The final minute of Tuesday, June 30, will last for 61 seconds when a leap second is inserted at midnight Universal Time. That corresponds to 10am Australian Eastern Standard Time on Wednesday, July 1.

Why the extra second? Traditionally, time was mapped out by astronomical observations such as the position of stars. But the earth's speed varies due to tidal forces and is, on average, slowing down very slightly.

That makes the planet a fairly unreliable timekeeper, explains Michael Wouters, head of time and frequency at the National Measurement Institute.

"[Astronomical time] is not a very good way of counting seconds because the rotation of the earth is not very stable," he says.

"As soon as good clocks were invented in the early 20th century, we realised the earth was not a very good clock."

The extra second is inserted every few years but not on a fixed timetable. Without the occasional correction, atomic time could be out of step with the earth's movements by up to half an hour over 500-600 years - changing the timing of sunsets and sunrises, for example.

"Eventually this would lead to a disagreement that you would notice," Dr Wouters says.

In terms of discernable consequences of the leap second, there will be very little. Technically, computerised clocks should read 09:59:60 to accommodate the extra second. But many systems work by "stepping back" and re-counting a second, and that's the part that can cause problems.

The most recent leap second on June 30, 2012 caused mild disruptions to some technology systems, including the large international websites reddit and LinkedIn. The check-in system at Qantas was temporarily affected, reportedly delaying about 50 flights.

The problem was related to the Amadeus reservation system used by the national carrier and other airlines. It is understood Qantas has consulted with the technology provider and is confident the issue will not strike again this year.

Dr Wouters explains that many computer systems take their time data from Australia's Network Time Servers, which are programmed to count the leap second. Those users include government departments, universities, major corporations and banks.

Mobile phones could derive their time from the phone network or from the manufacturer's own time server, Dr Wouters says, but, either way, users should not expect any disruption.

"I've never heard of a phone crashing because of a leap second," he says.

Leap seconds are regular but not fixed, occurring roughly every three to four years. The inclusion and timing of a leap second is a complex, global and bureaucratic affair: the International Earth Rotation and Reference Systems Service provides advice on whether time needs to be corrected, and the International Telecommunications Union makes the final decision on if and when the leap second occurs.

What to do with the rare gift of additional time? Perhaps a moment of contemplation on the futility of time; its seconds constantly slipping away, carrying us closer to death, save for the occasional and fleeting atomic reprieve.

Or maybe it is a chance to wonder at the miracle of science, so advanced in its methods that it has devised clockwork more exact than our fallible planet.

For stockbrokers, however, it may simply be a time to panic. Electronic trading can complete transactions within milliseconds and, according to reports, the last time there was a leap second, clients were able to make moves worth more than £18 million ($37 million) based on manufacturing data released a fraction of a second early by Reuters.

In the US, the Nasdaq will cease trading at 7.48pm on Tuesday, New York time, and shut down at 7.55pm, while other stock exchanges will delay transactions.

Futures exchanges in Australia, Singapore, South Korea and Japan will "smear" the leap second, Bloomberg reported. That means administrators will skew the clock to distribute fractions of a second across all 86,400 seconds of the day.

There is also debate about whether the leap second is warranted or necessary. Several nations, including the US, are lobbying the ITU to abolish the quirk because of its potential to cause technological disruption. A decision is expected at the organisation's November meeting.