The Time Shifters

From the back cover: They called it TTT-short for Time Teleportation Technique. Like all really great ideas, it was simple, and easy to operate-too easy, for Chuck Percival, who suddenly found himself drafted into an army dedicated to defending yesterday against today... and captained by strange men from tomorrow! Chuck found the idea of time travel intriguing, but he was by nature suspicious of people wanting to do him favors-and vice-versa. And time travel turned out to have a couple of nasty side effects that made the price of time jumping almost too high to pay. But like it or not, Chuck found himself in this particular army for the duration--a duration that might last several hundred years! What happened to him shouldn't happen to any devout twentieth century coward... including meeting and falling in love with his own great grandmother!