A story which features time travel in a universe in which time travel isn't commonplace. That is, universes such as Doctor Who have time travel as commonplace, so they won't be listed here.
Altered Destinies (Harry Potter) by DobbyElfLord
Harry has defeated Voldemort, but it was a costly victory. Aberforth Dumbledore presents a plan to go back and kill the infant Riddle, but Harry will have to stay there 10 years. Can Harry alter the wizarding world's destiny? WWII and Grindelwald
Have You Ever (Harry Potter) by Lady Moonglow
With the war looking bleak, the Golden Trio, Ginny, Draco, and Lavender go back to Tom Riddle's 7th Year to destroy Lord Voldemort once and for all. What Hermione DIDN'T count on, however, is a shared common room, a curse, and a little thing called love.
Absolutely Elsewhere (Harry Potter) by Wallyflower
Over the course of a week, Severus Snape solves a small mystery.
Counterclockwise (Harry Potter) by Georgina the Hyena
Percy has lost all of his siblings and his father in the war. But with a cunning plan and illegal time travel, he may just get them back.
(1) The Moment it Began (Harry Potter) by Sindie
If Snape could choose to live his life over, he would choose Lily over the Death Eaters. What if he got that chance?