Look at this, the NIST-F1 clock would neither gain nor lose a second in 60 million years !!!

NIST-F1 is a caesium fountain atomic clock in the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and serves as the United States' primary time and frequency standard. The clock took fewer than four years to test and build, and was developed by Steve Jefferts and Dawn Meekhof of the NIST physics lab.
The clock replaces NIST-7, a cesium beam atomic clock used from 1993 to 1999. NIST-F1 is ten times more accurate than NIST-7. NIST-F1 will be replaced by the NIST-F2.
Similar atomic fountain clocks, with comparable accuracy, are operated by other time and frequency laboratories, such as the Paris Observatory and the Physikalisch-Technische Bundesanstalt in Germany.
[edit] Accuracy
As of summer 2005, the clock's uncertainty was 5 x 10-16. It would neither gain nor lose a second in 60 million years.
http://en.wikipedia.org/wiki/NIST-F1