A decent quartz watch can easily out-perform a certified chronometer's automatic movement by only losing or gaining a second per day, but a cesium atomic clock is pretty much infallible. Since a second is literally defined as the time it takes for a cesium 133 atom to vibrate 9,192,631,770 times, actually measuring those vibrations directly instead of approximating a quartz equivalent yields perfect results. While there's some quartz watches that sync with the U.S. atomic clock, there's only one watch that actually has a built-in atomic clock: the Bathys Hawaii Cesium 133.
According to our friends at Hodinkee, this is the very first watch of its kind. Inside the considerably large housing — but small when you think about what's going in there — the Hawaiian watchmakers managed to fit a chip-scale atomic clock connected to a standard quartz watch movement.