Leap second
A leap second is a one-second adjustment occasionally inserted into or removed from Coordinated Universal Time (UTC) to account for the gradual slowing of Earth's rotation and maintain synchronization between atomic time and solar time, keeping the difference between UTC and UT1 (a measure of Earth's rotation) within ±0.9 seconds.[1] This discrepancy arises because the mean solar day is slightly longer than 86,400 seconds as defined by the cesium-based atomic second, due to tidal friction and other geophysical effects that cause Earth's rotation to decelerate over time.[2] UTC, established in 1972, combines the precision of International Atomic Time (TAI) with leap second corrections to ensure civil time remains aligned with astronomical observations for applications like navigation and astronomy.[3] The concept of the leap second emerged from international agreements in the 1960s and 1970s to resolve inconsistencies between ephemeris time (based on Earth's orbit) and atomic time, with the first leap second implemented on June 30, 1972.[4] Since inception, 27 positive leap seconds have been added to UTC—typically at irregular intervals averaging about one every 18 months initially but less frequently in recent decades—with the last occurring on December 31, 2016, making UTC 37 seconds behind TAI (the fixed offset of 10 seconds plus 27 leap seconds).[3] These insertions are determined by the International Earth Rotation and Reference Systems Service (IERS), which monitors Earth's rotation using very long baseline interferometry and other techniques, announcing decisions about six months in advance via Bulletin C.[3] No leap seconds have been added since 2016, partly due to a temporary speedup in Earth's rotation observed in recent years, which has even raised the possibility of a negative leap second in the distant future, though none has occurred.[3] Leap seconds are inserted at the conclusion of either June 30 or December 31 (Coordinated Universal Time), extending the final minute of the day to 61 seconds by adding a 23:59:60 UTC, after which the clock advances to 00:00:00 the next day.[5] This procedure, coordinated globally through time signal broadcasts and GPS, ensures minimal disruption, though it requires updates to timekeeping systems worldwide.[6] However, the irregular nature of leap seconds has caused technical challenges, including software bugs and synchronization issues in computing, finance, and telecommunications infrastructure.[4] Ongoing debates about the leap second's practicality have led to international efforts to phase it out; in November 2022, the 27th General Conference on Weights and Measures (CGPM), convened by the International Bureau of Weights and Measures (BIPM), adopted Resolution 4 calling for the suppression of leap seconds no later than 2035, after which UTC would drift gradually from UT1 (approximately 80 seconds by 2135) without discontinuous adjustments, relying instead on periodic offsets if needed for solar alignment.[7] This change aims to enhance the stability of global time standards for modern digital systems while preserving the benefits of atomic precision.[4]Fundamentals
Definition and Purpose
A leap second is an intercalary adjustment of one second added to or subtracted from Coordinated Universal Time (UTC) to keep it within 0.9 seconds of Universal Time 1 (UT1).[3] This mechanism ensures that UTC, which is based on International Atomic Time (TAI) adjusted by leap seconds, remains synchronized with the irregular rotation of the Earth as measured by UT1.[8] The primary purpose of leap seconds is to preserve UTC as a practical time scale that closely approximates mean solar time for applications in astronomy, navigation, and civil life, while upholding the uniform precision of atomic clocks.[9] Without such adjustments, the steady drift caused by atomic time running faster than Earth's rotation would cause civil time to diverge from solar events, such as day length and seasonal cycles.[10] Leap seconds were first introduced in 1972 to counteract the discrepancies emerging from the adoption of atomic time standards over ephemeris time tied to Earth's rotation.[11] For instance, by incorporating leap seconds, UTC maintains the alignment where 12:00:00 noon UTC approximates solar noon at the Greenwich meridian.[12]Relation to Time Standards
