Leap second

The IERS, which decides when one is needed, defines the device simply: "Leap seconds are used to adjust the time scale UTC to the apparent motion of the Sun, i.e. to Earth rotation", the purpose being to hold the difference UT1 minus UTC within 0.9 seconds. BIPM, which produces the time scale, states that "UTC is obtained from International Atomic Time (TAI) by the insertion of leap seconds", made "according to the advice of the International Earth Rotation and Reference Systems Service (IERS)".

Insertions are announced in IERS Bulletin C, issued twice a year by the IERS Earth Orientation Centre. Bulletin C Number 72, published on 6 July 2026, states that "NO leap second will be introduced at the end of December 2026" and gives UTC minus TAI as −37 s, a value in force since 1 January 2017, the date the most recent leap second took effect.

The 2035 decision is narrower than it is usually described. Resolution 4 of the 27th CGPM (2022), "On the use and future development of UTC", decides that "the maximum value for the difference (UT1-UTC) will be increased in, or before, 2035", and asks the CIPM, consulting the ITU, to propose a new maximum value ensuring the continuity of UTC "for at least a century" and to bring a resolution to the 28th CGPM in 2026. What this Codex does not do: say that the CGPM abolished the leap second. The words "leap second" appear only in the preamble, which notes that "the introduction of leap seconds creates discontinuities that risk causing serious malfunctions in critical digital infrastructure".

For a watch the consequence follows from the mechanism. A leap second is inserted into a time scale computed at the BIPM and announced in a bulletin; a watch running on its own oscillator receives nothing, keeps counting undisturbed, and ends one second from civil time until it is reset by hand.