Leap Seconds Abolished: What Continuous UTC Changes in 2027

By Jaspal Singh October 17, 2026 Updated: October 17, 2026

The world's timekeepers have voted to stop adding leap seconds to Coordinated Universal Time (UTC) from 20 May 2027. Clocks on phones and laptops will not visibly change. What changes is that servers, networks and navigation systems no longer have to survive a sudden one-second jump, and astronomical time is allowed to drift up to an hour away.

What Happened

On 15 October 2026, the 28th General Conference on Weights and Measures (CGPM), meeting in Versailles, adopted a resolution titled "On the technical actions needed to ensure the continuity of UTC". The International Bureau of Weights and Measures (BIPM) published it as Resolution 5 of the 28th CGPM. Its core decision is short: continuous UTC becomes effective on 20 May 2027, and the maximum allowed difference between UTC and UT1, the time scale that follows the Earth's actual rotation, is set at 3,600 seconds, or one hour.

Until now that difference was held under 0.9 seconds. Whenever the Earth's rotation fell far enough behind atomic time, the International Earth Rotation and Reference Systems Service (IERS) announced a leap second, and the last minute of a June or December day ran to 61 seconds. Twenty-seven leap seconds have been inserted since the system began in 1972. The most recent was on 31 December 2016, which is why atomic time (TAI) has run 37 seconds ahead of UTC since 1 January 2017.

The decision finishes work begun four years ago. In 2022 the previous conference's Resolution 4 committed to raising the tolerance "in, or before, 2035" and asked for a value that would keep UTC continuous for at least a century. The 2026 resolution picks that value and says it should hold for several centuries. Wired and Tom's Hardware both reported the vote this week, and Euronews noted that the BIPM counts 69 member states. The IERS had already said in its July 2026 Bulletin C that no leap second would be added at the end of December 2026, so the one inserted in 2016 now looks likely to be the last.

Why It Matters

Leap seconds were designed for a world of radio time signals and ships' navigators. For computers they have been a recurring hazard, because most software assumes that a minute has 60 seconds and that time never repeats or runs backwards.

The failures are well documented. During the June 2012 leap second, a Linux kernel timer bug locked up server CPUs across the industry; Meta's engineers later wrote that Reddit was unreachable for roughly 30 to 40 minutes. On 1 January 2017, Cloudflare's DNS service failed for some customers because code measuring response times subtracted one timestamp from another, got a negative number when the clock stepped back, and passed it to a random-number function that panics on negative input. At peak about 0.2% of DNS queries were affected, and the fix took until 06:45 UTC to reach every location.

The bigger worry looked ahead. In recent years the Earth has at times spun slightly faster, raising the prospect of the first negative leap second, where a minute would have only 59 seconds. No one has ever done that. The resolution cites a March 2025 workshop of the BIPM's time committee and the IERS that put the chance of a negative leap second before 2035 at about 30%. A step that has never been tested, landing on infrastructure that already stumbles on the familiar kind, is exactly the risk the vote removes.

For ordinary users there is no visible change. Daylight saving time is a separate, national matter and is unaffected. Your phone will keep showing the right civil time, because civil time is UTC plus your time zone offset, and UTC will simply stop jumping.

How It Works

There are two kinds of time involved. Atomic time is counted by hundreds of atomic clocks in national laboratories and averaged by the BIPM; it ticks at a perfectly steady rate. Earth time, UT1, is measured by watching the planet's rotation against distant radio sources. The Earth is an irregular clock: tides slowly brake it, while shifts in its core, oceans and atmosphere speed it up or slow it down by milliseconds a day. UTC is the compromise. It ticks at the atomic rate but, until now, was nudged by whole seconds to stay within 0.9 seconds of the Earth.

A leap second is applied as an extra second labelled 23:59:60. Plenty of software cannot represent that label at all. Unix time, which most servers use, simply pretends the second does not exist, so the system clock either repeats a second or is stepped backwards. That is how a duration calculation can come out negative, as it did at Cloudflare.

Big operators worked around this by "smearing" the second instead of inserting it. Google's published standard spreads each leap second over 24 hours from noon to noon UTC, so clocks run about 11.6 parts per million slow for a day and never show 23:59:60; Google says Amazon uses the same method in AWS. Meta used a 17-hour quadratic smear, and argued in a 2022 engineering post that the count should stay at 27. Smearing works, but different companies smear differently, so for hours at a time their clocks disagree with each other and with true UTC. Ending leap seconds makes that whole layer of workarounds unnecessary.

