Historically, time has been measured by the rising and setting of the sun, but the time on your computer and iPhone are based on far-more-accurate modern atomic clocks that determine time by the frequency of electrons in atoms. To do this, the passage of one second is measured by the amount of time it takes a cesium atom to oscillate exactly 9,192,631,770 times.
Atomic time and universal time were synchronized at the start of 1958 and have been diverging ever since. The second as measured by the atomic clock doesn’t change, but the rate at which the Earth spins does, due to tidal forces from the moon and other factors. Leap seconds have been implemented every few years since 1972 to keep the two systems in sync. Without the leap seconds, the chasm between the two measures of time would grow by a couple of thousandths of seconds per day, creating hours of difference over the centuries
The last leap second, which was added in June 2012, caused a variety of global hiccups. In Australia, up to 50 flights were delayed when the airline Qantas’ computerized reservation system collapsed, unable to adjust for the extra second. Several websites, including Reddit, Foursquare, and LinkedIn crashed when their Linux operating systems were caught off guard by the change. In 1998, the leap second caused a mobile phone blackout in the US, and some experts have expressed worries leap seconds could affect major power grids in the future.
Given the host of problems caused by the leap second, a growing chorus of countries is calling for its elimination. The United States is one of the most vocal opponents of the leap second. In November, the International Telecommunications Union, a United Nations agency, will vote on whether to keep the practice in place. If it decides not to, leap seconds will no longer be added after 2020.
Demetrios Matsakis is the Chief Scientist for Time Services at the US Naval Observatory, which sets the precise time for all forms of digital communication, including the internet, banking systems, GPS, military operations, and cell phone networks. He said it is impossible to predict what will happen when the leap second is added this year.
“I would suggest not to be in the air flying when the leap second is enacted.”
“There will definitely be failures of some systems — how significant, I don’t know,” he said. “I would suggest not to be in the air flying when the leap second is enacted.”
Matsakis said it is difficult to prepare computer systems for the added second. In addition, the fact that leap seconds must be input by hand leaves a lot of room for human error.
“A lot of modern systems don’t work well with leap seconds,” he said. “The biggest problem is the computers will think that time has gone backwards, and a lot of computer programs, like Oracle for example, think ‘somethings going wrong here’ when they see time going backwards, and they shut down.”
Some companies have attempted to work around the leap second. Google came up with a solution it calls the “leap smear” in which it divides the extra second into milliseconds and incorporates it into the system throughout the day. Matsakis said this may not be as effective as it appears.
“That is considered a problem too,” he said. “A lot of different groups have different ways of doing things, and they all have their own solutions. If their solutions are different, it could lead to the proliferation of time scales. That’s considered a bad thing—you always want people to be on the same time.”
The unpredictability of the speed of Earth’s rotation poses additional problems. Matsakis said someday, a second may have to be subtracted, which would cause even more issues. In his view, the best solution is to get rid of the practice of the leap second entirely.
"We should eliminate leap seconds because of the real-world practical impossibility of reliably implementing them, due to either their inherent nature or to general lack of knowledge of their very existence," he said.
He added that the only countries still opposing the elimination of the leap second are doing so for political reasons.
“I’m aware of only two nations that are adamantly against it: Russia and the United Kingdom,” he said. “In the United Kingdom, they still think the world runs on Greenwich Mean Time, and this would mean a departure from GMT, so that’s an issue of national pride. We always joke that it’s the last vestige of the British empire they’re trying to maintain.”
For Russia, eliminating the leap second would mean an expensive redesign of its GPS equivalent, which may be fueling its support for the practice. Matsakis maintains that, since time is subjective, we should probably all agree on the easiest version of it.
“There’s no correct time. Isaac Newton thought there was an absolute time but Einstein showed there wasn’t, that time is relative,” he said. “Whatever the clocks say is the time, is the time, and as long as we all agree on it we can synchronize our society.”