The "leap second" is needed to compensate for a slowdown in the Earth's rotation
Scientists say the "leap second" is needed to compensate for a slowdown in the Earth's rotation.
While clocks normally goes from 23:59:59 to 00:00:00, the extra second is being added in between - and hi-tech digital clocks will record that time as 23:59:60.
Peter Whibberley, a senior research scientist at the National Physical Laboratory in Teddington, said: "Leap seconds are needed to prevent civil time drifting away from Earth time.
"Although the drift is small - taking around 1,000 years to accumulate a one-hour time difference - if not corrected it would eventually result in clocks showing midday before sunrise."
The International Earth Rotation and Reference Systems Service in Paris decides when leap seconds are needed, and they are always announced about six months in advance.
That is because the extra second can be a nightmare for communication networks, financial systems and other applications that rely on precise timing, so they need to be programmed into computers to prevent mistakes.
This is the 27th time a leap second has been introduced since 1972.
Although they are normally added every two or three years, the last leap second was inserted just 18 months ago.
It's also possible for seconds to be removed, but this has never happened.
The leap seconds are usually introduced in the final minute of June or December, but they can be implemented in March or September on rare occasions.