- "As our investigations currently stand, the longest period that may 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 wizard's neck and revolved according to the number of hours the user wishes to relive."
- —Professor Saul Croaker[src]
The Hour-Reversal Charm (incantation unknown) is a very dangerous, unstable spell that can be used to reverse time up to five hours; going beyond the safety line of five hours can bring potential serious harm. According to Professor Saul Croaker, this is what is used on the Time-Turners, as their instability makes them more efficient when contained by a magical artefact.
- Pottermore (Mentioned only)