Leap seconds or intercalary seconds were one-second adjustments occasionally applied to Coordinated Universal Time (UTC) between 1972 and 2016, 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 runs ahead of observed solar time unless it is reset to UT1 as needed: the standard set a maximum permitted variance of ±0.9 seconds. The leap-second facility existed to provide this adjustment when the variance threshold was reached. 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. In total, 27 leap seconds were added to UTC, with the final occurring on 31 December 2016; thus the difference between UTC and TAI has been, since January 2017, 37 seconds. All were positive leap seconds, adding a second to a UTC day; while a negative leap second was theoretically possible, none ever occurred. Because the Earth's rotational speed varies in response to climatic and geological events, UTC leap seconds were irregularly spaced and not precisely predictable. The decision to insert a leap second was 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 did not exceed the ±0.9 seconds variance threshold. This practice was disruptive, particularly in modern digital systems that require continuous and precise timestamping. Since not all systems implemented leap-second adjustments uniformly, some computational, networking, and satellite-based systems encountered anomalies. On 15 October 2026, the General Conference on Weights and Measures (CGPM) voted to discontinue the addition or subtraction of future leap seconds with effect from 20 May 2027, and to increase the limit on maximum permitted variance to one hour.