A Time-Turner is a magical object shaped like an hourglass that will transport the wearer back in time. They were kept by the Ministry of Magic inside the Department of Mysteries until destroyed by members of Dumbledore's Army when ambushed by Death Eaters.
 Uses in the Books
 Third Book
When Hermione Granger applies for every subject possible for her third year at Hogwarts, it causes some problems with her overcrowded timetable. However the Head of Gryffindor House, Minerva McGonagall sent letters to the Ministry over the summer so that Hermione could use a Time-Turner to get to her lessons. When Hermione arrived at school McGonagall saw her privately gave her the Time-Turner with strict instructions as to its proper use and made her promise to tell no-one.
 Fifth Book
 Sixth Book
When Hagrid is upset that his three friends have dropped his Care of Magical Creatures Class, he is disappointed that they couldn't find a way. However after an offer to help his dying Acromantula friend Aragog from Hermione he admits he knew they would find it hard to fit him into their timetables even with the use of a Time-Turner. At this Hermione points out they couldn't have applied to the Ministry for one anyway as they had destroyed their entire stock in the summer, according to the Daily Prophet. This settles the matter for Hagrid and he accepts that there hadn't been a way for them to continue.
 Method and Effects
To use the Time-Turner, you need to wear it on a chain around your neck and turn the hourglass round, one turn measuring one hour backwards. This will also transport you to a different place.
 Time Travel Principle
The Time-Turner seems to conform to the Novikov self-consistency principle.
 Additional Canon
 The Films
Unlike in the books, the Time-Turner does not move you through space, just through time.
The inscription on the prop used reads:
My use and value unto you are gauged by what you have to do. I mark the hours every one nor have I yet outrun the sun.