The Bulletin C announce leap second. It is sent by email two times a year :

**https://hpiers.obspm.fr/iers/bul/bulc/Leap_Second.dat**

Be careful : The limit date of validity of a bulletinc C or Leap_Second.dat is "nearly" one year :

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 responsability 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.

- At the beginning of january to announce if there will be or will not be a leap second on 30 june at midnight
- At the beginning ot july to announce if there will ve or will not be a leap second on 31 december at midnight

Be careful : The limit date of validity of a bulletinc C or Leap_Second.dat is "nearly" one year :

- The limit date of validity of the bulletin C/Leap_second.dat published at the beginning of january is 28 december
- The limit date of validity of the bulletin C/Leap_second.dat published at the beginning of july is 28 june

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 responsability 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.