Doing Time by Jodi Taylor

At some time in the future, the secret of time-travel became available to all. Chaos ensued as people sought to take advantage. Because there will always be nutters who want to change history… And so the Time Police was formed. Internationally sanctioned thugs whose task it was to keep the timeline straight by any and all means possible. And they succeeded. The Time Wars are over. The Time Police won. But who will win the peace?