Satellite navigation shows why the old system was awkward. GPS time does not use leap seconds at all and currently runs 18 seconds ahead of UTC; receivers apply the offset from the navigation message. Russia's GLONASS, by contrast, follows UTC with its leap seconds, which made each insertion a coordinated operation. A continuous UTC means the offsets stop changing.

The gap between atomic and Earth time will now grow quietly. The rate varies, but at the pace of recent decades it would take a very long time, likely centuries, to reach the one-hour limit, which is why the resolution describes UTC staying continuous for several centuries. People who need Earth-rotation time, such as astronomers pointing telescopes or satellite operators, will keep getting the UT1 minus UTC value from the IERS, national labs and GNSS broadcasts, just as they do today. The difference is that this value will grow past one second.

What's Still Unknown

The resolution does not say what happens if the one-hour limit is ever reached. It asks the BIPM to keep monitoring UT1 minus UTC with the IERS and report regularly, but there is no defined correction method, and a future conference would have to decide one.

Radio time signals are a practical loose end. Some stations broadcast the UT1 offset in a code that only fits values under a second. The resolution notes that the International Telecommunication Union's Working Party 7A is studying an updated format, and that ITU rules allow a transition period until 2035, and no later than 2040, where equipment cannot be replaced sooner.

It is also unclear how quickly software will shed its leap-second code. Operating systems, NTP servers and the tz database's leap-second list will need updates that stop expecting new entries, and the hardest bugs may hide in older embedded devices that hard-coded assumptions about the six-monthly announcement cycle. Whether Russia's GLONASS and other systems adjust smoothly before 20 May 2027 will be watched closely by the timing community.

Frequently Asked Questions

When do leap seconds officially end?

Continuous UTC becomes effective on 20 May 2027 under Resolution 5 of the 28th General Conference on Weights and Measures, adopted on 15 October 2026. In practice the last leap second was already added on 31 December 2016, and the IERS has said none will be inserted at the end of December 2026. No further leap seconds are expected before the new rule starts.

Will my phone or computer clock change?

No. Devices take their time from network or satellite sources that follow UTC, and UTC will simply stop receiving occasional one-second adjustments. You will not see a jump, and you do not need to change any setting. Daylight saving time, which shifts clocks by an hour in many countries, is set by national governments and is not affected by this decision.

Why were leap seconds added in the first place?

Atomic clocks tick at a perfectly steady rate, but the Earth's rotation is irregular and, over the long term, slowing because of tidal friction. Leap seconds, introduced in 1972, kept UTC within 0.9 seconds of the time defined by the planet's rotation. That mattered to celestial navigation and astronomy, which relied on clock time matching the Earth's position.

What is a negative leap second?

It would remove a second instead of adding one, so the last minute of a day would end at 23:59:58 and jump straight to midnight. It would be needed if the Earth spun fast enough to get ahead of atomic time. It has never happened, and a 2025 expert workshop estimated a roughly 30% chance of one being required before 2035.

How did leap seconds break websites?

Most software assumes time only moves forward. When a leap second made system clocks repeat or step back, programs measuring elapsed time could get zero or negative durations. In 2012 a Linux timer bug overloaded servers and took Reddit offline for about 30 to 40 minutes. In 2017 a negative duration crashed part of Cloudflare's DNS service for some customers.

What is leap smearing?

Leap smearing spreads the extra second across many hours by making clocks run very slightly slow, so no clock ever shows the awkward 23:59:60. Google's standard smear lasts 24 hours from noon to noon UTC and slows clocks by about 11.6 parts per million. Amazon uses the same approach in AWS. Smearing avoids crashes but leaves different companies' clocks briefly out of step.

Does this affect GPS?

GPS time has never used leap seconds and currently runs 18 seconds ahead of UTC. Receivers convert using an offset broadcast in the navigation signal, so positioning itself is unaffected. The main benefit is that the offset stops changing. Systems that follow UTC directly, such as Russia's GLONASS, no longer have to coordinate occasional one-second jumps across satellites and receivers.

What happens if the gap reaches one hour?

The resolution does not say. It sets one hour, or 3,600 seconds, as the maximum difference between UTC and Earth-rotation time, and expects that limit to keep UTC continuous for several centuries. It asks the BIPM to monitor the difference with the IERS and report regularly. Any correction method would be left to a future General Conference to decide.