International Atomic Time (TAI)

In 1967, the Thirteenth General Conference of Weights and Measures adopted a resolution to replace the astronomical definition of the second - the second is the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of Caesium-133 atom.  The International Atomic Time (TAI) is the international reference scale of atomic time based on the seconds as defined above in the International System of Units (SI).

It is in the form of a continuous scale, i.e. in days, hours, minutes and seconds from the origin 1958 January 1 d 0 h 0 min 0 s.  The atomic time scale is accurate to a few billionths of a second as compared to a few thousandths of a second per day in Universal Time.