That's the Power of Love by Gr8t_78: https://archiveofourown.org/works/64657705/chapters/166097263

Summary:

It's 2003. Draco Malfoy, Hermione Granger, and Severus Snape travel back in time to put an end to the sinister presence in all their lives once and for all. Draco and Hermione decide to pose as seventh years alongside the students of Hogwarts in 1975 to create change and solve all their problems. But will it work? Maybe they had no idea what problems were till they did this.

https://archiveofourown.org/users/Gr8t_78