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Announcement of leap seconds in UTC and information on UTC-TAI.
Leap seconds can be introduced in UTC at the end of the months of December or June, depending on the evolution of UT1-TAI. Bulletin C is mailed every six months, either to announce a time step in UTC, or to confirm that there will be no time step at the next possible date.
TAI is the atomic time scale of BIPM; its unit interval is exactly one SI second at sea level. The origin of TAI is such that UT1-TAI is approximately 0 on 1958 January 1. The instability of TAI is about 6 orders of magnitude smaller than that of UT1. The terrestrial Dynamical Time TDT is presently
defined as TAI + 32.184s. Discussion is taking place in the IAU Working Group on Reference Systems (WGRS) about improved definition of time.
UTC is defined by the CCIR Recommendation 460-4 (1986). It differs from TAI by an integral number of seconds, in such a way that UT1-UTC stays smaller than 0.9s in absolute value. The decision to introduce a leap second in UTC to meet this condition is the responsibility of the IERS. 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.