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The "longest day" in 2015
30 June 2015 will have a supplementary second following the insertion of a positive leap second to Universal Coordinated Time (UTC). All atomic clocks in the world that represent UTC (currently in 73 institutes world-wide) will add one additional second to synchronize with the irregularities in the Earth's rotation time (known as UT1).

The procedure will be applied simultaneously to all clocks, and requires the duplication of the last second of the UTC day. Time 23 h 59 m 59 s of 30 June, UTC, will be followed by 23 h 59 m 59 s of 30 June, UTC. In some applications the question of which of the two seconds named "59" occurred first may arise.

This arrangement started in 1972, when UTC was defined as the atomic time reference synchronized to UT1 to better than 1 s. Leap seconds have been inserted when necessary since then, following announcements from the International Earth Rotation and Reference Systems Service (IERS). The leap second of 30 June will increase the total offset between International Atomic Time (TAI) and UTC to 36 s.

The future of this "stepped" UTC has been under discussion at the International Telecommunication Union (ITU) for fifteen years. The World Radiocommunication Conference in November 2015 will mark the conclusion of these discussions when a final consensus on the adoption of a uniform, continuous international reference time scale will be debated.

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