Everyone remembers one of the greatest reveals in the entire "Harry Potter" series; how was Hermione attending all of her classes and doing all of her homework in "The Prisoner of Azkaban"? It would seem impossible; there are, after all, only twenty-four hours in a day. Not even wizards can travel through time, right?
Wrong. It was revealed in less than one hectic page that Hermione had been traveling back trough time to pick up extra classes and, well, time.
"It's called a Time-Turner... and I got it from Professor McGonagall on our first day back. I've been using it all year to get to all my lessons. Professor McGonagall made me swear I wouldn't tell anyone. She had to write all sorts of letters to the Ministry of Magic so I could have one. She had to tell them that I was a model student, and that I'd never, ever use it for anything except my studies... I've been turning it back so I could do hours over again, that's how I've been doing several lessons at once, see?"
Harry and Hermione used the time-turner to go back three hours into the past; farther than Hermione had ever gone back before. In that time, they manage to keep Buckbeak from being killed, save both Harry and Sirius from the dementors (the former requires a more in-depth analysis to explain), and help the latter escape to somewhere tropical (as hinted at in "The Goblet of Fire").
Time-turners are not mentioned again until "The Order of the Phoenix"; when they are all smashed in the Department of Mysteries. J.K. Rowling gave us some insight as to why this happened:
"I went far too light-heartedly into the subject of time travel in Harry Potter and the Prisoner of Azkaban. While I do not regret it (Prisoner of Azkaban is one of my favourite books in the series), it opened up a vast number of problems for me, because after all, if wizards could go back and undo problems, where were my future plots?
I solved the problem to my own satisfaction in stages. Firstly, I had Dumbledore and Hermione emphasise how dangerous it would be to be seen in the past, to remind the reader that there might be unforeseen and dangerous consequences as well as solutions in time travel. Secondly, I had Hermione give back the only Time-Turner ever to enter Hogwarts. Thirdly, I smashed all remaining Time-Turners during the battle in the Department of Mysteries, removing the possibility of reliving even short periods in the future."
This plot device certainly would have created many problems if left unchecked; as perpetuated in this theory by a fan:
What would have happened? I think that Hermione would still have the time-turner; never having been able to go back to the point where she got the time-turner in order to not receive said time-turner.
What if a "time force" of Ministry unspeakables had been created in 1992 to stop Lord Voldemort from ever being born? Say the force went back in time to December 31, 1926 Wool's Orphanage and simply used an abortion spell on Merope before her baby could be born. Salazar Slytherin's line would probably have died out and there wouldn't have been a chosen one at all. "Harry Potter" would have been a boring story about how wizards can't seem to get along with muggles and we would all be die-hard "Twilight" fans.
Aside from being solid proof that the events in "Harry Potter" must have really happened (see Dumbledore's final line in the series), this situation would have been completely horrible for everyone. It is good; therefore, that J.K. Rowling smashed the the time-turners when she did.
Now if only I could grab a time-turner to stop Michael Gambon from shouting "HARRY!!!!! DID YA PUT YOUR NAME IN DA GOBLET OF FIAH!?!?!?"