The world is about to experience a minute that lasts 61 seconds

You’ll find the answer at 23:59 Greenwich Mean Time (GMT) on June 30, when the world will experience a minute that will last 61 seconds. The reason for the weird event is something called the leap second.

That’s when timekeepers adjust high-precision clocks so that they are in sync with the Earth’s rotation, which is affected by the gravitational tug of the Sun and the Moon.

Few of the planet’s 7.25 billion people are likely to be aware of the change... and even fewer will have set plans for how they will spend the extra moment.

But for horologists, the additional second is a big deal, and there is a wrangle as to whether it is vital or should be scrapped. “There is a downside,” admits Daniel Gambis, director of the Service of the Rotation of the Earth — the poetically named branch of the International Earth Rotation and Reference Systems Service (IERS), in charge of saying when the second should be added.

To be clear, the leap second is not something that needs to be added to that old clock on your mantelpiece. Instead, its importance is for super-duper timepieces, especially those using the frequency of atoms as their tick-tock mechanism.

At the top of the atomic-clock range are ‘optical lattices’ using strontium atoms, the latest example of which is accurate to 15 billion years — longer than the Universe has existed.

Outside the lab, caesium and rubidium clocks are the workhorses of Global Positioning System (GPS) satellites, which have to send syncronised signals so that sat-nav receivers can triangulate their position on Earth.

On Earth, big-data computers may be less manic than atomic clocks but still need highly precise internal timers. The Internet, for instance, sends data around the world in tiny packets that are then stitched together in micro-seconds. Some algorithms in financial trading count on gaining a tiny slice of a second over rivals to make a profit.

There have been 25 occasions since 1971 when the leap second was added in an effort to simplify Coordinated Universal Time (UTC), the official moniker for GMT.