On 31 December 2016, at 23:59:60 UTC, the world lived through a second that does not exist: nobody noticed, but the planet’s official clocks counted a 61-second minute. It was the twenty-seventh since 1972. Nearly a decade later it is still the last one, and behind that patch lie two irreconcilable ways of measuring time that have been failing to agree for half a century.
Two clocks that never quite match
The first clock is atomic. TAI (International Atomic Time) is built by averaging roughly 400 caesium and rubidium clocks spread across laboratories worldwide. Its unit depends on nothing astronomical: a second is the duration of 9,192,631,770 oscillations of the hyperfine transition of the caesium-133 atom. It is such a stable reference that its drift is measured in nanoseconds per day.
The second clock is the planet. Earth’s rotation is measured with space geodesy: VLBI (very long baseline interferometry, which points radio telescopes at distant quasars), laser ranging to satellites and to reflectors left on the Moon, and the orbits of the GNSS constellations. That is how UT1, the real rotation angle of the Earth, is obtained.
The trouble is that the Earth is not punctual. Its spin rate oscillates, so UT1 and TAI drift apart. The compromise is called UTC: it uses atomic seconds but is kept within 0.9 seconds of UT1. When the gap threatens to break that margin, a leap second is inserted. Today UTC runs 37 seconds behind TAI, and the difference between UT1 and UTC is published as DUT1, which never exceeds that limit.
Who decides the patch, and how far in advance
The body in charge is the IERS (International Earth Rotation and Reference Systems Service), coordinated from the Paris Observatory. Twice a year it publishes Bulletin C: if a leap second is coming, it gives six months’ notice. Bulletin D arrives on the same schedule with the DUT1 prediction.
On paper it is simple; in practice it means propagating one order to billions of devices. Unix systems rely on the leap-seconds.list file, which carries an expiry date to force updates. The NTP protocol transports it in a two-bit field called the leap indicator: 0 means “no change”, 1 warns that the last minute will have 61 seconds, 2 that it will have 59, and 3 is the “unsynchronised” alarm. PTP (IEEE 1588), the sub-microsecond synchronisation protocol used in industrial networks and stock exchanges, carries equivalent flags: leap61, leap59 and currentUtcOffsetValid.
The Earth is slowing down… and speeding up too
The length of the day is not constant. Tidal friction from the Moon lengthens it by roughly two milliseconds per century. On top of that come post-glacial rebound — the crust still rising since the last ice age, changing the planet’s moment of inertia — atmospheric circulation, which in El Niño years shifts mass towards the equator and slows the spin, and the coupling between the molten core and the mantle.
The result is a day of about 86,400.002 seconds on average, with oscillations of a few milliseconds. Between 2020 and 2024 the Earth spun unusually fast and days shorter than the theoretical 24 hours were recorded, which unsettled the predictions. A 2024 study in Nature (Agnew) attributed part of that braking to polar melting: water moving from the poles to the equatorial ocean increases the moment of inertia and slows the spin. That detail matters, because it changes the date on which the world would have to remove a second instead of adding one.
A second that breaks computers
Inserting a second is trivial for an astronomer and a nightmare for an operating system. POSIX defines time as seconds since 1 January 1970, assuming every day has exactly 86,400 seconds: 23:59:60 does not exist. Every jump forces a decision about what to do with it, and the decisions are not consistent.
Known incidents range from 2012, when a leap second took down part of Reddit’s and Mozilla’s infrastructure (Java and MySQL miscomputed the length of an interval), to 1 January 2017, when a bug of this kind made Cloudflare’s DNS resolvers return errors for thousands of sites for hours. The Linux kernel has had its own scares with high-resolution timers.
Industry has answered with two opposing strategies. Google started applying leap smear in 2008: instead of inserting the second in one go, it spreads its effect by slightly adjusting the clock frequency in the hours before and after. Amazon does something similar in its time sync service. Others prefer the exact jump and rely on the leap-second file. The problem is mixing them: when a client that smears its clock talks to a server that jumps, discrepancies of up to a second appear and last for hours, visible only in event logs.
The minute nobody wants in the markets
Where the matter stops being anecdotal is finance. European MiFID II rules (RTS 25) require trading firms’ clocks to be traceable to UTC with a maximum divergence of 100 microseconds for high-frequency trading and 1 millisecond for everyone else. An extra or missing second is, literally, an operational risk event: someone has to decide whether that instant is recorded once or twice, and order-logging systems have to agree. That is why markets rehearse, months ahead, a change that lasts one second.
The second that would have to go
Every leap second in history has been positive. Not one has ever been removed. If Earth’s rotation speeds up enough, a 59-second minute would be required, and there the code is far less tested: NTP contemplates the case in theory and PTP has its leap59 flag, but most systems have never faced a clock stepping backwards by one second. Any software that assumes monotonicity — databases with timestamps, logging systems, message queues — may behave strangely. The 2024 study puts that hypothetical scenario around 2029, although the very melting that drives it also pushes it back.
The agreed ending: 2035
In 2022 the General Conference on Weights and Measures (CGPM) took a historic decision: from 2035 at the latest, leap seconds will stop being inserted. UTC would remain a scale of atomic seconds but would be allowed to drift as much as a minute from UT1 before anyone intervenes. Astronomy, geodesy and navigation would still receive an updated DUT1, only as scientific data rather than as a patch on everybody’s clock. Until then, the BIPM and the ITU-R have to settle the details.
Until that happens, the leap second will remain one of those invisible pieces of the internet: an adjustment almost nobody perceives, happening every few years, forcing engineers around the world to review code written decades ago. The next time your clock reads 23:59:59, remember that somewhere on the planet that minute lasted 61 seconds — and that somebody had to decide that for you.
