Atomic clocks are the most accurate time and frequency standards known to humanity. Utilizing the hyper-fine transition frequencies of atoms such as Cesium-133 and Rubidium, these devices lose less than one second in tens of millions of years.
The global positioning system (GPS) that guides our modern world relies directly on atomic clocks onboard orbiting satellites. Because radio signals travel at the speed of light, an error of just one microsecond translates to a spatial deviation of 300 meters! Furthermore, Einstein's theories of special and general relativity must be continuously accounted for: time ticks faster at higher gravitational potential, and slower at high orbital velocities.