A high-precision atomic coordinate time standard, International Atomic Time (TAI) is based on the notional passage of proper time on Earth’s geoid. TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide. It is a continuous scale of time, without leap seconds, and it is the principal realization of Terrestrial Time (with a fixed offset of epoch). It is the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth’s surface and which has leap seconds.
How TAI Works
TAI is maintained by the International Bureau of Weights and Measures. It is based on a network of atomic clocks that measure the time elapsed between two specific points in the orbits of Earth’s moon. TAI is kept within 0.1 microseconds of the atomic time scale.
- Scientific applications: TAI is used for many scientific, technical, and engineering applications that require accurate and stable timekeeping, such as the GPS system, the Internet, and many other technologies.
Historical Context
TAI was first implemented in 1958 as a way to provide a more precise and uniform time standard for scientific and technical purposes. Before TAI, the standard for timekeeping was based on the rotation of the Earth, which is irregular and subject to small variations.
Relationship to UTC
UTC deviates from TAI by a number of whole seconds. As of 15 July 2026, UTC is exactly 37 seconds behind TAI; this has been the case since 1 January 2017, 00:00:00 UTC, immediately after the most recent leap second was put into effect. The 37 seconds result from the initial difference of 10 seconds.
Fun Fact
The acronym TAI stands for Temps Atomique International, a classic case of international compromise!
