"All attempts to travel back further than a few hours have resulted in catastrophic harm to the witch or wizard involved."
-- Professor Saul Croaker, Time expert for the Department of Mysteries (Pm)
A time-traveling device the use of which was strictly regulated by the Ministry (PA21). Inside the hourglass-shaped device there is a single One-Hour Reversal Charm (Pm).
All of them were thought to be destroyed during the Battle of the Department of Mysteries (OP35, HBP11, BP), but in 2017 the Ministry learned that other unregulated prototypes might exist in private hands (CC1.5, CC4.4).
- With the permission of Minerva McGonagall, Hermione Granger used a regulation Ministry Time-Turner to attend double classes during Third Year at Hogwarts (PA21).
- At Dumbledore's suggestion, Harry and Hermione illegally used the Time Turner to save the lives of Buckbeak the Hippogriff and fugitive Sirius Black, who would otherwise have had his soul sucked-out by a Dementor. During their time travel, Harry saved himself from Dementors as well by using a Patronus Charm (PA21, PA22).
J.K. Rowling explained why time-travel was not used to save the lives of other important characters:
Ellie: Why didn't Harry use the time-turner to save his parents?
JKR: Oh, that's a very good question, that. But it would take us into "Terminator" territory, if you've ever seen the "Terminator" films... but never mind. Well, the time-turner was a very difficult invention for me, because it created as many problems as it solved. And anyone who's read Order of the Phoenix may have noticed that during the climactic scene in which they chase through the Ministry of Magic, they shatter all the time-turners, thereby preventing them using those in the future (BP).