Global time stood still last night, delaying midnight by a second
An extra sliver of time - a "leap-second" - was added to the world's clocks to adjust for the inaccuracy of the spinning Earth.
Before the invention of 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 programmes 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 to keep our calendars lined up with the Earth's orbit around the sun.
Computer programmers try to take account of leap seconds but many systems could be caught out, warned atomic clock expert Prof Judah Levine, from the US National Institute of Standards and Technology.
"It's a major interruption mostly because there are a lot of systems that aren't prepared to handle the leap second correctly," he told National Geographic magazine.
The last leap second in 2012 temporarily disrupted a number of high-profile websites including Mozilla, Reddit, Gawker, LinkedIn, FourSquare and Yelp.