Leap second

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

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation. The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently would run ahead of observed solar time unless it is reset to UT1 as needed. The leap second facility exists to provide this adjustment. The leap second was introduced in 1972. Since then, 27 leap seconds have been added to UTC, with the most recent occurring on December 31, 2016.[1] All have so far been positive leap seconds, adding a second to a UTC day; while it is possible for a negative leap second to be needed, one has not happened yet.

Because the Earth's rotational speed varies in response to climatic and geological events,[2] UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS), to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 seconds.[3][4]

This practice has proven disruptive, particularly in the twenty-first century and especially in services that depend on precise timestamping or time-critical process control. And since not all computers are adjusted by leap-second, they will display times differing from those that have been adjusted.[5] After many years of discussions by different standards bodies, in November 2022, at the 27th General Conference on Weights and Measures, it was decided to abandon the leap second by or before 2035.[6][7]

  1. ^ Martin, Cassie (19 January 2024). "50 years ago, timekeepers deployed the newly invented leap second". 50 Years Ago. Science News. p. 4.
  2. ^ "IERS science background". Frankfurt am Main: IERS. 2013. Archived from the original on 29 August 2016. Retrieved 6 August 2016.
  3. ^ Gambis, Danie (5 January 2015). "Bulletin C 49". Paris: IERS. Archived from the original on 30 May 2015. Retrieved 5 January 2015.
  4. ^ James Vincent (7 January 2015). "2015 is getting an extra second and that's a bit of a problem for the internet". The Verge. Archived from the original on 17 March 2017.
  5. ^ Finkleman, David; Allen, Steve; Seago, John; Seaman, Rob; Seidelmann, P. Kenneth (2011). "The Future of Time: UTC and the Leap Second". American Scientist. 99 (4): 312–319. arXiv:1106.3141. doi:10.1511/2011.91.312. S2CID 118403321.
  6. ^ Cite error: The named reference AFP was invoked but never defined (see the help page).
  7. ^ Cite error: The named reference gibney was invoked but never defined (see the help page).