Leap second

Screenshot of the UTC clock from time.gov during the leap second on 31 December 2016

A leap second was the adding one extra second to time sometimes. This was done to fix the small difference between very exact time (kept by atomic clocks) and the less exact solar time (based on the Earth’s rotation). The time system we use (UTC) would get too far ahead of solar time without these fixes. Leap seconds were first used in 1972. From then, 27 leap seconds have been added, the last one on December 31, 2016. All of them added one second. Leap seconds were discontinued in October 2026.

Background

The Earth's rotation speed changes because of weather and natural events. This makes leap seconds hard to predict. A group called the International Earth Rotation and Reference Systems Service (IERS) decides when to add them, usually about six months before, so the difference between UTC and solar time never gets bigger than 0.9 seconds.

Discontinuation

Leap seconds cause problems, especially for systems that need very exact timing. Some computers do not update correctly, so their time can be wrong. After many years of talks, it was decided in November 2022 that leap seconds would be stopped by or before 2035. On 15 October 2026, the General Conference on Weights and Measures (CGPM) voted to officially stop using leap seconds.

References

Citations

  1. ↑ "50 years ago, timekeepers deployed the newly invented leap second". 2024-01-19. p. 4. Retrieved 2025-01-07.
  2. ↑ "IERS - IERS - Earth rotation". www.iers.org. Archived from the original on 2016-08-29. Retrieved 2025-01-07.
  3. ↑ "Bulletin C 49". Paris Observatory IERS Centers. 2015-01-05. Retrieved 2025-01-07.
  4. ↑ Vincent, James (2015-01-07). "2015 is getting an extra second and that's a bit of a problem for the internet". The Verge. Retrieved 2025-01-07.
  5. ↑ "The Future of Time: UTC and the Leap Second". American Scientist. 2017-02-06. Archived from the original on 2022-01-31. Retrieved 2025-01-07.
  6. ↑ France-Presse, Agence (2022-11-18). "Do not adjust your clock: scientists call time on the leap second". The Guardian. ISSN 0261-3077. Retrieved 2025-01-07.
  7. ↑ Gibney, Elizabeth (2022-11-18). "The leap second's time is up: world votes to stop pausing clocks". Nature. 612 (7938): 18–18. doi:10.1038/d41586-022-03783-5.
  8. ↑ "Resolution 5 of the 28th CGPM (2026) On the technical actions needed to ensure the continuity of UTC". Retrieved 15 Oct 2026.