NIST-F2 is a caesium fountain atomic clock that, along with NIST-F1, serves as the United States' primary time and frequency standard. NIST-F2 was brought online on 3 April 2014.
NIST-F1, a cesium fountain atomic clock used since 1999, has a fractional inaccuracy (ôf / f) of less than .
The planned performance of NIST-F2 is ôf / f < . At this planned performance level the NIST-F2 clock will not gain or lose a second in at least 300 million years.
The evaluated accuracy (u<sub>B</sub>) reports of various primary frequency and time standards are published online by the International Bureau of Weights and Measures (BIPM). The first in-house accuracy evaluation of NIST-F2 reported a u<sub>B</sub> of . In March 2014 and March 2015 the NIST-F2 cesium fountain clock reported a u<sub>B</sub> of in the BIPM reports of evaluation of primary frequency standards.
The last submission of NIST-F1 to BIPM TAI was February 2016.
At the request of the Italian standards organization, NIST manufactured many duplicate components for a second version of NIST-F2, known as IT-CsF2 to be operated by the (INRiM), NIST's counterpart in Turin, Italy. As of February 2016 the IT-CsF2 cesium fountain clock started reporting a u<sub>B</sub> of in the BIPM reports of evaluation of primary frequency standards.