Time technicians at the National Institute of Standards & Technology (Formerly the National Bureau of Standards) set a new level of precision in 1949 by inventing the atomic clock. It counted the oscillations of the nitrogen atom in an ammonia molecule--and was reliable to within one second in three years.
More recently, NIST switched to an atomic clock based on the vibrations of cesium atoms. It will need 300,000 years to gain or lose a single second. But NIST scientists are working on a still-better model: a single mercury ion will be trapped in a vacuum by laser beams and cooled to its lowest possible energy level. The atoms oscillations will then be so stable that the new timepiece should be accurate to within one second in 10 billion years--the total life span of stars similar to our sun.