"Professor Croaker’s law – the furthest someone can go back in time without the possibility of serious harm to the traveller or time itself is five hours. And we went back years. The smallest moment, the smallest change, it creates ripples. And we – we’ve created really bad ripples."
-- Scorpius Malfoy to Albus Potter (CC2.16)
A magical law that states that while using a Time-Turner, no one can travel back in history further than five hours without doing serious harm to themselves or others (CC2.16).
- Professor Croaker was an Unspeakable at the Department of Mysteries (GF7) who dedicated his life to the study of Time Travel (Pm).
- Croaker's caution was the reason the Ministry Time-Turners were used for only mundane tasks and were limited to a few hours only, such as Hermione repeating hours to take extra classes (Pm, PA21, CC2.16). He cited the unfortunate tale of Eloise Mintumble, who went back to 1402 and became trapped there. When she returned to the present time, her body had aged so much she soon died, and her time-travel had affected many lives including 25 descendants who were suddenly "unborn" (Pm).
- Lucius Malfoy thought Croaker's Law was "too vanilla" or restrictive, and had a Golden Time-Turner made for himself so he could travel back years without a problem, although he never actually used the device according to his son Draco (CC4.4).
- Harry's son Albus and Draco's son Scorpius broke Croaker's Law again and again while using an Experimental Time-Turner to go back years and attempt to save the life of Cedric Diggory (CC2.7, CC2.20, CC3.8). And indeed, they seem to have proven that Croaker was right - the consequences of changing even one thing in history had catastrophic results for the future, such as Albus being "unborn" and the Dark Lord surviving the Battle of Hogwarts while Harry Potter died.