Tonight (30 June) at 23:59:59 universal time (UTC) a minute will be a second richer to allow timekeepers of high-precision atomic clocks to sync time with Earth's slowing rotation.
Earth's rotational speed has been slowing down due to the moon's gravity, causing a leap second to be added, much like the leap day added to February once in four years.
Many million years ago a day was only 23 hours long. Since then clocks have gained an hour a day. Since 1820, the day has slowed down by 2.5 milliseconds.
The problem caused by a second is big when high-precision atomic clocks are keeping time.
Energy transitions at the atomic level are used to mark the precise time. The top-end ones using strontium atoms are believed to be accurate to 15 billion years.
The UTC (official name of GMT) was in fact devised to keep the atomic clocks and actual time in sync by allowing scientists to add an extra second at least once a decade.
Service of the Rotation of the Earth (SRE), a branch of the International Earth Rotation and Reference Systems Service (IERS), decides when the second should be added.
Caesium and rubidium atomic clocks are commonly used by Global Positioning System (GPS) satellites and play a vital role.
Computer systems could be caught out, atomic clock expert Judah Levine, from the US National Institute of Standards and Technology (Nist) in Boulder, Colorado, told National Geographic.
The internet, for instance, sends data around the world in tiny packets stitched together in micro-seconds. The last leap second in 2012 temporarily disrupted many websites.
Rivals in financial trading use algorithms to gain from microseconds and make a profit, reports ABC News.
Leap seconds were first introduced in 1972, by which time atomic clocks were ahead by 10 seconds. Scientists had to add 10 seconds to the world's astronomical clocks in one go back then.
Since then leap seconds have been added on 25 occasions.
If a leap second is not added every once in a while, there would be in 2000 years almost an hour's difference between UTC and the time taken by Earth for one rotation.