**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).