What is International Atomic Time (TAI)?

What is International Atomic Time (TAI)?

International Atomic Time — abbreviated TAI, from the French Temps Atomique International — is the continuous timescale that the world's atomic clocks keep together, counting the international second steadily forward without ever stopping or stepping.³ It is the physical reference against which other civil and scientific timescales are defined, but it is rarely shown on a clock face directly; most people meet it only through UTC, which is built from it.

The unit TAI counts is the SI second — the second of the International System of Units — defined by a fixed property of the caesium-133 atom rather than by the motion of the Earth or any astronomical body.⁴ That choice is what gives TAI its defining quality: every TAI second is the same length as every other, to a precision far beyond anything the Earth's rotation can offer.

TAI is not the reading of any single clock. It is a weighted average of hundreds of atomic clocks spread across dozens of national timing laboratories, combined into one scale that is steadier than any individual clock could be on its own.¹³ Its count was started so that it agreed with the time told by the Earth's rotation at the beginning of 1958, and it has ticked forward on atomic seconds ever since — drifting, slowly and inevitably, away from rotational time as the Earth's spin fails to keep perfect pace with the atoms.⁷

How does TAI differ from UTC?

TAI and UTC tick at exactly the same atomic rate, but UTC is held close to the Earth's rotation by occasional one-second steps, while TAI runs straight through without them.¹⁸ UTC is therefore TAI with a whole number of leap seconds subtracted: the two scales never run at different speeds, they only differ by an accumulated integer offset.⁸

That offset is 37 seconds in 2026. It is the sum of two parts: a 10-second seed value set when the modern UTC system began at the start of 1972 — carrying forward the drift that had built up between the atoms and the Earth since 1958 — and the 27 leap seconds inserted since.⁷⁶ Each leap second adds one more to the count; the underlying ticks never change.

The reason UTC needs those steps at all is that the Earth's rotation is slightly irregular, while atomic time is not. An international standard for time-signal broadcasts requires UTC to stay within 0.9 seconds of UT1, the timescale defined by the Earth's actual rotation, and a leap second is inserted whenever the two are about to drift further apart than that.⁸ TAI is under no such obligation, so it simply accumulates the difference.

The most recent bulletin from the body that monitors the Earth's rotation confirms that the offset holds at 37 seconds: no leap second will be inserted at the end of June 2026, so a TAI clock continues to read 37 seconds ahead of UTC until further notice.²

How is the SI second defined?

The SI second is defined by the caesium-133 atom: it is the duration of 9,192,631,770 cycles of the microwave radiation that makes a caesium atom switch between the two energy levels of its ground state.⁴ The current International System of Units states the definition by fixing that frequency — written ΔνCs — at exactly 9,192,631,770 hertz, so the second is no longer derived from any astronomical observation at all.⁴

Tying the second to an atom rather than to the Earth's spin was the change that made a scale like TAI possible. The Earth's rotation wanders by milliseconds within a single year and slows over centuries; a caesium resonance does not. An atomic clock realises this definition so reliably that the second became, in practice, the most precisely measured of all the base units — and the steady tick that TAI exists to count.⁴

Who maintains TAI, and how is it computed?

TAI is computed by the Bureau International des Poids et Mesures (BIPM) — the intergovernmental measurement bureau outside Paris that also maintains the definitions of the metre, the kilogram, and the second — from clock data sent in by national timing laboratories.³¹ No single laboratory owns TAI; it is a cooperative product of the whole network.

Each contributing laboratory keeps its own bank of atomic clocks and its own working realisation of UTC, written UTC(k) — UTC(NIST) at the United States' national measurement institute, for example. The bureau gathers the readings of all those clocks and forms a weighted average, giving more weight to the clocks that have proved most stable, to produce a single timescale steadier than any one of them.¹³ The mechanics of the weighting are intricate, but the idea is simple: averaging many good clocks cancels the small, independent errors of each.

The bureau publishes the results regularly, listing how each laboratory's clock differs from the official scale.³ The national realisations track each other and the official scale closely — typically to within billionths of a second — so that "UTC" measured in one country and another refer, for all practical purposes, to the same instant.¹

How does GPS time relate to TAI?

GPS time runs a constant 19 seconds behind TAI — GPS time equals TAI minus 19 seconds — and that gap never changes.⁵ The reason is that GPS time, like TAI, is a continuous atomic scale with no leap seconds; the only difference between them is a fixed offset set once and never touched.⁵

The offset is an accident of history. The GPS clock was started at the very beginning of 6 January 1980, a moment when TAI was already 19 seconds ahead of UTC, and because GPS time has carried no leap seconds since, it has held that 19-second gap to TAI ever since.⁵⁶ UTC, meanwhile, has gone on collecting leap seconds, so GPS time now leads UTC by 18 seconds — the 37-second TAI offset minus the 19-second head start.¹⁵

This is why a satellite-navigation receiver has to apply a small correction to turn its internal time into civil time: the navigation signal carries the current leap-second offset so the receiver can convert continuous GPS time into UTC.⁵ For the navigation itself, the continuity matters more than the calendar — a positioning system that timed signals to billionths of a second could not tolerate a clock that occasionally repeats or skips a second.

Why isn't civil time broadcast as TAI directly?

Civil time is broadcast as UTC, not TAI, because civil time is meant to follow the Sun, and TAI drifts away from it.¹ Atomic seconds are perfectly steady, but the Earth's rotation is not, so a clock kept on pure atomic time would fall further and further out of step with day and night over the decades — noon on a TAI clock would slowly creep away from the Sun's highest point.

UTC is the compromise that gives both properties at once. It ticks atomic seconds, so it is a stable physical reference, and it is nudged by leap seconds to stay within 0.9 seconds of the Earth's rotation, so it stays anchored to mean solar time and the cycle of day and night.⁸ That is the version of atomic time that broadcasts, computers, and the world's time zones actually use.

TAI and GPS time are reserved for the places where an unbroken count of seconds matters more than alignment with the Sun: astronomy, satellite navigation, and software that needs a strictly forward-moving clock. Code that must count seconds without the backward jumps a leap second causes generally works in atomic time rather than the civil time of Unix time.¹

That balance is about to shift. The world's measurement bodies voted in November 2022 to retire the leap second in or before 2035, widening the tolerance so that no further leap seconds are needed for at least a century.⁹ After that, UTC will be allowed to drift slowly away from the Earth's rotation — moving it closer in behaviour to a pure atomic scale like TAI, though it will keep the 37-second offset it has already banked.⁹

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