Global markets are bracing for an extra second - known as a leap second - which will occur tomorrow, with worries it may result in digital trading systems glitching and even failing.
The leap second, which is used to realign the world's clocks with mean solar time, will be introduced on 30 June at 23:59:59 Coordinated Universal Time (UTC) - equivalent to 19:59:59 in New York and 00:59:59 in London.
However, the insertion coincides with the opening of markets across Asia and Australia, raising the possibility of digital glitches and temporary suspension of trading while systems are rebooted.
Some $3.7bn (€3.3bn) will be traded at the bell in Japan, South Korea and Australia, according to Bloomberg Business, where the leap occurs exactly one second before markets open.
Greg Wood, president of the Futures Industry Association (FIA) market technology division, said: "The reason why it's a big deal is this is the first time it's occurring midweek.
"Adding this extra second, where there is going to be an additional sixty-first second that will have a timestamp of 23.59.60, has got people worried because we're not sure how systems will handle that."
The majority of computer systems are synchronised using Network Time Protocol (NTP), which encodes time using UTC as its basis.
Markets are planning to deal with tomorrow's irregularity in different ways. In New York, some markets including Nasdaq will close trading early while overnight traders will delay opening to try and negate any glitches.
In Australia and Asia, clocks will be recalibrated in advance, with the Japanese stock exchange slowing clocks for two hours beforehand so the exchange will be synchronised when the leap occurs.
In Brazil, the BM&F index adjusted its clocks outside of trading hours on 28 Sunday June, meaning that the country is now technically one second ahead of the rest of the world.
Wood says that this year's leap second comes at a difficult time, with global markets already facing massive instability as the Greek crisis reaches its climax.
"In the worst case, there could be systems that fail simply because they can't handle the adjustment to the leap second, and that could be at any level from internet services through to potentially components of a trading system," says Wood.
Leap seconds have been used since 1972 to correct for irregularities in the Earth's rate of rotation and realign UTC with mean solar time. There have been 25 such occurrences since 1972.
Tomorrow's leap will be the first to occur during active trading hours since the market has gone digital and reliant upon sub-second precision.
The last leap second was introduced on 30 Saturday June, 2012, and resulted in several notable glitches. Australian airline Qantas saw its booking system disrupted for more than two hours, while other high-profile sites including Reddit and LinkedIn also encountered problems.
The average length of a day, based on how long the Earth takes to rotate, is 86,400.002 seconds. As the rotation speed gradually slows, UTC begins to drift from mean solar time. Whenever this drift approaches 0.9 seconds, a leap second is inserted to realign the two systems.
The decision is taken by the International Earth Rotation and Reference Systems Service (IERS) and seconds are only ever introduced on 31 December 31 or 30 June, to coincide with the end or middle of the year.
If leap seconds were not used, UTC would gradually drift from mean solar time, meaning that sunrise could happen much later in the day as clocks would no longer correspond to the Earth's position relative to the sun.
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