ROBERT SIEGEL, HOST:
Today saw important deadlines for the Greek debt crisis and a deal on Iran's nuclear program. Negotiators may not have realized it, but they did have a little extra time to figure out those weighty issues. As NPR's Geoff Brumfiel reports, today contains an extra second.
GEOFF BRUMFIEL, BYLINE: To understand this thing called a leap second, here's an astronomy lesson. The earth spins on its axis. One spin is a 24-hour day. But that time period can change.
TOM O'BRIAN: The biggest reason is that the moon - the moon's gravity slows the earth's rotation down.
BRUMFIEL: Tom O'Brian is at the National Institute of Standards and Technology. In fact, the length of the day on Earth is very slowly getting longer.
O'BRIAN: It's the equivalent of about one or two-thousandths of a second per century. That might not sound like much, but that adds up over time.
BRUMFIEL: Things like major earthquakes and melting glaciers can also change the length of the day. Humans didn't notice until we developed super-accurate atomic clocks and computers. Since the 1970s, we've had to add leap seconds to keep our timepieces in sync with our wobbly planet. And so at 7:59 p.m. Eastern Time tonight, anyone who was watching the official U.S. time saw something weird.
O'BRIAN: Fifty-eight seconds, 59 seconds, and then - one time only - 60 seconds.
BRUMFIEL: In other words, this particular minute had an extra second in it. Now, leap seconds can cause trouble. Markus Kuhn is a computer scientist at Cambridge University in the U.K.
MARKUS KUHN: There is no standardized way of how computers in particular handle it.
BRUMFIEL: A previous leap second caused some computers to freeze.
KUHN: And a number of bigger computing centers had to shut down and had to reboot all their computers.
BRUMFIEL: Leap seconds have crashed airline reservation systems. They're believed to have briefly shut down Russia's GPS satellite system, and there's potential for even greater mischief as things like financial trading become ever more precise in their use of time. That's why some, like Tom O'Brian, say it's time to lose the leap second.
O'BRIAN: The U.S. believes that it would just be in the interest of everyone in the United States and across the world if we didn't have to deal with that extra complication and difficulty.
BRUMFIEL: But over centuries, the seconds would add up. The day would fall out of sync, and eventually, sunrise would come at noon. Supporters of the leap second say that rather than creating chaos for future generations, we should all live with an extra second every now and then. Geoff Brumfiel, NPR News. Transcript provided by NPR, Copyright NPR.