Global time will stand still on Tuesday, delaying midnight by a second.
An extra sliver of time - a "leap-second" - is being added to the world's clocks to adjust for the inaccuracy of the spinning Earth.
Before the invention of super-accurate atomic clocks, time was based on the Earth's rotation, one complete turn taking 24 hours.
Now a plethora of time-sensitive systems, including computer programs and financial markets, rely on the precise ticking of atomic clocks that measure the energy transitions of atoms.
The problem is that due to the moon's gravity the Earth is slowing down, and not in a regular way. So every now and then a leap second is added to allow astronomical time to catch up with atomic time. It is similar to the introduction of leap years keep our calendars lined up with the Earth's orbit around the Sun.
The latest pause - the 26th - will occur on June 30 at 23:59:59 co-ordinated universal time (07:59:59 Wednesday AEST). UTC provides a world-wide time standard free of time zones.
Computer programmers try to take account of leap seconds but many systems could be caught out, warns atomic clock expert Professor Judah Levine, from the US National Institute of Standards and Technology.
He told National Geographic magazine: "It's a major interruption mostly because there are a lot of systems that aren't prepared to handle the leap second correctly."
The last leap second in 2012 temporarily disrupted a number of high-profile websites including Mozilla, Reddit, Gawker, LinkedIn, FourSquare and Yelp.
In Australia, more than 400 Quantas flights were delayed as staff were forced to switch to manual check-ins.
Leap seconds were first introduced in 1972, by which time atomic clocks and astronomical clocks were already out of kilter by 10 seconds.
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