Atomic Time: TAI

International Atomic Time TAI is a laboratory timescale. Its unit is the SI second, which is defined in terms of a defined number of wavelengths of the radiation produced by a certain electronic transition in the caesium 133 atom. It is realized through a changing population of high-precision atomic clocks held at standards institutes in various countries. There is an elaborate process of continuous intercomparison, leading to a weighted average of all the clocks involved.

Though TAI shares the same second as the more familiar UTC, the two timescales are noticeably separated in epoch because of the build-up of leap seconds. At the time of writing, UTC lags about half a minute behind TAI.

For any given date, the difference TAI-UTC can be obtained by calling the SLALIB routine sla_DAT. Note, however, that an up-to-date copy of the routine must be used if the most recent leap seconds are required. For applications where this is critical, mechanisms independent of SLALIB and under local control must be set up; in such cases sla_DAT can be useful as an independent check, for test dates within the range of the available version. Up-to-date information on TAI-UTC is available from ftp://maia.usno.navy.mil/ser7/tai-utc.dat.