International Atomic Time (
TAI, from the French name ) is a high-precision atomic
coordinate time standard based on the notional passage of
proper time on
Earth's
geoid. It is the basis for
Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface, and for
Terrestrial Time, which is used for astronomical calculations. when the last
leap second was added, TAI is exactly 36 seconds ahead of UTC. The 36 seconds results from the initial difference of 10 seconds at the start of 1972, plus 26 leap seconds in UTC since 1972.