Time

"Mysterious thing, time. Powerful, and when meddled with, dangerous."

- Albus Dumbledore.

Time is the magic that allows to sequence events from the past through the present into the future, to compare their durations and the intervals between them, and to quantify the speed at which objects move and things change.

Time is one of the most complex and strongest kinds of magic and, as such, there is a room devoted to its study the Department of Mysteries, the Time Room. Time is governed by natual laws which magic can circumvent to some degree — although time-related magic is unstable, and serious breaches in the laws of time result in catastrophic events, the investigations conducted in the Department of Mysteries make it possible to go back in time for a period of no more than five hours (and, even so, the use of a time-travelling device such as a Time-Turner is hedged around with hundreds of laws).

It seems to be only possible to travel back in time, and not into the future.

Eloise Mintumble (1899)
In 1899, the Department of Mysteries conducted its last experience concerning time-travelling back in time for more than a few hours. Eloise Mintumble was sent to the year 1402, wherein she became stuck for a period of five days. When she was finally retrieved to the present, her body had aged five centuries, and, irreparably damaged, she died in St Mungo's Hospital for Magical Maladies and Injuries.

Her excursion to the past provoked a great disturbance to the life paths of all those she met, changing the present so dramatically that no fewer than twenty-five of their descendants vanished in the present, having been "un-born". Moreover, there were a few more alarming signs that time itself had been disturbed: Tuesday following her reappearance lasted two and a half full days, whereas Thursday shot by in the space of four hours.

The Ministry of Magic had a great deal of trouble covering up the mishap and, since then, the most stringent laws and penalties have been placed around those who study time travel.

Hermione Granger (1993-1994)
Hermione Granger received a Time-Turner 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.

Appearances

 * Harry Potter and the Order of the Phoenix
 * Pottermore