Temporal Reference System International Atomic Time https://www.commoncoreontologies.org/InformationEntityOntology TAI TAI is a time standard the values of which are generated by calculating the weighted average of the measurements of more than 400 atomic clocks and is based on the International System of Units (SI) definition of one second equals the time it takes a Cesium-133 atom at the ground state to oscillate exactly 9,192,631,770 times. TAI is extremely precise, but does not perfectly synchronize with Earth times, which is why TAI and UT1 are both used to determine UTC. TAI serves as the main realization of TT.