Charlotte Sometimes

Charlotte Sometimes is a children's novel by British writer Penelope Farmer. It is the story of Charlotte, a young girl sent to a boarding school where she begins travelling back in time every other night, switching places with a girl called Clare. Alternating between 1963 and 1918, the book takes a frightening turn when the girls find themselves stuck in the wrong decades.

Notable for its philosophical themes and questions of identity and self, the book inspired Robert Smith of The Cure to write the song Charlotte Sometimes.

Tropes:

 * Annoying Younger Sibling - Clare's little sister Emily initially fits this trope, though Charlotte later grows fond of her.
 * Boarding School - largely set in one.
 * Mistaken Identity - though in a decidedly non-hilarous manner. Charlotte and Clare are forced to figure out how to live each other's lives while not revealing to anyone what's going on.
 * Revised Ending - in the 1985 version of the book a few scenes are removed, including an emotional one at the end.
 * Time Travel
 * Trapped in the Past
 * World War One