Atomic Clock

An atomic clock is a time measuring device that measures the rate of decay of an element's isotope to keep time. Certain atomic isotopes decay with an extremely regular and measurable resonance frequency which can be used to mark incremental intervals. Atomic clocks are known for their extremely accurate and precise timekeeping.How Stuff Works: How atomic clocks work

The first atomic clock was built in 1949 at the National Bureau of Standards and used the Cesium-133 atom. In 2008, scientists debuted a new clock based on Strontium. The new clock is accurate to one second in 200 million years, making it more than twice as accurate as previous models.NIST news release: JILA Strontium clock (February 14, 2008)