WHEN is a minute not a minute?

Well at 2359 Greenwich Mean Time on June 30, or 9.59am AEST on July 1, the world will experience a minute that will last 61 seconds.

The reason? Something called the leap second. That’s when timekeepers adjust high-precision clocks so they’re in sync with earth’s rotation, which is affected by the gravitational tug of the sun and moon.

Few of the planet’s 7.25 billion people are likely to be aware of the change.

But for horologists, the additional second is a big deal, and there’s a wrangle as to whether it is vital or should be scrapped. The last modification, on June 30, 2012 was disruptive for many internet servers, including Qantas’ online reservation system, which went down for several hours.

The leap second is not something that needs to be added to the 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, unveiled in April, 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 satellites, which have to send synchronised 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 microseconds. 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 Co-ordinated Universal Time (UTC), the official moniker for GMT.

But over the past 15 years, a debate has intensified about whether the change should be made, given the hassle.

Daniel Gambis, from the International Earth Rotation and Reference Systems Service, which is in charge of saying when the second should be added, defends the change.

If it were dropped, in 2000 years there would be an hour’s difference between UTC and the time it takes for the earth to complete one complete turn, Gambis says.

“It would mean that, on a scale of tens of thousands of years, people will be having their breakfast at two o’clock in the morning.”