UTC (Coordinated Universal Time)
utc
The primary time standard by which the world regulates clocks and time, serving as the basis for civil timekeeping globally.
TAI (International Atomic Time)
tai
Definition
TAI (Temps Atomique International, or International Atomic Time) is a timescale produced by statistically combining data from approximately 450 atomic clocks maintained by over 80 national metrology laboratories. The International Bureau of Weights and Measures (BIPM) in Paris performs the computation. TAI is a continuous timescale with no leap seconds, running from its epoch of 1 January 1958 00:00:00 (UT2).
Relationship to UTC
UTC is derived from TAI by applying leap-second adjustments. As of 2026, UTC = TAI - 37 seconds. This offset is the sum of 27 leap seconds inserted since 1972 and the initial 10-second difference established when UTC was introduced. In a 2022 resolution, the General Conference on Weights and Measures (CGPM) decided that the maximum permitted difference between UT1 and UTC will be widened by 2035. The new limit and its implementation date were still undecided as of September 2026, but once the limit is widened, leap second insertions will effectively end and the TAI-UTC difference in force at that time will be retained.
How It Is Computed
TAI is calculated retrospectively. Data from participating clocks are collected over roughly one month, then processed statistically and published in a monthly bulletin called Circular T. No real-time TAI signal is broadcast; instead, each laboratory maintains its own real-time realization of UTC, called UTC(k), and that is what serves as a practical time reference. The real-time counterpart on the TAI side is TAI(k), which currently equals UTC(k) plus 37 seconds. In Japan, NICT maintains UTC(NICT) as the local realization of the standard.
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Related Terms
Atomic Clock
atomic clock
An atomic clock measures time by using the quantum-mechanical transition frequency of atoms as its reference, achieving accuracies on the order of one second in hundreds of millions of years and forming the backbone of modern timekeeping.
Leap Second
leap-second
A one-second adjustment applied to UTC to keep it aligned with the Earth's irregular rotation.
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