No more complaining that there’s not enough time to get it all done: On the last day of this year, you’ll have a whole extra second to finalize your New Year’s resolutions.

According to timekeepers at the International Earth Rotation and Reference Systems Service, the time determined by super-regular atomic clocks and the observed rotation of Earth have yet again become mismatched.

As long as all the world’s computer systems engineers have their ones and zeros in a row, there’ll be an additional breath of time at 6:59:59 p.m. ET on December 31, the U.S. Naval Observatory announced on July 6.

Atoms Versus Astronomy

Modern timepieces tick to the rhythm of Earth’s rotation based on the 24 hours it takes for the planet to complete one spin on its axis.

But on the level of seconds, the planet’s rate of spin fluctuates, mainly due to the gravitational effects of Earth’s moon. The push and pull of our orbital partner causes the planet’s massively heavy reserves of water to slosh around, which decelerates the spin between 1.5 and two milliseconds a day, on average.

Leap seconds are used to fill in the gap and make Earth’s observed speed align with our most accurate clocks—atomic devices that define a second by measuring the regular decay of radioactive elements such as cesium.

What Is a Leap Second? The International Earth Rotation and Reference Systems Service previously added a leap second on June 30, 2015. Why do we need to do this? And what kind of problems could result?

“If you don’t insert a leap second, eventually time based on those atomic clocks will be out of whack with solar time,” says science writer Dan Falk, whose 2008 book In Search of Time explores humanity’s relationship with time and our efforts to define and measure the temporal realm.

“It’s not a perfect solution. But Aristotle and Heraclitus were arguing about [time] 2,500 years ago, and we’re still arguing about it.”

For what it’s worth, the last time the atomic second and the astronomically defined second coincided was sometime in the 19th century, says Geoff Chester, a spokesman for the U.S. Naval Observatory.

Without leap seconds, the differences between official time as kept by atomic clocks and a day as defined by Earth’s rotation would slowly pile up. We’d face a two- to three-minute discrepancy between atomic time and astronomical time in 2100. By 2700, we’d get an extra half hour of sleep in the morning.

A leap second isn’t needed every year, because Earth rotates more regularly during some periods than others. A total of 26 leap seconds have been added since the practice began in 1972, when the world got 10 leap seconds to make up for lost time.

Time was then added almost every year through the 1980s. But since then some years have gone without a leap second, and this year is the first since 1998 that one has been added in consecutive years.

Time for Mayhem

Though people may not generally mind these slight tweaks to timekeeping, computers definitely do.

To a computer, a minute is always 60 seconds—no more, no less. And in today’s global digital networks, our telecommunication systems can be sensitive to even tiny changes in their internal clocks.

“Leap seconds are now a really big deal, so we have to give six months' notice as to when the next one will be introduced,” Chester says. Despite that lead time, about 10 percent of networks around the world fail in one way or another any time a leap second is added, he says.

In 2012, a worldwide airline booking system went down for several hours when its computers’ internal clocks could not reconcile the discrepancy with outside systems. The change also caused glitches for sites ranging from Reddit and Instagram to Yelp and Netflix.

Keeping up with accurate timekeeping even affects things beyond Earth.

“Think about Juno in orbit around Jupiter,” says Jean Dickey, a Jet Propulsion Laboratory physicist who has been studying the momentum of Earth’s rotation since the early 1990s. “With all our antennas on Earth, an error in time means an error in Earth rotation, which would end up being a navigation error. It could really wreak havoc with the mission.”

Though the U.S. government has stated it is in favor of abolishing the leap second, the International Telecommunications Union (ITU) voted last year to retain the practice, saying more study is needed to determine the impact on the world’s telecommunication systems.

We’ll probably get at least a few more leap seconds before the matter is settled, as the ITU won’t take up the matter again until 2023.