Time Bomb, the first of the two Ultra Thrillers, was published in 1992.

From the back cover: A dream that has long fired the human imagination has become reality: time travel. But as Tom Swift and the Hardy boys are about to discover, the dream can become a nightmare in the blink of an eye. An attack force of techno-thugs, under the command of the evil genius the Black Dragon, have seized control of a top-secret time-warp trigger! Frank and Joe and Tom leap into battle - shock troops in a war that stretches across the aeons to the edge of time. But whether chasing asteroids or dodging dinosaurs, they know they haven't a moment to lose. They must stop the Dragon before he carries out his final threat: turning the time machine into the ultimate doomsday device!

The book makes use of a tried-and-true sci-fi time-travel cliche: the first and last chapters begin with exactly the same text. But hey, I guess a sci-fi formula plot is a welcome change from a Casefiles formula plot...

A final note: Whoever packaged the book must've been a Hendrix fan - the text clip on the first page is titled "Purple Haze."