Into the Dim by Janet B Taylor

When fragile, sixteen-year-old Hope Walton loses her mom to an earthquake overseas, her secluded world crumbles. Agreeing to spend the summer in Scotland, Hope discovers that her mother was more than a brilliant academic, but also a member of a secret society of time travelers. Trapped in the twelfth century in the age of Eleanor of Aquitaine, Hope has seventy-two hours to rescue her mother and get back to their own time. Along the way, her path collides with that of a mysterious boy who could be vital to her mission . . . or the key to Hope’s undoing.