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 cesium 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 bureaux. 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.
Coordinated Universal Time (UTC) is the basis for legal time worldwide and follows TAI (see below) exactly except for an integral number of seconds, presently 32. These leap seconds are inserted on the advice of the International Earth Rotation Service (IERS) (website) to 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