The end of the leap second: CGPM Resolution 5 of 2026

Published 2026-10-20

At its 28th meeting, held in Versailles from 13 to 15 October 2026, the General Conference on Weights and Measures (CGPM) adopted Resolution 5, “On the technical actions needed to ensure the continuity of UTC”. It sets a date for the end of the leap second.

What was decided

• Continuous UTC takes effect on 20 May 2027.
• The maximum allowed difference between UTC and UT1, the time scale that follows the rotation of the Earth, becomes 3,600 seconds (one hour). According to the resolution, that keeps UTC continuous “for several centuries”.

What a leap second was

UTC, the time on your phone and computer, is kept by atomic clocks. The Earth does not turn at a perfectly steady rate, so atomic time and the time of the Earth’s rotation slowly drift apart. Since 1972 that gap was kept below 0.9 seconds by inserting a leap second when needed. 27 leap seconds were added, the last one at the end of 31 December 2016.

Why now

In 2022 the CGPM had already decided (Resolution 4) that the allowed difference would be increased in, or before, 2035. Resolution 5 brings that forward. It cites a workshop of the Consultative Committee for Time and Frequency (CCTF) and the IERS in March 2025, which estimated that the probability of a negative leap second, a second taken away instead of added, rises quickly and reaches 30% by 2035. A negative leap second has never been applied, and the resolution says it poses a high risk of causing anomalies and disruption to critical infrastructures.

What it means for your age

Nothing changes in how many years, months and days old you are. The age calculator counts with the clock of your device, and most computer clocks, including the JavaScript the calculator runs on, do not count leap seconds: in their reckoning every minute has 60 seconds. If you were born before 2017, the official count of seconds you have lived includes up to 27 leap seconds that such a clock does not show. From 20 May 2027 no new ones are added.

Source: Resolution 5 of the 28th CGPM (2026), BIPM.