International Atomic Time

International Atomic Time (abbreviated TAI, from its French name temps atomique international^[1]) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid.^[2] TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide.^[3] It is a continuous scale of time, without leap seconds, and it is the principal realisation 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.

UTC deviates from TAI by a number of whole seconds. As of 1 January 2017^[update], when another leap second was put into effect,^[4] UTC is currently exactly 37 seconds behind TAI. The 37 seconds result from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972.

TAI may be reported using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both Julian days and the Gregorian calendar are used. TAI in this form was synchronised with Universal Time at the beginning of 1958, and the two have drifted apart ever since, due primarily to the slowing rotation of the Earth.