The Chronograph By Innes, Harriet

"Life couldn’t be worse for Lorna Lomax. Dumped by her loser boyfriend on the eve of her sixteenth birthday, and at odds with her family and the stepfather she despises, Lorna has never felt more isolated. Then her dead father’s old Chronograph turns out to be a powerful instrument of time, and Lorna is catapulted into the Past to embark on a vital mission; to return the Chronograph to the Court of Time itself. Accompanied by Sebastian, a child of the Past, and Patrick, a mysterious youth from the Future, it should be a simple enough quest; except the evil Keeper of the Hours is in hot pursuit, while Patrick seems intent on accomplishing a mission of his own. Still worse, the Guardian of the Future is missing, leaving the prospect of nothing but barren emptiness as the Present Age ends. With the fabric of Time beginning to fray, the Three are soon faced with a battle to save not only the Chronograph, but Time itself – and nobody's identity is quite what it seems..."