Google, Meta, Amazon, and Microsoft have launched a campaign to finally be rid of the "leap second", joining the US National Institute of Standards and Technology and the Bureau International de Poids et Mesures in France in their opposition to the system launched in 1972.

What is a leap second? Let's start with the basics.

What is a leap second?

Quick question: how long is a day? If you said 24 hours, we're going to be pedantic and tell you no, you are wrong. So wrong, in fact, that if we left you in charge of satellites there would be utter chaos.

The Earth's rotation has altered significantly over time. Right now, the Earth rotates just over 365 times on its axis in the time it takes to orbit around the Sun. By looking at ancient corals, however, we can figure out that this wasn't always the case, and the Earth used to spin a lot faster than it does now. Hundreds of millions of years ago, in fact, the Earth made a full 420 spins in the time it took to go around the Sun.

While coral grows, it puts down a fine layer of calcium carbonate every day. Since corals grow more in the dry season than the wet, you can then count up the lines of calcium carbonate deposits between the seasons, ending up with a number of days in a year. It's like a more accurate version of tree rings.

Through this, scientists have figured out that 444-419 million years ago, the Earth spun 420 times in a full rotation of the Sun, while a few million years later it slowed to 410.

There are all sorts of factors that affect the speed of rotation, such as changing sea levels and shifts within the Earth, though the biggest factor is that the Moon is moving away from the Earth (who can blame it) and as the two bodies interact, the result is the Earth slowing down.

Normally, a leap second is introduced every now and then in order to account for the slowing of the Earth. These take place usually at the end of June or December. On these days, clocks add in an extra second in order to factor in the slowing of the Earth.

What is a negative leap second?

In 2020 the Earth's rotation sped up again. 

The record for the shortest day (since we began measurements with precise atomic clocks in the 1960s) was set in 2005. In 2020, that record was broken 28 times. Since records began, the average day has been getting longer, until 2020 when on average during the year, the days were about 0.5 milliseconds shorter.

If this were to happen again, the International Earth's Rotation Service may need to add a negative leap second. 

"It is certainly correct that the Earth is spinning faster now than at any time in the last 50 years," senior research scientist with the National Physical Laboratory’s time and frequency group, Peter Whibberley, told the Telegraph, adding that it may be that the change leads to the leap second system being removed altogether.

“It’s quite possible that a negative leap second will be needed if the Earth’s rotation rate increases further, but it’s too early to say if this is likely to happen."

What's the problem with leap seconds?

While a leap or negative leap second may not affect your own timekeeping significantly (you can't use it as an excuse for why you're late for work, for instance), it can have a big impact on satellite communications, which rely on solar time (measured by Earth’s rotation relative to the Sun) aligning with Coordinated Universal Time (UTC). It can also have an impact on all sorts of other computer systems that aren't ready for the change.

In 2012, a big outage famously took place on Reddit when the Linux operating system tit was using failed to take into account the leap second. 

"As an industry, we bump into problems whenever a leap second is introduced," Meta wrote in a blog explaining why they're supporting the removal of the leap second system. "And because it’s such a rare event, it devastates the community every time it happens. With a growing demand for clock precision across all industries, the leap second is now causing more damage than good, resulting in disturbances and outages."

The company – along with other big hitters like Google, Amazon, and Microsoft – claims that the leap second now does more harm than good.

"While the leap second might have been an acceptable solution in 1972, when it made both the scientific community and the telecom industry happy, these days UTC is equally bad for both digital applications and scientists, who often choose TAI or UT1 instead," Meta wrote.

Tech companies have tried various ways to get around the problem, and at Meta they use a "leap smear"; spreading fractions of the leap second out over the course of a day. It may work, but it's less than an ideal solution. They, and others, would like to scrap the leap second altogether.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.