Time-Turner

A Time-Turner is a magical device for short-term time travel; Hermione's looks like a gold hourglass pendant on a necklace. McGonagall gives it to her in Prisoner of Azkaban so that she can attend several classes at once, and near the end of the book, on Dumbledore's suggestion, she and Harry use it to save Sirius Black and Buckbeak. In The Cursed Child, Croaker's Law limits legal Time-Turners to five hours, and an illegal one drives the plot.