The adjustment between Atomic & Earth Time


Paris, 5 January 2012
Bulletin C 43
To authorities responsible
for the measurement and distribution of time

on the 1st of July 2012

A positive leap second will be introduced at the end of June 2012.
The sequence of dates of the UTC second markers will be:

2012 June 30, 23h 59m 59s
2012 June 30, 23h 59m 60s
2012 July 1, 0h 0m 0s

The difference between UTC and the International Atomic Time TAI is:
from 2009 January 1, 0h UTC, to 2012 July 1 0h UTC : UTC-TAI = – 34s
from 2012 July 1, 0h UTC, until further notice : UTC-TAI = – 35s

Leap seconds can be introduced in UTC at the end of the months of December or June, depending on the evolution of UT1-TAI.

Bulletin C is mailed every six months,
either to announce a time step in UTC or
to confirm that there will be
no time step at the next possible date.

Information provided by: INTERNATIONAL EARTH ROTATION SERVICE (IERS), Paris, France


The Coordinated Universal Time (or UTC) replaced Greenwich Mean Time (GMT) as the reference time scale derived from The Temps Atomique International (TAI) calculated by the Bureau International des Poids et Mesures (BIPM) in Paris, France using a worldwide network of atomic clocks. UTC differs from TAI by an integer number of seconds; it is the basis of all activities in the world.

UT1 is the time scale based on the observation of the Earth’s rotation. It is now derived from Very Long Baseline Interferometry (VLBI). The various irregular fluctuations progressively detected in the rotation rate of the Earth lead in 1972 to the replacement of UT1 as the reference time scale . However, it was desired by the scientific community to maintain the difference UT1-UTC smaller than 0.9 second to ensure agreement between the physical and astronomical time scales.

Since the adoption of this system in 1972, firstly due to the initial choice of the value of the second (1/86400 mean solar day of the year 1900) and secondly to the general slowing down of the Earth’s rotation, it has been necessary to add 21s to UTC.

The decision to introduce a leap second in UTC is the responsibility of the International Earth Rotation Service (IERS). According to international agreements, first preference is given to the opportunities at the end of December and June, and second preference to those at the end of March and September. Since the system was introduced in 1972, only dates in June and December have been used.



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