International Atomic Time (TAI) serves as a continuous time scale based on the uniform second defined by cesium atomic clocks, providing a stable reference without interruptions for astronomical adjustments.[3] Coordinated Universal Time (UTC) builds upon TAI by incorporating leap seconds to align with solar time, while Universal Time 1 (UT1) represents the irregular rotation of Earth, measured through astronomical observations.[10] The precise relationship between these scales is UTC = TAI − (10 + LS), where 10 seconds is the fixed offset established on January 1, 1972, when UTC was initialized, and LS denotes the cumulative number of leap seconds inserted into UTC since then.[3] As of November 2025, 27 leap seconds have been inserted, resulting in a current difference of TAI − UTC = 37 seconds.[3] This adjustment ensures UTC remains synchronized with UT1 within a tolerance of ±0.9 seconds, preventing excessive drift between atomic and astronomical time.[10] Leap seconds are governed internationally by the International Telecommunication Union (ITU), which sets the standards for time signal dissemination, and the International Earth Rotation and Reference Systems Service (IERS), which monitors Earth's rotation and announces necessary insertions approximately six months in advance.[13] The IERS calculates the UT1−UTC difference using data from global observatories to maintain the ±0.9-second limit, ensuring UTC's reliability as a hybrid scale.[13] UTC, with its leap second mechanism, forms the foundation for civil timekeeping worldwide, serving as the legal standard in most countries for daily schedules, contracts, and official records. It is also essential for broadcasting time signals via radio and satellite systems, enabling precise coordination in telecommunications and navigation.Historical Development
Origins and Early Proposals
The recognition of Earth's irregular rotation dates back to observations of tidal friction and geophysical events that cause variations in the length of the day, prompting the need for a more uniform time scale independent of rotation. In the early 20th century, astronomers noted that the mean solar day was not constant due to these factors, leading to proposals for a new standard based on Earth's orbital motion rather than rotation. In 1952, the International Astronomical Union (IAU) adopted Ephemeris Time (ET), proposed by figures such as André Danjon and Gerald Clemence, as a uniform scale defined by the tropical year at 1900.0, intended to replace Universal Time for scientific ephemerides while maintaining alignment with solar phenomena.[14][15] The 1960s marked a pivotal shift with the development of highly precise atomic clocks, beginning with Louis Essen's construction of the first practical cesium beam atomic clock in 1955 at the National Physical Laboratory in the UK, which achieved unprecedented stability compared to astronomical timekeepers. This innovation enabled the calibration of atomic time against astronomical standards, highlighting the superior uniformity of atomic seconds over Earth's variable rotation. In 1967, the 13th General Conference on Weights and Measures (CGPM) defined the SI second in terms of the cesium-133 hyperfine transition, establishing the foundation for International Atomic Time (IAT), a precursor to TAI computed from multiple atomic clocks starting in October 1967. Debates at the 1967-1968 CGPM centered on reconciling the precision of atomic time with the need for civil time to remain aligned with solar day lengths, weighing scientific accuracy against practical synchronization with noon.[16][17][10][18][19] By 1970, the International Telecommunication Union (ITU) Radiocommunication Sector recommended an adjustable Coordinated Universal Time (UTC) scale, incorporating atomic seconds while allowing leap second insertions to keep UTC within 0.1 seconds of UT1, addressing the growing divergence between atomic and astronomical time. This proposal built on Essen's suggestions for occasional adjustments to maintain phase between the two systems. In the late 1960s, initial trials involved ephemeral step adjustments of up to 0.1 seconds and slight variations in second length (frequency numbering) in international radio broadcast signals, such as those from the UK and US, to coordinate time transmissions and keep signals aligned with Universal Time without disrupting uniformity. These experiments paved the way for the formal leap second mechanism adopted in 1972.[20][17][21][22]Implementation Timeline and List
The leap second system was officially adopted on January 1, 1972, establishing Coordinated Universal Time (UTC) with an initial 10-second offset from International Atomic Time (TAI) to align civil time with Earth's rotation while maintaining atomic precision.[3] The first adjustment occurred shortly thereafter, marking the start of periodic insertions to account for discrepancies. Since 1972, 27 positive leap seconds have been inserted into UTC, with the most recent on December 31, 2016, bringing the total TAI-UTC difference to 37 seconds.[23] No further insertions have occurred as of November 2025. The complete list of insertion dates and corresponding cumulative TAI-UTC offsets is as follows:| Insertion Date | TAI-UTC (seconds) |
|---|---|
| 1972-06-30 | 11 |
| 1972-12-31 | 12 |
| 1973-12-31 | 13 |
| 1974-12-31 | 14 |
| 1975-12-31 | 15 |
| 1976-12-31 | 16 |
| 1977-12-31 | 17 |
| 1978-12-31 | 18 |
| 1979-12-31 | 19 |
| 1981-06-30 | 20 |
| 1982-06-30 | 21 |
| 1983-06-30 | 22 |
| 1985-06-30 | 23 |
| 1987-12-31 | 24 |
| 1989-12-31 | 25 |
| 1990-12-31 | 26 |
| 1992-06-30 | 27 |
| 1993-06-30 | 28 |
| 1994-06-30 | 29 |
| 1995-12-31 | 30 |
| 1997-06-30 | 31 |
| 1998-12-31 | 32 |
| 2005-12-31 | 33 |
| 2008-12-31 | 34 |
| 2012-06-30 | 35 |
| 2015-06-30 | 36 |
| 2016-12-31 | 37 |