Have you ever heard of "Atomic Clock" ? Well, heres your description..
An atomic clock is a type of clock that uses an atomic resonance frequency standard as its timekeeping element. They are the most accurate time and frequency standard known, and are used as primary standards for international time distribution services, to control the frequency of television broadcasts, and in global navigation satellite systems such as GPS.
Atomic clocks do not use radioactivity, but rather the precise microwave signal that electrons in atoms emit when they change energy levels. Early atomic clocks were based on masers. Currently, the most accurate atomic clocks are based on absorbtion spectroscopy of cold atoms in atomic fountains such as the NIST-F1.
National standards agencies maintain an accuracy of 10−9 seconds per day (approximately 1 part in 1014), and a precision set by the radio transmitter pumping the masers. The clocks maintain a continuous and stable time scale, International Atomic Time (TAI). For civil time, another time scale is disseminated, Coordinated Universal Time (UTC). UTC is derived from TAI, but synchronized, by using leap years, to UT1, which is based on actual rotations of the earth with respect to the solar time.
And the latest Atomic Clock using cesium standard deviates only one second per 100,000 years!
Sources info from: http://en.wikipedia.org/wiki/Atomic_clock