Harry Potter and the Impossibility of Time Travel

*slightly edited 2018/10/14* I started writing the fic post OotP but pre HBP. It takes place three years after the Trio left Hogwarts. On Harry's 21st birthday, the final battle finally takes place, but Voldemort still has an ace up his sleeve to finally get rid of Harry - he sends Harry and his friends to the past and now it is up to them to find a way home and defeat Voldemort.