At the 13th meeting of the CGPM (1967), the current definition for the duration of a second was defined. The definition fixes the length of the second in terms of a characteristic of the caesium atom and implies that the length of a second is related to the length of the mean solar day in 1900.
When atomic time was adopted, some communities of users, in particular those using celestial navigation, requested that atomic time be synchronized with the rotation of the Earth. To compensate for the Earth's irregular velocity of rotation, the International Telecommunication Union (ITU) defined in 1972 a procedure for adding (or suppressing) a leap second as necessary, to ensure that the difference between the international time reference and rotational time remained less than 0.9 s. The resulting time scale is Coordinated Universal Time (UTC). This time scale is calculated at the International Bureau of Weights and Measures (BIPM - Bureau International des Poids et Mesures) with the contribution of 69 national institutes that operate about 400 atomic clocks. (The NMISA is one of these institutes.) The CGPM endorsed the definition of UTC.
Because the Earth's rotation speed varies, leap seconds are irregularly spaced and unpredictable. The insertion of a leap second is normally published a few months in advanced by the International Earth Rotation and Reference System Service (IERS).
Leap seconds since 1972
Proposal to abolish leap seconds
Network synchronisation is also difficult at the time of a leap second, since many computer operating systems are not leap second aware. It is possible that computers will have a timing error for many hours after the leap second event.
From about the year 2000, discussions started related to the requirement for a continuous time scale. Working Party 7A of Study Group 7of the International Telecommunication Union (ITU) exhausted technical considerations and studies. Consensus could not be reached on other than technical grounds. The matter was submitted to the Radio Assembly (RA) for resolution. No decision could be reached and it was submitted to the World Radiocommunication Conference 2012 (WRC-12) for discussion. The Conference postponed the decision to WRC-15.
WRC-12 issued Resolution 653 inviting WRC-15 to consider the feasibility of achieving a continuous reference time-scale, whether by modification of Coordinated Universal Time (UTC) or some other method, and take appropriate action, taking into account ITU-R studies. During the WRC-15 (2 to 27 November 2015), it was decided that further studies are required on the impact and application of a future reference time-scale, including the modification of coordinated universal time (UTC) and suppressing the so-called "leap second". The decision by WRC-15 calls for further studies regarding current and potential future reference time-scales, including their impact and applications. A report will be considered by the World Radiocommunication Conference in 2023. Until then, UTC shall continue to be applied as described in Recommendation ITU-R TF.460-6 and as maintained by the International Bureau of Weights and Measures (BIPM). WRC-15 also calls for reinforcing the links between ITU and the International Bureau of Weights and Measures (BIPM). ITU would continue to be responsible for the dissemination of time signals via radiocommunication and BIPM for establishing and maintaining the second of the International System of Units (SI) and its dissemination through the reference time scale.
ITU Press Release, last accessed 30 November 2015.