What is a leap second?
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.
Leap seconds are added to, or taken away from, the UTC time scale to link it to the rotation of the Earth.
There is ongoing debate over whether or not to abolish leap seconds and allow solar time to gradually drift away from atomic time.
Although some users do experience problems caused by leap seconds, they have already been in use for 40 years and the international community needs to be sure the long term consequences are understood before making any changes to a system that works for the vast majority of users.
The decision as to whether or not to abolish leap seconds was deferred until at least 2015 after a recent meeting of the Radiocommunication Assembly in Geneva, Switzerland.
In the meantime, the UTC time scale will continue to be linked to solar time. The most recent leap second was inserted on 30 June 2012. This was the 25th leap second added since their adoption in 1972.