On 27 days in the past century, time has gone backwards for a second.
Or, at least, our clocks have.
These extra "leap seconds" were added to keep our ultra-precise atomic clocks aligned with the Earth's slightly less reliable rotation.
But we have probably already leapt our last second.
Later this week, delegates will meet near a French palace in Versailles to decide whether to abandon the leap second for good.
In its place, if the vote succeeds, it will ask people centuries in the future to observe a "leap hour".
How Earth's spin affects clocks
By traditional definition, a second is an 86,400th of a day.
But Earth's rotation is not perfectly uniform. Our 24-hour days can vary in length by a thousandth of a second (a millisecond).
There are a number of reasons for this inconsistency.
Earth's rotation is generally slowing down, thanks mostly to the pull of the Moon. Astronomers see this in historical records of solar eclipses from more than 3,000 years ago.
Astronomers found that written records of solar eclipses in ancient China and the Middle East did not match astronomical models – until the Earth’s slowing rotation was factored in. (Wikimedia Commons: Kalan, CC BY 3.0)
But over shorter time periods, our planet occasionally spins faster.
Since about 2020, Earth has sped up.
Lucia McCallum, a geodesist at the University of Tasmania, said a number of things can affect the Earth's spin.
"Atmosphere, wind, big ocean currents, glaciers melting, big volcanoes, redistribution of mass in the inner Earth," Dr McCallum said.
This slight variation means the traditional definition of a second isn't precise enough for technologies like GPS, which needs to measure fractions of milliseconds to work.
Atomic time more precise
In the late 1960s, the governing body of the International Bureau of Weights and Measures (BIPM), which standardises the world's measurements, adopted a more precise definition of a second based on measurements of caesium atoms.
This is what underpins coordinated universal time (UTC).
UTC is tracked by atomic clocks, currently the most accurate measures of time, although newer "nuclear clocks" may soon surpass them.
The National Measurement Institute maintains atomic clocks, which are the most accurate methods of keeping time. (Supplied: National Measurement Institute)
Michael Wouters, leader of time standards at Australia's National Measurement Institute (NMI), said the atomic second was as close to the traditional second as possible.
"There was no exact value to begin with, because it was changing all the time."
Dr McCallum helps measure the speed of Earth's rotation, using telescopes pointed at extremely distant black holes.
"We use those black holes as anchor points," Dr McCallum said.
Radio telescopes, like this one at Hobart's Mt Pleasant Observatory, can observe an ultra-distant black hole and compare their signals to establish how quickly the Earth is spinning. (Supplied: Jim Lovell)
These observations are vital for satellite navigation, and they are also used to tell when Earth's rotation falls out of sync with UTC, prompting a leap second.
Since their introduction in 1972, leap seconds have been added to our clocks 27 times, most recently at the end of 2016.
How leap seconds mess with modern life
Leap seconds have become increasingly difficult to deal with in the computer age.
Darryl Veitch, a researcher in computer networking at the University of Technology Sydney, said leap seconds break some of software's assumptions about time, such as that it only moves forwards and there are 60 seconds in a minute.
But Dr Wouters said people "forgot about this problem, and they wrote software that then crashed when time went backwards".
Several things slow down the Earth's rotation, but the influence of the Moon and tides is a major factor. (Supplied: NASA)
Unlike time zones or daylight saving, leap seconds are irregular, scheduled only six months in advance.
Previously, they have caused platforms like internet technology provider Cloudflare and social media site Reddit to crash.
In 2022, the BIPM's governing body decided to abandon the leap second by 2035 and start discussing a different method of keeping Earth's days in sync with UTC.
But the decision needed to be fast-tracked when Earth started speeding up, because it meant the next leap second may need to be "negative".
Increasingly interconnected systems mean that a timekeeping error in one place can snowball. (Unsplash: Lukas Blazek)
Dr Wouters said a negative leap second was less likely to cause problems because it involves time jumping forwards rather than backwards, but Professor Veitch was not so sure.
"It's never happened before, and therefore the software for that has never been tested," Professor Veitch said.
Since leap seconds will soon be retired, introducing a negative one seems an unnecessary risk.
"It's being cautious about an unusual event that's prompting the accelerated timetable," Dr Wouters said.
Nor are there any clear economic or practical arguments for preserving leap seconds, beyond keeping software engineers busy.
"Some people have a philosophical objection to [losing leap seconds], and that's because we're disconnecting the way we count time from our actual experience of it," Dr Wouters said.
"But clocks themselves are a very artificial thing."
Vote may create 'leap hour'
The governing body of the BIPM will vote on leap seconds at its meeting on 12 –15 October.
The leading proposal is to let UTC drift from Earth's rotation until it is an hour out of sync, and then observe a "leap hour" to bring it back.
Ceasing leap seconds means that Earth's day and night will slowly fall out of sync with official time, but it will take more than a century for a minute of difference to build up. (ESO: Anton Yankovyi, CC BY 4.40)
Earth's rotation currently loses less than a second per year, so it could take more than a millennium for this leap hour to become necessary.
Our current timekeeping systems are less than a century old, and it is likely they will look very different by then.
"I would imagine in 100 years, software would be completely automated, and we just won't have the kinds of problems we have now," Professor Veitch said.
Dr McCallum said it does not matter whether UTC is seconds behind Earth's days.
"What we still have to maintain is measuring Earth's rotation."
If the vote succeeds, Dr Wouters said there would be one difference to his work.
In the past, leap seconds were often added on December 31, so some staff at the NMI had to work on New Year's Day to check there were no time measurement problems.
"I'm not too unhappy that that won't be necessary anymore."