Why We Have a Leap Second, and Why It's Unlikely to Wreak Chaos

The last time a second was added to the world's clock, websites crashed. This time should hopefully be different.

National Journal

For just a moment on Tuesday, time will stand still.

At 8 p.m. on the East Coast, an extra second will tick on clocks the world over. 7:59:59 will become 7:59:60 before reaching 8:00:00.

When the last leap second was added in 2012, several websites—including Reddit, Gawker, and Amazon's Web services—crashed. The Australian airline Qantas saw its online booking system fail; passengers had to be checked in by hand and more than 400 flights were delayed. These systems ran on code that couldn't comprehend why time appeared to stop.

Ahead of Tuesday's leap second, some news outlets have worried that similar glitches may come up, and even might affect global financial markets (prices change second by second, so it's essential that a buyer's and seller's computers are in agreement about what time a transaction takes place).

But John Lowe, one of America's top timekeepers, is not expecting the apocalypse.

Lowe is the group leader of the time and frequency services division at the National Institute of Standards and Technology (NIST). NIST is the U.S. government body responsible for setting our measurement standards—what an inch is, what an ounce is, what a second is, and so on. Lowe is in charge of making sure all the nation's clocks sync up. "We disseminate time," he says.

(For as mind-blowing as that job description sounds, there are some downsides: "With my friends and family, if I'm ever late, I get endless needling about it," Lowe says. "Don't you know what time it is?" they ask. Of course he does.)

Lowe says that system administrators have had six months to prepare for the leap second. "So I would hope that most people have checked their computers and made sure they are prepared for this," he says. "But you never know."

It's most likely that the critical systems that run our government and big corporations that store our data won't miss a beat during the leap second, he says. Google is employing a novel "leap smear," slowly adding milliseconds throughout the day until the clock catches up at 8. The New York Stock Exchange Arca Equities is ending trading at 7:55 p.m., just to be safe. Some websites that have not anticipated the event may crash. Even if they do, the fix will be simple.

"If they haven't prepared anything, it may take a momentary shutdown to just reset everything and come back up," Lowe says. "In real terms, that is probably the most of what's going to happen.

But why even risk that? We need to insert a leap second because human timekeeping has become so precise, it's more reliable than the Earth.

If you thought the length of a day was defined by the amount of time it takes to complete one rotation around Earth's axis, you'd be wrong.

Time on Earth is calibrated by something much tinier, and much more precise: the vibrations of the cesium atom as it jumps from one energy state to another.

Atomic cesium clocks keep near-perfect time, only losing or gaining a second every 300,000,000 million years. The rotation of the Earth is a less trusty timekeeper. Its speed can vary (even large earthquakes can throw it off) and it is gradually slowing down***.

Cesium is much more stable. It vibrates at a constant, predictable frequency. A second is defined as the amount of time it takes for a cesium atom to vibrate 9,192,631,770 times. There are 86,400 seconds in a day.

Here's the problem. Although the Earth's rotation no longer determines the length of the day, we still like it to turn in sync with the atomic clock. One complete rotation of the Earth takes around 86,400.002 seconds, explains NASA. Those extra .002 seconds accumulate, and every two years or so we add a second to make up the difference. Since the 1970s, 25 of these seconds have been added. Almost enough time for just one more Super Bowl ad.

It's also possible that this leap second may be the last.

In November, The Wall Street Journal reports, the International Telecommunication Union plans to vote on whether the world's time systems should continue to acknowledge leap seconds.

That's because leap seconds may be more of a headache than they are worth. The atomic clock and the rotation of the Earth are only getting out of sync by one minute every century. In 2,000 years, we'd only be off by 20 minutes.

Though time originated as a measurement of heavenly bodies, humans have transcended those old notions. The length of a second is an arbitrary human choice. For our computers to run, for our society to function, all that matters is we all agree on the definition of a second, and that definition is stable. The continuous buzzing of the cesium atom is stable. The Earth's rotation is not.

"No longer is the concept of noon when the sun is directly over your head—most people have gotten long far away from concerning themselves with that," Lowe says. "The idea that we do these leap seconds is probably not that important anymore."

Time is now of our making.


***Some news accounts have attributed this rotational slowing as the reason to add the leap seconds, but they are wrong, Lowe says. The Earth is slowing down extremely gradually: adding about 1.7 milliseconds every century. "Yes we are slowing down, but not at the kinds of rates that would constitute a leap second," Lowe says. The real reason for the leap second: "When they originally defined the second based on the cesium atom, they missed it by a few hertz," Lowe says.

It's a mistake so common, Lowe says, "Neil deGrasse [Tyson] even got this one wrong."