Time’s up for the leap second
Timekeepers meeting in Versailles have voted to stop adding leap seconds to the world’s clocks from May 2027. Official time and the spinning Earth will now be allowed to drift apart by up to an hour, and the reason is our planet has started spinning a little faster.
The hour of power.
Tuesday’s NerdNews profiled the next generation of incredibly precise timekeeping devices: clocks that take their beat not from the electrons circling an atom, but from the nucleus of the atom itself. They are potentially 10 times more accurate than our currently incredibly precise best timepieces.
Today NerdNews considers time on a much larger physical scale, time dictated by the rotation of the Earth itself.
Turns out a “day” is a little bit more complicated a concept than you might think.
A second opinion.
As Tuesday’s piece made clear, a second as measured by an atomic clock is an incredibly precise thing, based on 9,192,631,770 vibrations of a caesium-133 atom. Averaged out over hundreds of atomic clocks in labs around the world, we call it UTC, Coordinated Universal Time, and it powers telecommunications and the internet with phenomenal precision.
The Earth, by comparison, is a lousy timekeeper. A massive factor in this is that the Earth does not spin at a constant speed. Tides from the Moon, winds and movements in the Earth’s molten core mean that the Earth rotates at subtly varying speeds across a year. As a result, after one complete orbit of the Sun, time as determined by the rotation of the Earth, known as UT1, could be out by fractions of a second compared with atomic time.
To reconcile this difference, in 1972 scientists introduced the leap second. Just this month, the world’s time nerds gathered in Versailles and voted to replace the leap second with a leap hour.
So what was a leap second?
So since 1972, whenever UT1 drifted too far from UTC, timekeepers have inserted a leap second, an extra 61st second at the end of a minute. There have been 27 of them, all positive.
The most recent one arrived at about 11am on New Year’s Day 2017 in Sydney (10:30am in Adelaide) and you almost certainly didn’t notice. Computers did. Software is written assuming a minute has 60 seconds, and in 2012 a leap second helped knock out Qantas’s check-in system and delay flights around the country.
Spin doctors.
Just in case astronomical time wasn’t complicated enough for you, in the mid-2010s the Earth started spinning faster. You might remember in particular 5 July 2024, the fastest day ever recorded, coming in 1.66 milliseconds short of 24 hours!
The problem is, if this pattern continued unchecked, we’d eventually have to remove a leap second from the year, as opposed to our traditional adding one in. This is potentially far more difficult to do. Adding a leap second means clocks at Greenwich Mean Time tick 11:59pm and 58 seconds, 59 seconds, 60 seconds, then midnight (and the rest of the world follows suit at whatever local time may be). Removing one means jumping straight from 58 seconds to midnight, and that has been compared to a potential mini Y2K.
“A negative leap second has never been added or tested, so the problems it could create are without precedent.” — Patrizia Tavella, director of the time department, International Bureau of Weights and Measures.
Even with climate change reducing the risk of a negative leap second, a mere 10% likelihood of it being employed still had experts in a lather.
Hour glass half full.
It’s possible to question the value of the leap second in 2026 in ways that simply weren’t possible in the 1970s and 80s. Before GPS, navigation and even astronomy relied on Earth time and atomic clock time being closely in sync. The two clocks being out by a single second could put your position almost half a kilometre out at the equator.
Now that navigation has moved across to GPS, and astronomers can simply look up the exact difference between the two clocks, we don’t need to add messy, sometimes challenging, often announced at fairly short notice, leap seconds in and out of our time.
As a result, at the 28th General Conference on Weights and Measures in Versailles this month, delegates voted that from World Metrology Day, 20 May 2027 (set your watches!), no leap seconds will be added or subtracted. The two clocks will be allowed to drift apart by up to an hour before anything is done about it. This brought forward the original 2035 deadline.
Why the rush? That scary negative leap second.
“After 30 years of debates, we are ready.” — Noël Dimarcq, president of the Consultative Committee for Time and Frequency, International Bureau of Weights and Measures.
“The maximum value for the difference |UT1-UTC| will be 3 600 seconds (1 hour), ensuring the long term continuity for UTC for several centuries” — Resolution 5 of the 28th General Conference on Weights and Measures (2026).
The vote wasn’t unanimous, and while reports haven’t said who opposed it, Russia have for a long time been holdouts on this change. Their GLONASS satellite system has leap seconds inbuilt. Other traditional opposition has come from people simply acknowledging that throughout history we’ve based time on the position of the Sun in the sky. The leap hour represents a fundamental change in that philosophy.
If a leap second confused you, and you therefore find a leap hour 3,600 times as concerning, fear not. Experts estimate it will be centuries before any timekeepers need to make this sort of decision.
If you think about it, we already live with clock time and sun time being quite different. The sun rises about 22 minutes earlier in Kalgoorlie than in Perth, even though they share the same time zone.
So for those of you hankering after one more leap second, I’m sorry. There’s none scheduled before the change kicks in. Turns out that bonus second none of us noticed, around 11am on 1 January 2017 (Sydney time, 8am in Perth), was the last one anyone will get.
Further Reading:
• “Scientists vote to end leap seconds, redefining how the world keeps time”, Emma Gometz, Scientific American, 2026.
• “A global timekeeping problem postponed by global warming”, Duncan C. Agnew, Nature 628, 333–336, 2024. https://doi.org/10.1038/s41586-024-07170-0
• “Earth Rotation Records Spur October Vote to Avert Negative Leap Second”, Tech Times, 2026.
• “A vote is being held on whether to abandon the leap second”, Science Media Centre Spain, 2026.
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