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Leap Seconds
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Leap Seconds

A leap second is a one second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep UTC within 0,9 seconds of mean solar time.

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

30 June 197231 December 197231 December 197331 December 1974
31 December 197531 December 197631 December 197731 December 1978
31 December 197931 June 198130 June 198230 June 1983
30 June 198531 December 198731 December 198931 December 1990
30 June 199230 June 199330 June 199431 December 1995
30 June 199731 December 199831 December 200531 December 2008
30 June 201230 June 201531 December 2016

Proposal to abolish leap seconds

The irregulatiry and unpredictability of leap seconds is problematic for several areas, especially computing. To calculate the number of elapsed seconds correctly, a leap second loopup table must be maintained. It is not possible to correctly calculate the time interval for a future date more than six months into the future.

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.