The uncertainty of NIST-F1 is less than 2 x 10-15, which means it would neither gain nor lose a second in 20 million years! The graph above shows how NIST-F1 compares to
previous atomic clocks built by NIST. It is approximately three times more accurate than NIST-7, a cesium beam atomic clock that served as the United State's primary time and frequency standard from 1993-1999.