leapsecond

A leap second is a one-second adjustment to Coordinated Universal Time (UTC) inserted to keep UTC within 0.9 seconds of UT1, which is based on Earth's rotation. Because Earth's rotation is irregular and gradually slows, atomic time would drift apart from apparent solar time without occasional corrections.

Leap seconds are announced by the International Earth Rotation and Reference Systems Service (IERS), typically via Bulletin C, with several months of notice. They are added by inserting a second at the end of June 30 or December 31, producing a time of 23:59:60 UTC for one second before returning to 00:00:00 of the next day. Most leap seconds have been positive, adding one second; negative leap seconds are technically possible but have not occurred.

Historically, leap seconds were introduced in 1972. Since then, 27 leap seconds have been added, with the most recent occurrences in 2015 and 2016 (June 30, 2015, and December 31, 2016). There has been ongoing discussion about abolishing leap seconds or modifying their implementation, led by international bodies such as the IERS and ITU, and plans have been considered for transitions in the coming decades.

The leap second can pose challenges for computer systems and networks that rely on a continuous clock. Timekeeping standards include UTC, UT1, International Atomic Time (TAI), and civil time; leap seconds synchronize UTC with Earth's rotation while preserving the integrity of TAI. The topic remains a subject of policy and technical debate as authorities consider long-term approaches to harmonize civil time with astronomical time.

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