Pull to refresh
Logo
International timekeepers vote to abolish the leap second

International timekeepers vote to abolish the leap second

Rule Changes

69 countries set continuous UTC for May 2027, letting Earth's rotation drift up to an hour from atomic time

Yesterday: CGPM votes to end the leap second

Overview

Updated Yesterday

Delegates from 69 countries voted Thursday in Versailles to abolish the leap second. Starting May 20, 2027, Coordinated Universal Time (UTC) will run continuously, and Earth's rotation will be allowed to drift up to a full hour from atomic time before a leap hour resets it.

Since 1972, leap seconds have been added roughly once every few years to keep UTC within a second of solar time. The irregular jumps, announced only six months ahead, force data centers, satellite systems, and financial markets to patch around time steps. Without adjustments, researchers estimate a negative leap second now has a 30% chance of being needed by 2035.

Why it matters

Ending leap seconds removes the software failures and outages that periodic time jumps have caused across GPS, telecom, and financial networks.

Questions about this story

Free account needed to ask — your question is kept and asked for you right after sign-up. Answers are public.

No questions yet — be the first to ask.

Key Indicators

69
Member states voting on the resolution
BIPM member states met at the 28th CGPM in Versailles to decide the future of the leap second.
30%
Estimated chance of a negative leap second by 2035
A March 2025 CCTF and IERS workshop projected the risk, prompting the accelerated end of leap seconds.
1 hour (3,600 s)
New maximum difference allowed between UTC and Earth rotation
Raised from 0.9 seconds, the leap-hour threshold needs no intervention for more than a millennium.
~1,000+
Years expected before a leap hour is needed
Scientists estimate the hour-wide tolerance covers up to 3,000 years of Earth's rotational drift.

Voices

Curated perspectives — historical figures and your fellow readers.

Ever wondered what historical figures would say about today's headlines?

Sign up to generate historical perspectives on this story.

People Involved

Organizations Involved

Timeline

November 2022 May 2027

5 events Latest: Yesterday
Tap a bar to jump to that date
  1. Continuous UTC takes effect

    Upcoming Milestone

    Coordinated Universal Time stops accepting leap second insertions on World Metrology Day, ending a 55-year practice.

  2. CGPM votes to end the leap second

    Latest Decision

    28th CGPM adopts Resolution 5: continuous UTC from May 20, 2027, with a leap hour replacing leap seconds, allowing up to 3,600 seconds of drift.

  3. Pre-vote reports note Russian, UK opposition

    Context

    CNN and Al Jazeera report Russia wants to delay change to 2040 because GLONASS uses leap seconds.

  4. Workshop warns of negative leap second risk

    Report

    CCTF and IERS experts estimate a 30% probability a negative leap second will be needed by 2035, spurring urgency.

  5. 27th CGPM plans to redefine UTC by 2035

    Decision

    The conference set a goal to abandon leap seconds by 2035, with the exact mechanism to be drafted later.

Scenarios

1

Continuous UTC launches on schedule, no last leap second

Likely Resolves by May 20, 2027

Discussed by: BIPM Resolution 5; Scientific American; CBC

The resolution explicitly decided no further leap seconds before the May 20, 2027 start date. Technical stakeholders such as the International Telecommunication Union (ITU) and IEEE have pushed for exactly this: a clean cutoff with no intermediate step, letting systems retire leap second handling code.

2

Russia blocks or delays alignment of GLONASS

Possible Resolves by May 20, 2027

Discussed by: CNN; Al Jazeera

Russia pushed to defer the change to 2040 because GLONASS builds leap seconds into its transmitted time. Should Moscow refuse to align GLONASS with continuous UTC, Russian receivers could show a widening offset from much of the rest of the world's time infrastructure, testing the new standard's universality.

3

Leap hour procedure stays undefined as systems diverge

Unlikely Resolves by End of 2027

Discussed by: The Register; heise online

With no leap hour needed for over a thousand years, BIPM and IERS may defer defining how one would be inserted. In the meantime, systems like GPS already run 18 seconds ahead of UTC because they ignore leap seconds. A years-long gap without clear leap-hour rules could leave future civil time and satellite time drifting apart.

Historical Context

3 moments from history that rhyme with this story — and how they unfolded.

1972

Leap seconds introduced (1972)

Coordinated Universal Time adopted leap seconds in 1972 to stay within one second of Earth's rotational time, UT1. Since then, 27 leap seconds have been inserted at irregular intervals, most recently on December 31, 2016.

Then

Atomic time and solar time held together within a second, preserving the link between scientific clocks and the position of the Sun in the sky.

Now

The irregular insertions, announced only months ahead, became a recurring maintenance burden for data centers, satellite systems, and financial networks.

Why this matters now

The 2026 vote dismantles the exact system built in 1972, replacing a seconds-level alignment with an hour-level tolerance.

October 1582

Gregorian calendar reform (1582)

Pope Gregory XIII issued a papal bull replacing the Julian calendar, dropping 10 days from October of that year to correct drift in the date of Easter. Catholic countries adopted it immediately; Protestant and Orthodox nations held out for generations.

Then

The date jumped from October 4 to October 15, 1582, correcting a centuries-long calendar error.

Now

Full adoption took centuries, with some countries switching only in the 20th century.

Why this matters now

Changing a global time reference is slow and uneven. The leap second vote shows a similar international coordination challenge, though modern communications make it far faster than the calendar shift.

1999-2000

Year 2000 problem (1999-2000)

Computer systems storing years as two digits risked failing at midnight on January 1, 2000. Governments and companies worldwide spent an estimated $300-500 billion reprogramming software ahead of the rollover.

Then

Widespread failures largely did not occur because of the massive preparation effort.

Now

The episode established a template for costly, coordinated global fixes to time-related software risks.

Why this matters now

BIPM's resolution states preparing for a negative leap second would cost industries sums similar to their Y2K preparations, making the leap hour choice a way to avoid that burden.

Sources

(10)