Why A Top US Naval Scientist Is Terrified Of The Leap Second

June 30 will be exactly one second longer than usual. Here's why it happens, and how it could crash a plane

Jun 29, 2015 at 3:23 PM ET

On June 30, humanity will “spring forward” by tacking a much-needed second onto the tail end of the month. John Oliver has already planned elaborate leap-second festivities, but it may be a bit early to celebrate—experts caution that the leap second could lead to worldwide computer glitches.

Since the year 1820 we’ve all been slightly late. If you consider “one day” the amount of time it takes our planet to rotate once, every day should last exactly 86,400.002 seconds. But if you’re tapped into the atomic clock (which, by the way, you totally are) then your days last a mere 86,400.000 seconds, based on some electromagnetic tomfoolery within the element Cesium.

Every day the problem gets a tiny bit worse—two thousandths of a second worse, to be precise. Part of the problem is that Earth’s rotation is actually slowing down over time, as our little planet stubbornly tugs against the gravitational force of both the sun and the moon. Weather patterns mess with Earth’s rotation, too. Tides and typhoon season toss a few milliseconds into the mix, the Earth’s inner core constantly changes, throwing our rotation into flux and El Niño, a wave of warm water that floods the Pacific, occasionally adds one thousandth of a second all by itself.

Since humanity first tried a “leap second” on for size in the early 1970s, it’s been a rocky, unpredictable road. Between 1972 and 1999, we had one leap second every year. But since the new millenium, we’ve seen only four leap seconds in 15 years. It turns out (a tad ironically) that the decision to add a leap second is made, well, at the last second.

And that’s where computers lose their sh*t.

Virtually all computer systems are coordinated with the atomic clock. Right now, coders around the world are gleefully hacking away at their keyboards, telling their various programs—from online shopping platforms to stock market interfaces and GPS navigation systems—to assume 60 seconds in every minute. The fact that this rule doesn’t hold up four times every 15 years at random makes preparing your program for the inevitable quite complicated. June 30, 2015, is effectively a software hiccup waiting to happen.

As Slate points out, computer glitches aren’t a huge problem. Every few months Facebook goes down for one reason or another, we all freak out, and then everything is fine. Y2K was averted. But if you’re trading stocks when the software that communicates exchanges reboots, a tiny glitch can cost millions. And if you’re trying to land an airplane when your hi-tech GPS freaks out because seconds and minutes temporarily lose their meaning, a harmless blip could spell disaster.

Then again, it’s probably just so many scare tactics. There’s no reason to stock up on water or go into hiding for another Y2K scare. Realistically, it’s not like the experts are freaking out, right?

“There will definitely be failures of some systems—how significant, I don’t know,” Demetrios Matsakis, the chief scientist for Time Services at the U.S. Naval Observatory, told Motherboard. “I would suggest not to be in the air flying when the leap second is enacted.”

Oh crap—never mind. To the bunker!