Leap second

A leap second (sometimes called intercalary second) is a one-second adjustment occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between International Atomic Time (TAI), as measured precisely by atomic clocks, and 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 (at that time UTC already lagged behind TAI by 10 seconds because of earlier methods of managing the difference). Since then, 27 leap seconds have been added to UTC, with the most recent occurring on 31 December 2016; thus the difference between UTC and TAI is now 37 seconds. All have so far been positive leap seconds, adding a second to a UTC day; while a negative leap second is theoretically possible, it has not yet occurred.

Because the Earth's rotational speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and not precisely predictable. The decision to insert a leap second is made by the International Earth Rotation and Reference Systems Service (IERS), typically about six months in advance, to ensure that the difference between UTC and UT1 does not exceed ±0.9 seconds.

This practice has proven disruptive, particularly in modern digital systems that require continuous and precise timestamping. Since not all systems implement leap-second adjustments uniformly, some computational, networking, and satellite-based systems may encounter anomalies.