Today, everyone gets an extra second of time. Just before midnight according to UTC (coordinated universal time), the world's atomic clocks will have an extra second inserted. After 11:59:59, June 30, and before 00:00:00, July 1, will appear a "leap" second.
"That means that there are two seconds with the name '59,' because the name '60' does not exist," explained Felicitas Arias, the director of the time department at the International Bureau of Weights and Measures who was referred to me by a colleague as the "Queen of Time."
Leap seconds have a pretty fundamental purpose: they keep our clock-time in sync with solar time. But they're increasingly controversial. With the need for an international time standard, countries across the globe are arguing over whether we should keep adding leap seconds as needed, or forget about them. It's currently at a bit of a stalemate.
We've been telling time using the Sun's position according to the Earth's rotation for millennia. Greenwich Mean Time—the former international time standard—referred to solar time at London's Royal Observatory. But then came atomic clocks, and in the 1960s UTC began.
"But at the same time, the astronomers wanted to have a strong link between both clocks, so they asked that the difference between both clocks [solar and UTC] should not exceed one second," recalled Daniel Gambis, Head of the Earth Orientation Centre at IERS (the International Earth Rotation and Reference Systems Service). He's the guy who announces when we should add leap seconds.
The leap seconds are needed because atomic time isn't exactly the same as solar time. The Earth's rotation is slowing ever so slightly, which takes the two out of sync. Atomic time is too accurate.
"Since we cannot reset the Earth, we add the additional second in UTC," said Gambis.
It might all sound sensible, if a little trippy. But coordinating the addition of a second in all the world's clocks isn't easy. Our technology has a tendency to get screwy when time isn't continuous.
"We're always worried about system disruptions at the insertion of a leap second," said Arias. "The systems that today are transferring information, these are systems relying on time synchronisation, and the insertion of a leap second makes an abnormal event in those systems, so you don't know exactly what might happen."
The leap second, which always falls at the end of June or December, was announced six months in advance to give people time to prepare.
A second's blip might not seem like a big deal, but it can mean a lot in systems where time is critical. Stock markets, for example, are scrambling to keep in sync. And at the last leap second in 2012, a bunch of web services including Reddit experienced leap-related glitches.
Reddit told me in a statement that, this time, "Reddit engineering has been working diligently to reduce possible downtime. We have been collaborating with other industry experts to mitigate any possible effect of the leap second on our systems. Our small-scale tests are promising, and we'll be watching during the event to quickly respond to any unforeseen issues that may arise."
When I asked the US National Institute of Standards and Technology (NIST) if we should be worried about systems disruption around this leap second, Time and Frequency Services Group Leader John Lowe responded in an email, "That is a question for industry, ask them if they are prepared..."
This threat of a systems glitch is the main argument against leap seconds. It's a big reason why the US, along with countries including France, Italy, and Japan, wants to get rid of leap seconds.
Arias argued for that side of the battle. "My position, and the position of many administrations and many of my colleagues that are working in time metrology, is that the leap second is an artefact that was created in the 70s because there was a request from celestial navigation; there was a need to be referenced to Earth's rotation time," she said.
"Civil timekeeping today is not as close to solar time as many people believe."
But with GPS and other navigation systems, we don't need that anymore. And so given the incompatibility of modern technology with the leap second, it's more hassle than it's worth. "So it is time to make a change, and to make a change that will be affordable to all systems," said Arias, adding that users will be given years to prepare if leap seconds are dropped.
The other side of the debate, however, argues that we should keep leap seconds. We've always used the Sun as a guide for time—should we really give that up because computer programmers can't deal?
"The list of advantages of this process is much bigger than the list of the drawbacks," said Gambis, who, as an astronomer, thinks leap seconds are a good compromise between the Sun's movement and atomic time.
"Since we are on Earth, all our activities are on Earth, it's important to be in phase with the motion of the Sun and the motion of the Earth," he added.
That's the view of countries including the UK. Ahead of a public dialogue launched in 2014, former Science Minister David Willetts proposed that, "without leap seconds we will eventually lose the link between time and people's everyday experience of day and night."
It's true that synchronising our timekeeping with the Sun seems like a given; it's easy to forget that time is not just there, it's a construct we make. But Arias defended against the suggestion that a second here or there would throw our calendars off in any meaningful way.
"Civil timekeeping today is not as close to solar time as many people believe," she said, pointing out the discrepancies that arise owing to time zones covering large areas, daylight savings, and the exact solar time on a given day versus the average solar time (used in the likes of Greenwich Mean Time).
"In many countries you have differences of about two hours, in places of the territory, between civil time and solar time," she said.
The technological argument to lose leap seconds isn't unanimous, though. Some systems have the opposite problem: they're adapted to include leap seconds. GLONASS, Russia's answer to GPS, follows UTC. If the international time standard were to change, Russian representatives claim that the system would have significant difficulties.
With staunch arguments for and against the leap second, and a need to keep everyone in sync, it's all come to a bit of a stand-off.
But a decision will be made. After years of discussion and setbacks, it's down to the International Telecommunication Union to thrash it out at the World Radiocommunication Conference in November.
While Arias and Gambis have different opinions, they're both trepidatious about the potential outcome.
Gambis complained that the decision was now in the hands of administrative workers, not scientists. "I guess it's a mess now," he said. "The debate will be a mess, and it will be a kind of lobbying between different parties."
Arias also expressed concern that some proposals were being made by people who didn't have the necessary technical knowledge, such as one idea to have two timescales as a reference. "This is very dangerous," she said, pleading that administrations close to a decision abstain from supporting this option.
"This is equivalent to having two kilograms, two metres—two seconds," she said. "This will increase confusion. The reference is unique as the unit is unique."
It's unlikely the 2015 leap second will be our last. Any decision would take a while to implement so, while they're not regular like the extra day in a leap year, we'll probably have at least one more even if we decide to get rid of them.
Until then, use that extra time wisely.