Why the Leap Second Is Disappearing

Why it’s suddenly everywhere

1. 01

   On October 15, 2026, delegates from 69 countries at the 28th General Conference on Weights and Measures (CGPM) in Versailles passed Resolution 5: from May 20, 2027, UTC will run continuously — no leap second will ever be inserted or deleted again.

2. 02

   The official trigger was fear of a "negative leap second": Earth’s rotation has been speeding up, and a March 2025 CCTF–IERS workshop put the probability of needing one by 2035 at 30%. Nobody has ever deleted a second from global time, and no software has been tested for it.

3. 03

   The resolution sets a new leash of 3,600 seconds — one full hour — on how far UTC may drift from astronomical time, effectively postponing the problem to a "leap hour" thousands of years from now.

4. 04

   Russia voted against. Its GLONASS satellites bake leap seconds directly into the navigation system, so Moscow had argued for pushing the change to 2040.

The timeline

1. 1967The atom beats the Earth

   After Louis Essen and Jack Parry build the first practical caesium atomic clock at the UK’s NPL in 1955, the second is redefined as 9,192,631,770 caesium vibrations — severing it from Earth’s rotation.

2. 1972The leap second is born

   To keep atomic UTC within 0.9 seconds of astronomical time (UT1), the world starts inserting extra seconds. The famous timestamp 23:59:60 appears for the first time.

3. 2012-06-30The night half the web crashed

   A leap second knocks out Reddit, Mozilla, LinkedIn, Foursquare and Yelp as Linux/Java systems choke on a 61-second minute; Qantas check-in systems fail and flights are delayed.

4. 2016-12-31The 27th — and final — leap second

   The last ever adjustment is applied. Cloudflare’s DNS breaks anyway, because one code path assumed time never runs backward. UTC now sits 37 seconds behind atomic time.

5. 2022-11The countdown starts

   The 27th CGPM votes to stop adjusting UTC by 2035. Meta, Google, Microsoft and Amazon have campaigned for this; Meta engineers call every leap second "a major source of pain" in a widely shared blog post.

6. 2024-03Climate change buys time

   Duncan Agnew publishes in Nature that melting ice in Greenland and Antarctica is slowing Earth’s spin enough to delay the first negative leap second from ~2026 to ~2029 — but it is coming.

7. 2026-10-15The abolition vote

   The 28th CGPM in Versailles moves the date up: continuous UTC from May 20, 2027, with a one-hour tolerance band. Russia votes no; the resolution passes.

The backstory

Two clocks that never agreed

Humanity runs on two clocks that fundamentally cannot sync. One is the Earth itself: a spinning, wobbling planet whose rotation is slowed by lunar tides, stirred by its molten core, and nudged by shifting oceans and jet streams. The other is the atom: since 1967, the second has been defined by caesium vibrations, and International Atomic Time (TAI) ticks with a uniformity no planet can match. Coordinated Universal Time (UTC) — the time your phone, stock exchange and power grid all use — was the compromise between them: it ticks at the atomic rate, but every few years, when Earth lagged more than 0.9 seconds behind, timekeepers at the IERS would order a one-second stutter to let the planet catch up.

Fifty-four years ago that looked like a reasonable deal. Computers were rare, and astronomers wanted their telescope software to keep working off civil clocks. The cost of the compromise only became visible as the world digitized and began synchronizing itself down to the nanosecond — the same arms race now driving telecom networks toward exotic hardware like hollow-core fiber. A leap second creates a minute with 61 seconds, and a remarkable amount of software is built on the quiet assumption that such a thing cannot exist. The 2012 crash that took down Reddit and Qantas bookings became the canonical horror story; Google\u2019s answer was the “leap smear” — secretly slowing its clocks across a day so the extra second never formally appears — and Meta spent 17 hours smearing each event, later publishing a viral engineering post arguing the leap second should simply be left in the past. Different smear algorithms at different companies made global time subtly inconsistent — exactly what a time standard is supposed to prevent.

The accelerating Earth

The twist that turned a slow-burn debate into an emergency is that Earth sped up. For reasons geophysicists are still untangling — core dynamics, glacial rebound, shifting water mass — the planet has been rotating faster over the past decade, recording its shortest days ever measured. If the trend continued, timekeepers would have to do something never attempted: a negative leap second, deleting 23:59:59 from a day. In March 2025, a CCTF–IERS workshop of Earth-rotation experts estimated the probability of needing one would hit 30% by 2035.

The BIPM\u2019s official resolution is blunt about what that means: the preparation cost for critical infrastructure would be “similar to their preparations for the millennium bug” — for an event that has never been tested at scale. There is even a climate subplot: Agnew\u2019s 2024 Nature paper showed that melting polar ice, by redistributing mass toward the equator like a skater extending her arms, delayed the negative leap second by roughly three years. Global warming, in effect, bought the world a little time to abolish time\u2019s oddest unit.

What actually changes on May 20, 2027

Practically, almost nothing — which is the point. The last leap second was inserted in 2016, UTC sits 37 seconds behind TAI, and none is expected before the switch. From May 20, 2027 (World Metrology Day), UTC simply stops chasing the planet. The gap to astronomical time is allowed to grow to one hour — a gap experts say would take thousands of years to accumulate — at which point a “leap hour” would be handled the way daylight-saving changes already are. Astronomers and satellite operators who need UT1 will read it from IERS bulletins and GNSS messages, as many already do.

The holdout was Russia, whose GLONASS constellation applies leap seconds inside the navigation system itself; Moscow wanted until 2040 to replace aging satellites and voted against the resolution. It passed anyway. And buried in the same Versailles meeting were two quieter agenda items that show where timekeeping goes next: a future redefinition of the second based on optical clocks, and a lunar reference time scale for a Moon economy. The leap second\u2019s obituary, in other words, arrived in the same week humanity started drafting clocks for other worlds.

Key facts

Vote date

2026-10-15

Effective

2027-05-20

Leap seconds total

27 (all positive)

Last one

2016-12-31

TAI minus UTC

37 s (frozen)

New tolerance

1 hour

Negative-leap risk

30% by 2035

Voted against

Russia

What people are saying

Common questions

Will my phone or computer notice anything?

No. No leap second was scheduled anyway before 2027, so abolishing them simply removes a future risk. The practical work — updating standards documents, NTP/PTP software assumptions and the ITU radio codes that broadcast the UT1–UTC difference — happens inside infrastructure over the next few years, invisibly to users.

Will clocks stop matching sunrise and sunset?

Not in any human lifetime. The drift accumulates at roughly a second or two per year at most. By 2100 UTC may be about a minute ahead of astronomical time, and the resolution allows a full hour of drift — a gap that would take millennia to arrive, if it ever does.

What was so scary about a negative leap second?

Every leap second so far has been added, so all real-world testing assumes a 61-second minute. Deleting a second breaks different code paths — anything assuming timestamps always increase, or that minute 60 always exists. A 2025 expert workshop estimated a 30% chance one would be needed by 2035, and the BIPM compared the required preparation to the Y2K effort.

Who opposed abolishing it, and why?

Russia voted against, because GLONASS applies leap seconds inside the satellite navigation system itself, and replacing that hardware takes time; Moscow wanted 2040. The UK historically defended the link to Greenwich mean solar time but came around — the leap-hour compromise keeps Earth time officially tethered, just very loosely.