International Atomic Time (TAI): What It Is and How It Differs From UTC

International Atomic Time (TAI, from the French Temps Atomique International) is the world’s most precise time standard, built purely from atomic clocks with no corrections ever applied. UTC — the standard actually used to set clocks and computers — is derived from TAI, then adjusted with occasional leap seconds to stay roughly in step with the Earth’s rotation.

How TAI is measured

TAI is calculated from a global network of roughly 400 atomic clocks in laboratories worldwide, coordinated by the International Bureau of Weights and Measures (BIPM) — the same network behind UTC. It counts seconds by measuring the vibration frequency of caesium-133 atoms, a rate so stable it drifts by less than a second over tens of millions of years.

Unlike UTC, TAI never has anything added or removed. It’s a pure, continuous count of atomic seconds since its epoch in 1958.

TAI vs. UTC: the leap-second gap

UTC equals TAI minus a whole number of leap seconds — 37, as of the most recent leap second added in 2017. Earth’s rotation is slightly irregular and gradually slowing, so without correction, UTC (tied to atomic time) would slowly drift apart from UT1 (time based on the Earth’s actual rotation). A leap second, inserted occasionally into UTC, keeps that drift within 0.9 seconds.

TAI never receives these adjustments, so the gap between TAI and UTC only grows over time — it can’t shrink, because leap seconds are only ever added, never subtracted.

Where TAI actually gets used

Almost nobody sets a clock to TAI directly. GPS satellites run on GPS Time, a fixed 19-second offset from TAI that — unlike UTC — never gets leap seconds either, which is why GPS Time and UTC have also drifted apart since GPS launched. TAI otherwise mostly matters to metrologists, physicists, and anyone maintaining the systems that keep UTC itself accurate.