The new cesium clock called NIST-F2 is so accurate it isn't meant to gain or lose a second in the next 300 million years, said the US Department of Commerce's National Institute of Standards and Technology (NIST).

The International Bureau of Weights and Measures said the new US atomic clock is now the most accurate time standard in the world. NIST-F2 is expected to lose about one trillionth of a second per day.

NIST-F2 will operate in tandem with its predecessor, NIST-F1, which has been setting the standard for US civilian time since 1999. Both clocks keep time using a "fountain" of cesium atoms to determine the exact length of a second.

NIST-F2 had been in development for a decade and is three times more accurate than the F1, which has been in use since 1999. NIST will continue operating both clocks at its campus in Boulder, Colorado.

Cesium fountain clocks measure the frequency of a specific transition in the cesium atom, which is clocked at 9,192,631,770 vibrations per second. This standard is used to define the amount of time in one second.

The key difference in the new clock and its predecessor is the original F1 clock operates near room temperature and the F2 operates at a much colder minus 316 degrees Fahrenheit (minus 193 Celsius).

The colder operating temperate reduces some of the very small measurement errors that must be corrected for in the F1 clock.

"If we've learned anything in the last 60 years of building atomic clocks, we've learned that every time we build a better clock, somebody comes up with a use for it that you couldn't have foreseen," said NIST physicist Steven Jefferts, lead designer of NIST-F2.