Time-Turner

"I mark the hours, every one, Nor have I yet outrun the Sun. My use and value, unto you, Are gauged by what you have to do."

- Inscription on Hermione Granger's Time-Turner

The Time-Turner was a device capable of time travel that resembled an hourglass on a necklace. The number of times one turns the hourglass corresponds to the number of hours one travels back in time. It is extremely important that the user of a Time-Turner not be seen by past or future versions of themselves unless, of course, said versions are aware of their usage of a Time-Turner. A possible scenario is a wizard or witch killing their past or future selves by mistake. It is unknown if there is a limit to how far into the past a Time-Turner can travel.

History
"Hermione’s immense workload finally seemed to be getting to her. Every night, without fail, Hermione was to be seen in a corner of the common room, several tables spread with books, Arithmancy charts, rune dictionaries, diagrams of Muggles lifting heavy objects, and file upon file of extensive notes; she barely spoke to anybody and snapped when she was interrupted."

- Hermione's workload due to her Time-Turner

Hermione Granger received one from Professor McGonagall in 1993, so that she could attend more classes in her third year than time would allow. Since McGonagall made her swear to not tell anyone about it, she did not mention it to Harry or Ron until the end of the school year, when she and Harry used it to travel back in time and save Sirius Black and Buckbeak from certain death. Special permission from the Ministry of Magic had to be sought to allow Hermione to use one, but her academic record ensured that permission was given.

Hermione found her third year stressful with the extra class load, and therefore decided to drop Divination, which she despised, and Muggle Studies, which she did not find very useful, given that she was a Muggle-born. This allowed her to have a normal schedule once again, and she returned her Time-Turner. Ron was disappointed that Hermione did not tell her friends about it, despite her promise to McGonagall.

"I can't stand another year like this one. That Time-Turner, it was driving me mad. I've handed it in."

- Hermione Granger at the end of her third year

The entire stock of Time-Turners, located in the Time Room, in the Ministry of Magic was destroyed during the Battle of the Department of Mysteries in 1996.

Behind the scenes

 * It is possible that Barty Crouch Jr., Percy Weasley, and Bill Weasley used Time-Turners, as they were all each known to have achieved twelve O.W.L.s.
 * It is unknown when or if the Ministry of Magic replaced the Time-Turners that were broken during the battle in 1996, or if it is even possible to recreate or replace a Time-Turner.
 * In the film version of Harry Potter and the Prisoner of Azkaban, Harry and Hermione were seen by the executioner twice, having walked past him scowling, then ran past him again while "retracing their steps."
 * In the book, when the Time-Turner is used it takes the person back to the location where they were present at the time they'd gone back to. However, in the film version, when the Time-Turner is used it leaves the person in the same place they were when they turned time back. The video game version has the user appearing at an entirely random place (i.e. Harry and Hermione use it in the Hospital wing and appear at the Forbidden Forest).
 * The possibility of time travel within the Harry Potter universe may seem to allow many plot holes, but characters appear to use them for trivial tasks that have no effect on existence as a whole. The one notable use of a Time-Turner within canon, the Rescue of Sirius Black and Buckbeak, obeys the Novikov Self-Consistency Principle. This theory of time-travel, stating that "Nothing can be changed because anything a traveler does merely produces the circumstances they had noted before travelling," is incidentally reminiscent of J. K. Rowling's employment of self fulfilling prophecy, but while prophecies within canon are relevant only to the degree that characters place relevance on them, as the books state Harry Potter and Tom Riddle could walk away and void the prophecy if they chose to, the same cannot be said for the series if any major events within the books or the Harry Potter universe as a whole have been related to the Time-Turner's misuse.
 * This does not, however, refute the idea that people can change the past through the use of an object like the Time-Turner.
 * It is unknown what the effect of excessive Time-Turner use might produce. The user might still age while within an hour produced by the Time-Turner, and if so, then for wizards or witches such as Hermione, constant use might age them faster, adding days, weeks, or even months onto their internal chronological clock. It is also unknown how this applies to later on in life. If Hermione was perhaps a month older by using the Time-Turner during her third year, then biologically her seventeenth birthday might arrive a month earlier than her calendar birthdate might indicate, which might cause the Trace to have been lifted earlier as well.
 * In LEGO Harry Potter: Years 1-4, there are grandfather clocks throughout Hogwarts which allow Hermione to use her Time-Turner to transport herself and her teammates back in time to complete certain objectives. One of these includes a mission where Harry and Hagrid hatch Norberta's egg.
 * It is also a possibility that it is impossible to change the past, leaving a Time-Turner to only let users see what actually happens at another place. Examples that can explain this is that in the actual time, Harry was also hit by a snail shell, and Hermione still heard the crack of a branch. Which was what happened in the past.
 * The Time-Turner is one of the many collectables offered by the Noble Collection.
 * Part of the inscription on Hermione's Time-Turner - "nor have I yet outrun the Sun" - seems to suggest that Time-Turners can only travel backwards in time, not to the future. Alternatively, this may suggest a limit on how far back the Time-Turner may travel. It is possible that Time-Turners can only travel as far back as the beginning of the day.
 * Hermione Granger's Time-Turner is David Heyman's favourite prop from the films.

Appearances

 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (film)
 * Harry Potter and the Prisoner of Azkaban (video game)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Half-Blood Prince
 * LEGO Harry Potter: Characters of the Magical World
 * LEGO Harry Potter: Years 1-4
 * The Wizarding World of Harry Potter 
 * J. K. Rowling's Official Site