"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. We have been able to encase single Hour-Reversal Charms, which are unstable and benefit from containment, in small, enchanted hour-glasses that may be worn around a witch or wizards neck and revolved according to the number of hours the user wishes to relive."
-- Saul Croaker, Department of Mysteries (Pm)
The Hour-Reversal charm reverses time by one hour. This unstable charm must be kept inside an object, and is contained within small hourglasses to create Time-Turners (Pm).