Airlines, trading floors and technology companies are braced for chaos tonight as world timekeepers prepare to add a leap second to global clocks.
Immediately before midnight, dials will read 11:59:60 as clocks hold their breath for a second to allow the Earth's rotation to catch up with atomic time.
When the last leap second was added in 2012, Mozilla, Reddit, Foursquare, Yelp, LinkedIn and StumbleUpon all reported crashes and there were problems with the Linux operating system and programmes written in Java.
In Australia, more than 400 flights were grounded as the Qantas check-in system crashed.
Experts at Britain's National Physical Laboratory (NPL), who will officially add the second to UK time, warned that markets that are already jittery from Greece could suffer transaction delays if their software was not prepared.
"There are consequences of tinkering with time," said Peter Whibberley, Senior Research Scientist in the Time and Frequency group at NPL, who is known to colleagues as 'The Time Lord'.
"Because leap seconds are only introduced sporadically, it is difficult to implement them in computers and mistakes can cause systems to fail temporarily."
European markets are largely closed when the event is scheduled to occur, but the change will hit trading floors in the US, Japan, Australia, South Korea and Singapore.
To make matters more complicated, stock exchanges are approaching the leap second differently, with Asian markets planning to trade as normal, while US exchanges are expected to stop activity early.
Google has even developed a special technique to deal with what it refers to as a 'leap smear' and has been gradually adding milliseconds to its system clocks prior to the official arrival of the leap second.
Atomic time is constant, but the Earth's rotation is gradually slowing down by around two thousandths of a second per day.
Leap seconds are therefore essential to ensuring civil time does not drift away from time based on the Earth's spin.