Universal time will read the unusual 11:59:60 before it hits midnight.
International timekeepers periodically add a "leap second" to make up for a gradual slowdown in the Earth's rotation.
Experts at the International Earth Rotation and Reference System Service make the adjustment when the planet's movement falls out of sync with atomic clocks used to measure time.
The time it takes the Earth to rotate on its axis — the definition of a day — is now about two milliseconds longer than it was 100 years ago, said Geoff Chester, spokesman at the U.S. Naval Observatory. It adds up to nearly three-quarters of a second a year.
This is the first leap second since January 2009 and the 25th overall since the process started in 1972. Scientists said the next one probably won't be needed until 2015 or 2016.
Roberto Abraham, who teaches astronomy at the University of Toronto, said without the leap seconds, things would eventually be "out of sinc."
"So after 500 years, you would find the clock says it's noon, but the sun is two hours away from its maximum height, he said.
Abraham said theoretically, he wouldn't hold it against anyone if their response was "Who cares?" But on the other hand, society would be affected when it comes to farming, religious festivals and cultural practices.
"I do agree that, at some level, you don't want to just toss out thousands of years of history just because some guy invents a better widget for measuring time," said Abraham.
Scientists said there should be no noticeable affect or inconvenience on computers or any other technology that requires precise timekeeping because they adjust for these leap seconds.
Earlier this year, official timekeepers from across the world discussed whether to eliminate the practice of adding leap seconds. They decided they needed more time to think and will next debate the issue in 2015.
_With files from The Associated Press