Leap Seconds Explained
A leap second is an extra second added to UTC, occasionally, to keep atomic time in step with the Earth's slightly irregular rotation.
Why they exist
The Earth does not turn at a perfectly steady rate, while atomic clocks do not drift. If nothing were done, the two would slowly move apart, so a second is occasionally added to UTC, always at the end of 30 June or 31 December.
How many and what next
Since the system began in 1972, 27 leap seconds have been added, the last at the end of 2016. In 2022 the international body responsible for measurement agreed to stop using leap seconds by 2035, because they cause problems for computer systems.
UK clocks are unaffected in practice. When a leap second is added, the national time services simply count one extra second at midnight UTC.
Frequently asked questions
When was the last leap second?
The last one was added at the end of 31 December 2016.