The Time-Turner ability is available at the end of Year 3
and can be used by Hermione. When in Year 4 you will be in the Gryffindor dorm room, here you will see a Grand-father clock, walk up to it and press
to activate the Time-Turner ability. There are more clocks in the game (I assume just throughout Hogwarts) but that one is available during story.
Other Grand-father clocks locations are:
The Bell Tower
Garden by Hagrids shack/hut