is the time scale that is used worlwide to coordinate technical and scientific
activities. It is a compromise between the highly stable atomic time and
the irregular Earth rotation.
Because of the secular decelleration of the Earth's rotation, TAI, of which the unit of time, corresponds the second of the mean solar day of the epoch 1820, presents an continuous increasing (parabolic) shift with respect to UT1. If legal time was based upon TAI, coincidence with solar day could not be maintained (in a couple of year TAI - UT1 can increase by a few seconds). Therefore international community, for reckoning time, used the intermediate time scale UTC.
UTC is defined by the CCIR Recommendation 460-4 (1986). It differs from
TAI by an integer number of seconds, in such a way that UT1-UTC stays smaller
than 0.9 s in absolute value. The decision to introduce a leap second in
UTC to meet this condition is the responsibility of IERS (Bulletin
C). According to the CCIR Recommendation, first preference is given
to the opportunities at the end of December and June, and second preference
to those at the end of March and September. Since the system was introduced
in 1972 only dates in June and December have been used.