On December 31, 2016, metrologists around the world—especially those living under Greenwich Mean Time (GMT)—will add a single second to their New Year’s Eve countdown. International timekeepers have decreed, after some debate, that the atomic clock is due a leap second.

Expect UTC to keep the leap second for at least another eight years.

The ITU World Radiocommunication Conference (WRC-15), in session in Geneva, has decided that Coordinated Universal Time (UTC) should retain the “leap second”, and has deferred any future decision on the subject until the 2023 World Radiocommunication Conference, that will again consider a new reference time scale to be considered by in 2023. ITU has the responsibility for distributing UTC.

The issue arises due to the established practice of keeping UTC in step with sidereal (astronomical) time, or in other words, with the time we use day-to-day. As the length of both the day and the year (Earth’s rotation and orbit periodicity) varies, adjustments are required to our timebases.

This has become an increasingly significant issue as more and more communications (and other) global systems are synchronous on a microsecond (or finer) scale. Providers (e.g. GPS) and users of synchronized time must work in lock-step whenever adjustment (leap) seconds are added or subtracted from UTC. The alternative, widely proposed, is for UTC to maintain a continuous timebase going forward, and not to add/subtract adjustments: which would imply that over longer timescales, it would drift markedly apart from everyday time.

The ITU World Radiocommunication Conference (WRC-15), in session in Geneva, has decided that Coordinated Universal Time (UTC) should retain the “leap second”, and has deferred any future decision on the subject until the 2023 World Radiocommunication Conference, that will again consider a new reference time scale to be considered by in 2023. ITU has the responsibility for distributing UTC.

The issue arises due to the established practice of keeping UTC in step with sidereal (astronomical) time, or in other words, with the time we use day-to-day. As the length of both the day and the year (Earth’s rotation and orbit periodicity) varies, adjustments are required to our timebases.

This has become an increasingly significant issue as more and more communications (and other) global systems are synchronous on a microsecond (or finer) scale. Providers (e.g. GPS) and users of synchronized time must work in lock-step whenever adjustment (leap) seconds are added or subtracted from UTC. The alternative, widely proposed, is for UTC to maintain a continuous timebase going forward, and not to add/subtract adjustments: which would imply that over longer timescales, it would drift markedly apart from everyday time.

On June 30th at precisely 23:59:59, the world’s atomic clocks will pause for a single second. Or, to be more precise, they’ll change to the uncharted time of 23:59:60 — before ticking over to the more worldly hour of 00:00:00 on the morning of July 1st, 2015. This addition of a leap second, announced by the Paris Observatory this week, is being added to keep terrestrial clocks in step with the vagaries of astronomical time — in this case, the slowing of the Earth’s rotation. And it’s a bit of a headache for computer engineers.

Leap seconds are like the Y2K bug in that they threaten to throw out of sync time as measured by computers and time as measured by atomic clocks. But while Y2K was a single instance (computer systems that were used to abbreviating the year to two digits were confused by “2000″ and “1900″), the addition of leap seconds are a regular problem. The first was added back in 1972

Unfortunately, when the last leap second was added back in 2012, more than a few sites had trouble keeping pace. As reported by Phys.org, Foursquare, Reddit, LinkedIn, and StumbledUpon all crashed when the leap second ticked unexpectedly into place. In the case of Reddit, the problem was eventually traced back to a Linux subsystem that got confused when it checked the Network Time Protocol only to find an extra second.

“Almost every time we have a leap second, we find something. It’s really annoying, because it’s a classic case of code that is basically never run, and thus not tested by users under their normal conditions.”

Instead, companies have been forced to create their own workaround, with Google’s “leap smear” perhaps the best-known example. As the company’s site reliability engineer Christopher Pascoe explained in a blog post, the usual fix is to turn back the clocks by one second at the end of the day, essentially playing that second again. However, says Pascoe, this creates problems: “What happens to write operations that happen during that second? Does email that comes in during that second get stored correctly?” Google’s solution is to cut the extra second into milliseconds and then sprinkle these tiny portions of time into the system imperceptibly throughout the day. “This [means] that when it became time to add an extra second at midnight,” says Pascoe, “our clocks [have] already taken this into account, by skewing the time over the course of the day.”

Of course, not every website or company has the sort of engineering resources needed to implement something like a “leap smear,”

2015 is a wonderful year for anyone who has a special interest in time or simply experiences its passage.

In March, we enjoyed this century’s Pi Instant, when clocks on 3/14/15 touched that rare Pi moment at 9:26:53.58979323….

But today (30 June 2015) is special because it includes a Leap Second. The fourth such event this century

The time standard, known as Coordinated Universal Time (UTC), as measured by atomic clocks, differs slightly from mean solar time, or Universal Time (UT1), which is proportional to the rotation angle of the Earth on its axis.

In Recommendation ITU-R TF.460-6, the Planetary Council (aka ITU-R World Radiocommunication Conference) has decreed that when UTC and UT1 would differ by more than 0.9 seconds, a leap second should be added (or substracted) from UTC. Implementing this recommendation falls to the Planetary Time Committee (aka International Earth Rotation And Reference Systems Service, or IERS).

The last time the IERS added a leap second, things basically went haywire – largely because OS schedulers failed to stop scheduled processes or started processes prematurely, hanging systems around the world. This year, the industry is confident that there’s nothing to worry about.