Harry Potter and the Prisoner of Azkaban introduced us to the Time-Turner, a magical device that allows a user to go back in time for a few hours. Hermione Granger used one throughout her third year at Hogwarts so she could attend more classes than would otherwise have been possible.
At the end of the third story Hermione and Harry Potter used her Time-Turner to save the lives of both Sirius Black and Buckbeak the hippogriff. However, this was the last time we saw a Time-Turner in any of the movies that followed.
Video Of Hermione and Harry Using A Time-Turner
Here it is, the moment Harry (and we) learn about Time-Turners.
Why Is This A Movie Mystery?
Did you ever wonder why no one used a Time-Turner to prevent the deaths from the Final Battle at Hogwarts in Harry Potter and the Deathly-Hallows pt. 2? If you have, you aren't alone. LOADS of people have wanted to know why Time-Turners weren't used. Surely these magical devices could have saved the lives of Fred Weasley, Remus Lupin, Nymphadora Tonks, Lavender Brown, and others. Right? Well, not exactly...
All The Time-Turners Were Destroyed
Remember at the end of the fifth movie when Harry, Hermione, Ron Weasley, Ginny Weasley, Luna Lovegood, and Neville Longbottom go to the Ministry of Mysteries to save Sirius Black from Lord Voldemort? They were ambushed by Death Eaters shortly after Harry removed his prophecy from the Hall of Prophecy somewhere deep within the Ministry. They battled with the Death Eaters and most, if not all, of the prophecies were destroyed.
Well, it turns out that the prophecies weren't the only thing destroyed when they were trying to escape from the Death Eaters. The fight in the book is far more detailed than it is in the movie and involves several other rooms.
At one point a Death Eater attacked our heroes in one of these rooms, and he ended up with a baby's head and mind during the fight after he fell into a jar that contained Time. Can you guess what else happened during their battle with the Death Eater?
The jet of red light flew right over the Death Eater's shoulder and hit a glass-fronted cabinet on the wall full of variously shaped hourglasses. The cabinet fell to the floor and burst apart, glass flying everywhere, then sprang back up onto the wall, fully mended, then fell down again, and shattered -
[A short time later...]
The baby-headed Death Eater was screaming and banging into things, toppling grandfather clocks and overturning desks, bawling and confused, while the glass cabinet that Harry now suspected had contained Time-Turners continued to fall, shatter, and repair itself on the wall behind them.
Harry Potter and the Order of the Phoenix
Early on in the next book, Harry Potter and the Half-Blood Prince, during a conversation between Hagrid and Hermione, we find out that Harry's suspicion about the glass cabinet was correct.
"Ar, I always knew yeh'd find it hard to squeeze me inter yer timetables," [Hagrid] said gruffly, pouring them more tea. "Even if yeh applied for Time-Turners - "
"We couldn't have," said Hermione. "We smashed the entire stock of Ministry Time-Turners when we were there last summer. It was in the Daily Prophet."
Harry Potter and the Half-Blood Prince
After writing Harry Potter and the Prisoner of Azkaban, author J.K. Rowling realized that she might have made a mistake by introducing Time-Turners into her series. She didn't regret including one in the third story but she didn't want Time-Turners to be seen as an easy way to solve the problem of Voldemort. Therefore, she decided to have them all destroyed so that no one would be tempted to use them at any other point in the series. 
So now you know that it none of the characters who died in the final story could have been saved by a Time-Turner because there were none left.
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