NPL-CsF2

A caesium fountain clock, responsible for keeping United Kingdom's atomic time is declared as the most accurate long-term time keeper in the world.

Scientists claimed the clock would lose or gain less than a second in 138 million years, which makes it the most accurate clock in the world.

http://www.ibtimes.com/articles/204944/20110828/world-s-most-accurate-clock-belongs-to-britain-united-kingdom-time-keeper-long-term-cesium-fountain.htm