The most precise wall clock is likely a cesium atomic clock. These clocks use the resonant frequency of cesium atoms to measure time and can be accurate to within a billionth of a second. The most accurate of these clocks are the National Institute of Standards and Technology (NIST) F-1 and NIST-F2 Cesium Fountain Atomic Clocks. These clocks are accurate to within one second every 20 million years.