International Atomic Time is the unit interval of the second, and its submultiples, and is realized according to its atomic definition.
By international agreement, the second is the duration of nine billion, one-hundred ninety-two million, six-hundred thirty-one thousand, seven-hundred and seventy periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
Submultiples of the second include a millisecond, which is one-thousandth of a second, a microsecond, which is a millionth of a second, and a nanosecond, which is a billionth of a second.
Other atomic frequency standards include the hydrogen maser, rubidium clocks, and quartz frequency standards, which are controlled by comparison with a cesium standard, either directly, or by means of radio transmissions.
The U.S. atomic time standard is kept by the U.S. Naval Observatory in Washington D.C., and is derived from a master clock consisting of a bank of atomic cesium clocks whose measurements are averaged to produce the most reliable atomic time standard.