A Time-Turner is a special timepiece in which an Hour-Reversal Charm has been encased, for added stability. It is a device used for time travel, and it resembles an hourglass on a necklace. The number of times one turns the hourglass corresponds to the number of hours one travels back in time (although, it should be noted that the longest period that may be relived without the possibility of serious harm to the traveller or to time itself is around five hours).
- "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[src]
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.
The entire stock of Time-Turners, located in the Time Room, in the Ministry of Magic were rendered useless during the Battle of the Department of Mysteries in 1996. While not "destroyed" per se, due to the way one of them fell when their counter was knocked over, the entire stock was trapped in an endless loop of falling over, un-falling, and then re-falling, in an endless cycle for all eternity, and are thus unable to be used. 
Time-related magic is unstable, and serious breaches in the laws of time result in catastrophic events. Possible scenarios include a wizard or witch killing their past or future selves by mistake, or altering one's life path in such a drastic fashion that it can result in temporal anomalies such as un-births. As such, the Ministry of Magic seeks the strictest guarantees if it permits the use of Time-Turners: Time-Turner possession is hedged around with literally hundreds of laws, and the most stringent laws and penalties are in place to prevent their misuse.
The longest period that can be travelled back in time without serious chance of harm to the traveller or time itself is around five hours. 
Behind the scenes
- In the film adaptation of Harry Potter and the Prisoner of Azkaban, while the importance of not being seen while travelling back in time is stressed, Harry and Hermione pass by the Clock Tower Courtyard while following their past selves. However, the executioner was sitting there, sharpening his axe, so he must have seen two pairs of Harrys and Hermiones, though he was too busy sharpening his axe to notice them.
- 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 adaptation, when the Time-Turner is used it leaves the person in the same place they were when they turned time back. The GBA 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 traveller does merely produces the circumstances they had noted before travelling," is incidentally reminiscent of J. K. Rowling's employment of self-fulfilling prophecy. However, references to catastrophes that can take place when time travelling (a reference to a wizard travelling to the past and being killed by his past self in Prisoner of Azkaban, or Eloise Mintumble's time-travelling mishap in Pottermore in which several people end up un-born in the present) seem to go against Novikov Principle, indeed creating paradoxes.
- Knowing that time-related magic is unstable, there might be different ways to experience time through magic. This would explain catastrophic events as with Eloise Mintumble, and also paradoxes as result of poorly performed spells.
- 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.
- The Time-Turner is one of the many collectibles offered by the Noble Collection.
- Hermione Granger's Time-Turner is David Heyman's favourite prop from the films.
- You can hear quiet ticking as a background sound during nearly the whole Time-Turner episode of the film.
- Harry Potter and the Prisoner of Azkaban (First appearance)
- 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 (Mentioned only)
- LEGO Harry Potter: Characters of the Magical World
- LEGO Harry Potter: Years 1-4
- Harry Potter for Kinect (Mentioned and appears on a illustrative photograph)
- The Wizarding World of Harry Potter
- J. K. Rowling's Official Site
Notes and references
- ↑ This quote comes from the "Harry Potter Sticker Kit", containing a plastic replica of the Time-Turner used on the film.
- ↑ Harry Potter and the Order of the Phoenix
- ↑ Blue Peter (CBBC) interview with JK Rowling, confirming that all the MoM time-turners were destroyed.
- ↑ Pottermore
- ↑ Assuming an average usage of 3 hours per day, 5 days per week, for a period of 9 months, this would add up to approximately 24 extra days.
- ↑ http://www.youtube.com/watch?v=e3FP-nmkFL0&feature=relmfu