Time's Divide (The Chronos Files, 3, Band 3)

The Cyrists are swiftly moving into position to begin the Culling, and Kate’s options are dwindling. With each jump to the past or the future, Kate may trigger a new timeline shift. Worse, the loyalties of those around her―including the allegiances of Kiernan and the Fifth Column, the shadowy group working with Kate―are increasingly unclear.
Kate will risk everything, including her life, to prevent the future her grandfather and the Cyrists have planned. But, when time runs out, it may take an even bigger sacrifice to protect the people she loves.