- "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 on the hazardous nature of this charm[src]
The Hour-Reversal Charm is dangerous and unstable and going beyond the safety threshold of five hours may bring serious harm to the flow of time. According to Professor Saul Croaker, this enchantment is what is used to bewitch Time-Turners, as their instability makes them more efficient when contained by a magical artefact.
- Pottermore (First mentioned)
- Harry Potter and the Cursed Child (Mentioned only)
- Harry Potter and the Cursed Child (play) (Mentioned only)