Empire of Time — Daniel Godfrey

For fifteen years, the Romans of New Pompeii have kept the outside world at bay with the threat of using the Novus Particles device to alter time. Yet Decimus Horatius Pullus -- once Nick Houghton -- knows the real reason the Romans don't use the device for their own ends: they can't make it work without grisly consequences.
This fragile peace is threatened when an outsider promises to help the Romans use the technology. And there are those beyond Pompeii's walls who are desperate to destroy a town where slavery flourishes. When his own name is found on an ancient artifact dug up at the real Pompeii, Nick knows that someone in the future has control of the device. The question is: whose side are they on?