James Burrus/NIST

At 8 p.m. Eastern Time tonight, the whole world gets an extra second to the minute—a "leap second"—to make up for the Earth spinning at a slower rate.

Just like a Leap Year is designed to compensate for the earth moving around the sun at a slower pace, the leap second is like a tune-up in time keeping, making up for the Earth spinning just slightly more slowly on its own axis.

Here's an explanation from the UN time keeping agency:

An observatory in France, the International Earth Rotation service, views the cosmos and keeps track of when we're getting out of synch by more than 0.9 seconds—that's when a leap second adjustment is made, says John Lowe, head of the Time and Frequency Service at the National Institute of Standards and Technology (NIST).

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The NIST are the official atomic time keepers. Their atomic clock, which runs by measuring the frequency of emissions from atoms, sets official standard time measurements and has been used by computer systems around the world since 1972.

To keep their clock in synch with Coordinated Universal Time, which is measured through astronomy, they'll be adding the extra second this evening. The last time a leap second came around, in 2012, a few sites and browsers crashed spectacularly—most notably Reddit, Mozilla, and some other Linux-based systems.

Lowe says these glitches occur most commonly when, in a network, computers are not all synchronized to handle the extra second in the same way. "The real problem comes when there are multiple terminals trying to talk to each other. Some of them get the extra second and some of them do not," he told Fusion.

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Leap seconds are mandatory according to the International Telecommunication Union (the United Nation's time keeping organization) and they're not without controversy.

"The last time a leap second adjustment was made in 2012, we learnt of some glitches in digital systems and networks, such as for airline bookings," said agency spokesperson Sanjay Acharya. "We can’t predict what may happen this time. The proponents for the abolishment of the leap second argue that a continuous reference time scale will be more reliable and robust in the digital age."

One of the issues with using the leap second as a way of regulating time is that it's impossible to predict when they're "necessary" more than six months out.

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"Programmers and software guys can just put that in without being concerned about ever having to re-visit them. But the problem with leap seconds is that those are done by astronomical observation," Lowe told Fusion. The agency will consider dropping the leap second requirement at their meeting in November this year, in favor of using a system like the atomic clock universally.

To avoid mishaps, the New York Stock Exchange is closing early today, along with a few other global markets.