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Although time seems to move faster the older we get, in reality days are getting increasingly longer every year—albeit by infinitesimal amounts. According to NASA, days have lengthened by an average of two milliseconds since 1820. It’s a tiny elongation to be sure, but it’s been big enough to cause problems in the high-precision world of timekeeping.

For centuries, the Earth’s rotation served as the basis for measuring hours, minutes and seconds. After World War II, however, came the advent of atomic clocks, which are based on extremely predictable electromagnetic vibrations in cesium atoms and so accurate that they would lose only one second of time over the span of 1.4 million years. Measured by atomic clocks, Coordinated Universal Time (UTC) became the primary international timekeeping standard in 1961.

While a day measured by atomic clocks is exactly 86,400 seconds, the Earth’s rotation, as measured by Universal Time 1 (UT1), is not as precise. The average length of time it takes for the planet to complete one rotation is 86,400.002 seconds. The discrepancy is due to the fact that the Earth’s rotation is actually slowing due to a braking force applied by the gravitational pulls among the Earth, sun and moon.

Over the course of one year, that two-millisecond discrepancy between UTC and UT1 adds up to nearly a one-second difference. Over a number of decades, there would be a one-minute imbalance; over thousands of years, clocks would be off by an hour. Just as extra days are added to calendars every four years to compensate for the fact that the Earth’s revolution around the sun takes slightly longer than 365 days, the International Earth Rotation and Reference Systems Service since 1972 has periodically added extra seconds—called leap seconds—on either June 30 or December 31 to compensate for the mismatch between atomic clocks and the Earth’s actual rotation.

Instead of moving from 23:59:59 on June 30 to 00:00:00 on July 1, the insertion of the leap second will cause the UTC to move to 23:59:60 in between. (Since leap seconds occur simultaneously around the world and UTC runs four hours ahead of Eastern Daylight Time, this will occur at 8 p.m. on the East Coast of the United States.) Thus, June 30 will last 24 hours and one second.

Before today, 25 leap seconds have been inserted since 1972. Twenty-one extra seconds were added between 1972 and 1998, but today’s leap second will only be the fourth since then. Why have leap seconds become less common? The reason is the Earth’s rotation is irregular, with some days a tiny fraction longer and shorter than others. The spin of the planet fluctuates based on oceanic tides, natural disasters such as earthquakes and volcanic eruptions, dynamics of the Earth’s core and even climate changes such as the melting of polar ice caps. According to NASA, atmospheric variations due to El Niño can cause the Earth’s rotation to slow and add as much as one millisecond to the length of the day.

That unpredictability is causing problems in a world that is becoming increasingly connected by technology and reliant upon the Internet. Some computers have difficulty processing a 61-second minute and can’t complete normal operations. The addition of the last leap second in 2012 caused several prominent web sites—such as LinkedIn, Reddit and Gawker—to crash, and there are fears the disruption could be even worse this year. Several major financial markets, reliant on trades timed to the split second, are ending trading early or opening late in an attempt to avoid problems.

In contrast to the predictability of leap years, leap seconds are impossible to plan for over the long-term. The astronomers at the International Earth Rotation and Reference Systems Service who monitor the Earth’s rotation to the time kept by atomic clocks decide only six months in advance if a leap second needs to be added. That means computer programmers are unable to write codes anticipating when leap seconds will occur.

“In the short term, leap seconds are not as predictable as everyone would like,” said Chopo Ma, a geophysicist at NASA’s Goddard Space Flight Center and a member of the directing board of the International Earth Rotation and Reference Systems Service, in a statement. “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.”

The technological disruption has led to calls for the abolition of leap seconds with proponents arguing that the benefit is not worth the cost. Some scientists have argued for leap seconds to be replaced with the insertion of leap hours every few centuries, which could be planned for far in advance. The International Telecommunication Union will consider the abolition of leap seconds at its next meeting in November.

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