Time will stop again on June 30, but there is no need to worry, as it’s only for a nanosecond. Researchers will add a sliver of time-a leap second-to the world's clocks.
Just as leap years keep our calendars lined up with Earth's revolution around the sun, leap seconds adjust for Earth's rotation. This kind of fine-tuning wasn't much of an issue before the invention of atomic clocks, whose ticks are defined by the cycling of atoms. Cesium-based clocks, one kind of atomic clock, measure the passage of time much more precisely than those based on the rotation of our planet, so adding a leap second allows astronomical time to catch up to atomic time.
Most of us won't notice the addition, which happens at 23:59:59 coordinated universal time (UTC), or 7:59 p.m. ET, unless we deal in timescales shorter than a second, or if we use a computer program that crashes because it can't handle the leap second. It's happened before: The 2012 leap second brought down Reddit, Gawker Media, and Mozilla.
"It's a major interruption mostly because there are a lot of systems that aren't prepared to handle the leap second correctly," says Judah Levine, a physicist at the National Institute of Standards and
Technology (NIST) in Boulder, Colorado. Leap seconds occur irregularly, which makes it hard for programmers to test their fixes, he explains.
Leap seconds don't come on a regular schedule because Earth's rotation varies, says Demetrios Matsakis, chief scientist for time services with the United States Naval Observatory in Washington D.C. Our planet is slowing down, but it does so in unpredictable ways. So some periods require more leap seconds than others.
The International Earth Rotation and Reference Systems Service continuously monitor our planet and will recommend adding leap seconds to the International Telecommunications Union (ITU). The ITU makes the ultimate decision on whether to add a leap second or not.