"As our investigations currently stand, the longest period that be relived without the possibility of serious harm to the traveller or to time itself is around five hours."
-- Saul Croaker, Department of Mysteries (Pm)
Mintumble was a witch who studied the properties of time. In 1899 she attempted to travel back through time and was trapped for several days in the year 1402. When she was rescued and returned to the present, her body aged centuries with the trip and she died a short time later in St Mungo’s. Mintumble’s interactions with persons in 1402 severely disrupted the laws of time. As a result, twenty-five people in the present simply vanished, having never been born. Time itself was seriously affected: the next Tuesday lasted for over two days and then Thursday only lasted a few hours.
Because of the terrible ramifications of this accident, the Ministry of Magic curtailed all such time travel experiments and severely limited any investigations into time. The most a person can safely travel back through time is a few hours through the use of a Time Turner.