NEW YORK - Today, time will stand still. Just for a second - a leap second.

Since 1967, when clocks went atomic, human timekeeping has been independent of Earth's rotation. The problem is, the planet is slowing down but clocks are not.

So every few years, to get everything back in sync, scientists add a second. They have done it 25 times since 1972. The last time was in 2012, but that was on a weekend. Today will be the first leap second during trading hours since markets went electronic.

It is scheduled for 8pm in New York, just when markets in Asia are opening, and exchanges around the world are taking no chances. United States stock markets are ending some after-hours trading early and others from Sydney to Tokyo are recalibrating their clocks ahead of time.

The wider world, too, has to be ready. In 2012, Qantas Airways and Reddit, among others, suffered glitches on their websites. As modern life has become more and more digital, the potential for snags when clocks go from 11:59:59 to 11:59:60, instead of straight to 12:00:00, has risen.

The time warp has the potential of making mischief with law enforcement, voice and data services, utilities and weapons systems. About 10 per cent of large-scale computer networks will encounter hiccups due to the leap second, said Mr Geoff Chester, public affairs officer for the US Naval Observatory in Washington, which keeps time for the world's biggest military.

"With the leap second, you count 61 seconds in a minute, and that's where the problems lie," Mr Chester said.

Earthlings have to make sure NTP, or Network Time Protocol, used by computer systems, meshes with Coordinated Universal Time, or UTC, which is determined by the oscillations of cesium-133 in atomic clocks.

There are, generally, three ways to do that, said Mr Dave Sohn, senior product manager at Spectracom Corp, which helps companies integrate global positioning, navigation and timing.

Clocks can stop for a second, or take a tick backwards, he said. Alternatively, clocks can slice up the leap second and spread it around in fractions. This is a practice known as dilution or, more colourfully, the "leap smear".

Google used the method during past leap seconds. Amazon said it is planning to add time to all 86,400 seconds of the day before the event so that at midnight, its systems will have caught up.

To lessen any possible impact of the leap second, NYSE Arca Equities said evening trading will close five minutes early.

Futures exchanges in Australia, Singapore, South Korea and Japan will smear the leap second after the event, according to the Futures Industry Association (FIA).

In the meantime, the Paris-based International Earth Rotation and Reference Systems Service will keep track of the gradual slowing of the planet, caused in part by drag created by the moon. Millions of years ago, days were 22 hours long.

"We need to stop having this fire drill every few years," said Mr Greg Wood, president of FIA's division that oversees market technology.

He compared the leap second to Y2K, the anticipated computer catastrophes related to the calendar turning to 2000 from 1999, which were ultimately minor. At the very least, he said, "I'm in favour of it not occurring mid-week."

BLOOMBERG