Say you meet a mad-scientist type who tells you he has discovered a time machine and offers you the chance to go back to Hollywood in its glamorous 1930s heyday, but on one condition: You prevent Ronald Reagan from becoming President. Would you do it? Gabriel Prince, 40ish single travel writer, snatches the opportunity — and does indeed cause Reagan’s early death in Time on My Hands by Peter Delacorte. But the question at the heart of the book — Are there parallel universes of time where different events are occurring simultaneously, producing different outcomes and different worlds? — is never satisfactorily addressed, resulting in more of a brainteaser than a novel. B