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What we do

Mission of Japan Standard Time Group

International Atomic Time (TAI),
Coordinated Universal Time (UTC), and Leap Seconds

Definition of a second: The duration of the unit “second” was first defined based on an ephemeris time, which is tied to the rotation and the orbital motion of the earth. The definition in terms of frequency of atomic radiation was replaced with the International System of Units (SI) in 1967.
The second is defined in the SI as “the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.”

The time scale created by the atomic clocks is referred to as atomic time.
TAI was inaugurated its origin at 0:00, January 1, 1958, at 0:00 by synchronizing to UT2.
The TAI is decided by calculating a weighted average of the times of atomic clocks in the world.

Since the rotation of the Earth is influenced by natural phenomena such as tidal friction, time difference between UT and UTC is apt to occur. Therefore one second is either inserted into or deleted from UTC in order to keep the time difference between UTC and UT below 0.9 seconds.
This is referred to as a “leap second.”
Leap second adjustment was introduced to UTC after the special adjustment carried out in 1972. Leap second adjustments were performed 27 times from 1972 to January 2017. All adjustments were done by inserting a second to the UTC so far, and now the UTC lags 37 seconds behind the TAI.

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