Tuesday June 30 will be one second longer than any other day this year – and the longest day since 2012.
That's because a "leap second" will be added to account for the fact that the Earth's rotation is gradually slowing down, NASA explains.
A day lasts 86,400 seconds, according to coordinated universal time (UTC), but days haven't actually been that long since about 1820, due to the gravitational tug-of-war between the Earth and the moon slowing the Earth's rotation – now, the average day is approximately 86,400.002 seconds long, NASA says.
It may not seem like a lot – it's actually less than the blink of an eye – but the difference adds up to almost a second every year, which is why scientists are adding a "leap" second to June 30 to keep time in sync with the mean solar day, which measures the passage of time based on the sun's position in the sky, CBC News says.
NASA has a video that explains more on how scientists measure time:
So, as the clock approaches midnight on Tuesday, it will strike 23:59:60 before rolling over to 00:00:00 on July 1.
This won't have much of an affect on the average person, NASA explains, but it could affect some computer systems – it becomes problematic when computers are networked together and can have issues agreeing on what time it is.
In 2012, when the last leap second was introduced, it caused problems across the Internet – briefly brought down websites like Reddit and Gawker, as well as disrupting Qantas Airlines' booking system, leading to the cancellation of more than 400 flights, CBC News notes.
Twenty-five leap seconds have been added since 1972 – when the first leap second was implemented. Tuesday's leap second will be only the fourth to be added since 2000, NASA notes.
“In the short term, leap seconds are not as predictable as everyone would like,” Chopo Ma, a geophysicist at NASA’s Goddard Space Flight Center, explains. “The modeling of the Earth predicts that more and more leap seconds will be called for in the long-term, but we can’t say that one will be needed every year.”