UTC 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.

Universal Time UT1 is the time of the Earth clock, which performs one revolution in about 24h. It has short term instabilities at the level of 10^-8 and the duration of the day is slowly decaying (almost 0.002 s/century). UT1 is one of the products of IERS, based on VLBI observations.

The Temps Atomique International (TAI) is the atomic time scale derived by the Bureau International des Poids et Mesures (BIPM); its unit interval is exactly one second of the Systeme International d'Unites (SI) at sea level. The origin of TAI is such that UT1-TAI was 0 on 1958 January 1. The instability of TAI is about 6 orders of magnitude smaller than that of UT1.

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.

The relationship of UTC with TAI and the corresponding offsets and step adjustments of UTC are available. DUT1 is the difference UT1-UTC, expressed with a precision of 0.1 s, which is broadcasted with the time signals. The changes in DUT1 are announced by IERS (Bulletin D).