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Temps Atomique International (TAI) or International Atomic Time is a very accurate and stable time scale. It is an average of the time kept by many caesium clocks (atomic clocks) all over the world, and has been available since 1955.
True high-precision TAI times can only be determined after the fact, as atomic time is determined by the reconciliation of the observed differences between an ensemble of atomic clocks maintained by a number of national time bureaus. This is done under the auspices of the International Bureau of Weights and Measures. However, atomic clocks are so accurate that only the most precise time computations need to use these corrections, and most time service users use atomic clocks that have been previously referenced to TAI to estimate TAI times for most purposes.
TAI was synchronised with Universal Time such that TAI = UT1 on January 1 1958. TAI runs independently of Universal Time (which is really a measure of the Earth's revolutions), so the synchronisation is not maintained.
Coordinated Universal Time (UTC) is the basis for legal time worldwide and follows TAI exactly except for an integral number of seconds, presently (mid 2005) equal to 32. These leap seconds are inserted on the advice of the International Earth Rotation and Reference Systems Service (IERS) to keep UTC synchronised with UT1. The leap seconds ensure that, on average over the years, the Sun is overhead within 0.9 seconds of 12:00:00 UTC on the meridian of Greenwich. Civil time in various regions around the Earth generally differs from UTC according to the time zone; mostly the increments are in whole hours, but in a few regions, such as parts of India as well as French Polynesia, and Newfoundland, half-hour increments are used. Nepal and parts of Australia and New Zealand have whole hour plus 45 minutes offsets from UTC.
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