Timekeeper Conspiracy (Time Wars) by Simon Hawke

Pre-Loved Paperback At first the discovery of minus time seemed like a blessing. Countries at war could fight their battles in the past rather than destroy the present. But now the situation was desperate. Humanity faced the danger of a temporal split, with incalculable consequences, as a result of attempts to change the past. So Captain Lucas Priest, a seasoned veteran of the Temporal Corps, was dispatched to seventeeth-century France. In the world of Cardinal Richelieu and the Three Musketeers, he must infiltrate the terrorist underground called the Timekeepers and put an end to their mad schemes forever...whatever forever means!.