"What's The Point Of Anything In Harry Potter If They Can Go Back In Time And Undo It? Idiots!"
J.K. Rowling used to be popular, but she fell out of favor when it recently came to light that she wrote those awful Harry Potter books. Those stories made no sense! Azkaban introduced time travel in the form of little devices called time turners, and this should have removed all risk and drama because if something bad happens, you can just go back and fix it. That's what happens in some major superhero movies, that's what happens in crazy cartoons, and that's what logic says should happen in Harry Potter.
Whenever someone dies, why not go back in time and save them? Or go all the way back and save Harry's parents? Or do a fetus deletus on Voldemort back when he was still in the womb?
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Or, hey Ministry, they resurrected Voldey 15 minutes ago. Can we get a reset on that?
Time turners can't change the past.
Though there's an ungodly amount of fanfiction where time turners change the past -- including the famous Potter play The Cursed Child, which is fanfiction because people other than J.K. Rowling wrote it -- the books and movies are firm about this. Characters go back in time an hour or two (there's no indication you can go back further than that) and sometimes think they're supposed to change stuff, but they change nothing. They don't slip into an alternate timeline either. They go to their own past, where all actions are all fixed.
Harry sees a figure scare off the Dementors, then he goes back in time and scares the Dementors off, because that figure was him all along. The gang hear an axe fall and Hagrid cry, then they go back in time and save Buckbeak, which was what made the executioner drop that axe and made Hagrid cry for joy. The movie goes even further than the book, making some rocks hit the kids then having them go back in time and be the ones who threw those rocks.
I feel kind of silly recounting the plot as though you couldn't understand this story written for small children unless I explain it. But the fact is, many people who read the book or saw the movie came away thinking time turners let you change the past. They don't. Time turners just let you be in two places at once, which was why they routinely let Hermione attend simultaneous classes. They're great for productivity but aren't gamechangers when it comes to saving the world.
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You could go back and bet on Quidditch outcomes you already know, but that's why time turners are regulated.