Time will stop in the night of June 30 and July 1, but don’t worry: It will only be for a second. 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.

With the leap second on June 30, coordinated universal time (UTC) will move from 23:59:59 to 23:59:60, and then to 00:00:00 on July 1. Most of us won’t notice the addition 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. The last leap second was added at 23:59:60 UTC on June 30, 2012.

According to timekeepers at the International Earth Rotation and Reference Systems Service, the time determined by super-regular atomic clocks and the observed rotation of Earth have yet again become mismatched. The International Earth Rotation and Reference Systems Service continuously monitors 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. The last leap second was added in 2012, but in the early 1980s, time scientists were adding them every year.

Atoms Versus Astronomy

Modern timepieces tick to the rhythm of Earth’s rotation based on the 24 hours it takes for the planet to complete one spin on its axis.

But on the level of seconds, the planet’s rate of spin fluctuates, mainly due to the gravitational effects of Earth’s moon. The push and pull of our orbital partner causes the planet’s massively heavy reserves of water to slosh around, which decelerates the spin between 1.5 and two milliseconds a day, on average.

Leap seconds are used to fill in the gap and make Earth’s observed speed align with our most accurate clocks — atomic devices that define a second by measuring the regular decay of radioactive elements such as cesium.

Leap seconds were first introduced in 1972, and at that point, atomic clocks and astronomical clocks were already off by ten seconds so researchers added ten seconds all at once in 1972 to the world’s astronomical clocks. So far, a total of 25 leap seconds have been added. This means that the Earth has slowed down an additional 25 seconds compared to atomic time since then. However, this does NOT mean that the days are 25 seconds longer nowadays.The only difference is that the days a leap second was added had 86,401 seconds instead of the usual 86,400 seconds.

Though people may not generally mind these slight tweaks to timekeeping, computers definitely do. To a computer, a minute is always 60 seconds—no more, no less. And in today’s global digital networks, our telecommunication systems can be sensitive to even tiny changes in their internal clocks.

Unfortunately, leap seconds present a big problem for computers which are programmed to count 60 seconds per minute, not 61. The unexpected addition to the calendar can – and may – hobble computer systems around the world, potentially leading to outages, problems with utilities, navigation and communications systems.

The last leap second that occurred in 2012 is believed to have crashed systems running the Linux and programmes written in the Java coding language, as well as popular online services like Reddit, Yelp, LinkedIn, FourSquare, Gawker, Mozilla and StumbleUpon, among other sites and apps.

Google developed what it calls one of its “coolest workarounds” after a 2005 leap second made some of its computer systems to stop accepting new commands. To avoid the leap second issue, Google gradually adds a couple of milliseconds to its servers’ clocks throughout the day when a leap second is to occur — just enough to stave off disaster by the end of the day but not enough to trip any alarm bells when the adjustments are made.

Although the ‘leap’ glitch is not as complex as the Y2K that scared the world at the beginning of the new millennium, it has potential to cause a few hiccups to some networks Tuesday/Wednesday night. Key financial services around the world are frantically trying to avoid a leap second that threatens to disrupt functioning including global trading during the June 30 event. To circumvent the potential for disaster, several exchanges are ending trading early or opening late on June 30 or July 1.

In an environment where market values fluctuate and deals are done in tiny fractions of time, a rogue second could potentially affect millions of dollars, but for most internet users, the leap second should be added without too much of a hitch. Good luck, everyone!

Hat Tip: Jane J. Lee, 2015, National Geographic, 26 June.