Astronauts on the ISS age ~0.007 seconds less in 6 months—and GPS needs 38 microseconds/day fixes—because time is personal. Einstein shredded the idea of a universal now: at ~99% light speed your clock crawls, muons outlive their microsecond lifetimes, and a 5-year trip can leave your twin 50 years older. Backward travel is the thorny part—Gödel’s closed timelike curves, wormholes, and near-horizon black holes fit the math, but paradoxes and Hawking’s chronology protection may slam the door.