Time Twister — Frank Asch

Journal # 3 of the Cardboard Genius finds Alex Archer's powers of invention tested once again. His intergalactic spaceship, Star Jumper, is ready for deep space travel, but his co-pilot Zoe Breen finds a glitch: When Star Jumper returns from her voyage, more than fifty years will have passed on earth! It's time for Alex to invent a time machine. Its first live subject will be Alex's little brother, Jonathan! The imp's been cooperative since Alex promised he could come on the voyage -- a promise Alex has every intention of breaking. Time travel proves to be full of serious mishaps -- one of which threatens the very existence of the universe! And little Jonathan proves that he's learned a thing or two from his older brother and invents a time-bending device of his own. But once time is twisted, will the Cardboard Genius be able to twist it back?