National Physical Laboratory

Leap seconds

The atomic clocks housed at NPL are nearly a million times better at keeping time than the rotation of the Earth. Leap seconds are used to provide a link between the extremely stable time scale based on atomic clocks and the more variable time scale of the solar day.

A leap second is added to Coordinated Universal Time (UTC) when the Earth's rotation becomes out of sync with the time kept by the atomic clocks held at national measurement institutes such as NPL. When time based on the Earth's rotation lags behind UTC, a second is added to the atomic time scale. It is also possible for a second to be removed from the UTC time scale, although such a negative leap second has never been required.

When are leap seconds added?

When a leap second is needed, it will be inserted in the last minute of either December or June, or exceptionally in March or September, immediately prior to midnight or 00:00:00 hours UTC.

The decision as to whether a leap second is required is taken by the Earth Orientation Center of the International Earth Rotation and Reference Systems Service (IERS), approximately six months in advance.

The most recent leap second was inserted on 31 December 2016

This was the 27th leap second added since their adoption in 1972

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