them tightly secured in the ministry, is because of such things. Had the professor, or dumbledore, or hermione been caught using the time time-turners, they surely would've been arrested on-spot.