International Atomic Time International Atomic Time Temps Atomique International TAI or International Atomic Time is a very accurate and stable time scale It is a weighted average of the time kept by about atomic clocks including a large number of caesium atomic clocks in over national laboratories worldwide It has been available since and became the international standard on which UTC is based on January as decided by the General Conference on Weights and Measures CGPM The International Bureau of Weights and Measures is in charge of the realization of TAI.
The highest precision realization of TAI times can only be determined retrospectively as the timescale is defined by periodic comparisons among its participating atomic clocks However these corrections are usually only needed for applications that require nanosecond scale accuracy Most time service users use realtime estimates of TAI provided by atomic clocks that have been previously referenced to the composite timescale GPS is a commonly used realtime source of time traceable back to TAI.
Coordinated Universal Time UTC is the basis for legal time worldwide and always differs from TAI by an integral number of seconds In mid UTC was behind TAI by seconds The difference is due to leap seconds which are periodically inserted into UTC due to slight irregularities in the Earth's rate of rotation While TAI is a continuous and stable timescale UTC has intentional discontinuities to keep it from drifting more than seconds from a timescale defined by the Earth's rotation Roughly speaking solar noon the time at which the sun is directly overhead would drift away from without leap second corrections is computed by the International Earth Rotation and Reference Systems Service IERS TAI was defined such that TAI on January.
Because UTC is a discontinuous timescale it is not possible to compute the exact time interval elapsed between two UTC timestamps without consulting a table that describes how many leap seconds occurred during that interval Therefore many scientific applications that require precise measurement of long multi year intervals use TAI instead TAI is also commonly used by systems that can not handle leap seconds.
See also
 Terrestrial Time
 Coordinated Universal Time
 Universal Time
 Sidereal Time
 Time and frequency transfer
 Clock synchronization
 Network Time Protocol
External links
 Bureau International des Poids et Mesures
 website
 NIST Time and Frequency FAQs