The relative frequency stability and the accuracy of atomic time scales, like International Atomic Time TAI, is now of order 1 × 10-15 thanks to progresses in clock technology and in clock comparison techniques. Cold atom primary Cs standards have a stated accuracy of 1 × 10-15 and a stability in the 10-16 region. Other cold atom clocks provide even better prospects, as well as clocks based on trapped ions. Frequencies based on optical and microwave transitions can now be compared with a similar or even better uncertainty thanks to femtosecond comb technology. Clock comparison techniques based on GPS (see http://maia.usno.navy.mil/gpst.html), or on dedicated Two Way technology provide adequate performance when averaging data over one or a few days, and should be improved to accompany the progresses of clocks.

Since 1999, a number of organizations initiated a review on the future of the UTC system (insertion of leap seconds between TAI and UTC to keep |UT1 – UTC| < 0.9s). Several working groups have been initiated, notably by the International Telecommunications Union (Special Rapporteur Group (SRG) in the Working Party 7A), by the International Union of Radio Science, and by the IAU following Resolution B2(2000). No immediate conclusion may be foreseen but a consensus should be reached over the next triennium.