Even in the magical world, time travel is extremely dangerous. Ministry of Magic-issued Time Turners have an hour-lock placed on them, allowing the user to travel back in time a maximum of 5 hours, deemed the safe limit by the Ministry. Hermione Granger was issued a Time Turner in her third year, to allow her to take more classes than time would allow. While she did eventually drop a couple of subjects to return to a regular class-load, the Time Turner proved invaluable near the end of the school year, when she and Harry used it to save more than one innocent life.
Official Harry Potter licensed merchandise