The stage play Harry Potter and the Cursed Child changed the way Time-Turners work in the wizarding world and retconned the plot of Harry Potter and the Prisoner of Azkaban. The sequel to the Harry Potter series sees the return of the small time-traveling devices, which were already thought to cause plot holes since they could be used to zip back and time and solve all of Harry's problems.
SCREENRANT VIDEO OF THE DAY SCROLL TO CONTINUE WITH CONTENT Time-Turners were first introduced in Harry Potter and the Prisoner of Azkaban when it was revealed that Hermione Granger had been using one all year to attend multiple classes at once. At the book's climax, Harry and Hermione used her nifty tool to go back in time, save Buckbeak from being executed, and cast a powerful Patronus that chased the Dementors away from Harry and Sirius' past selves.
When Hermione used the Time-Turner to travel back in time and attend a different class, she wasn't creating a timeline in which she attended Ancient Runes instead of Arithmancy. Instead, she used time travel to allow herself to be in two places simultaneously. Therefore, when she was sitting in Arithmancy class, another version of herself was also sitting in Ancient Runes, even before she ever used the Time-Turner. She couldn't change the past because it had already happened.
The Time-Turner Retcon Ruined Harry's Big Lesson In Prisoner Of Azkaban When Harry and Hermione went back in time to save Buckbeak and Sirius Black in Harry Potter and the Prisoner of Azkaban, the function of time travel was the same as it had been when Hermione time traveled to attend classes. They hadn't realized it when they first believed Buckbeak was executed, but they had already saved him.
This was an important lesson for Harry in the Harry Potter series. The time paradox that the Time-Turner created each time it was used demonstrated that the past could not be changed. So, though Harry might have been tempted to go back in time and save his parents, it wasn't possible because it had already not been done. This squashes the claim that Time-Turners could have ended the entire Harry Potter franchise from book 1.