Why Leap Seconds Are Being Phased Out—and What Changes

The change aims to make timekeeping systems more reliable, without requiring people to reset everyday clocks.

ScienceOctober 16, 20263 min readHow Sureno writes this

The short version

Leap seconds are being phased out because irregular one-second adjustments can cause problems for computer systems and other technical infrastructure. ³
The replacement allows UTC, the shared global time standard, to differ from time based on Earth’s rotation by as much as an hour before a correction. ¹³
The Verge reports that the change is planned to take effect on May 20, 2027, earlier than the previous target of 2035. ²
Most people are not expected to need to adjust everyday clocks; the change concerns the rules used to keep official time. ¹³

What is a leap second?

Coordinated Universal Time, usually shortened to UTC, is the shared time standard used around the world. It is kept close to time measured by Earth’s rotation. Because Earth does not rotate at a perfectly steady rate, the two measures can drift apart. ¹⁴

A leap second is an occasional one-second adjustment to UTC. It has been used to keep that difference small. But adding an unusual second can be difficult for computer systems that expect time to move in regular steps. ¹³

Why are leap seconds being phased out?

The main reason is to reduce the chance of technical problems. Software and infrastructure that depend on precise time may not all handle an irregular minute in the same way. The Guardian reports that systems can face a minute with an extra second, or one with a second removed. ³

Instead of making small corrections often, the new system allows a wider gap between UTC and time based on Earth’s rotation. That means fewer disruptive adjustments, while still keeping a link between the two ways of measuring time. ¹³

What is a leap hour?

A leap hour is a much less frequent correction. Under the new plan, UTC could differ from time based on Earth’s rotation by as much as one hour before timekeepers make an adjustment. One hour is 3,600 seconds. ¹³

The name can sound as if everyone will soon need to add an hour to their clocks. That is not what the reports describe. The Guardian says the first leap hour is not expected for thousands of years. ³

When will leap seconds end?

The Verge reports that the change is planned to take effect on May 20, 2027. It says this brings forward a deadline that had been set in 2022: stop using leap seconds on or before 2035. ²

The reports describe a change to the official timekeeping system, not a date when people must reset their watches or household clocks. The practical effects are expected to fall mainly on organizations and systems that rely on precise time standards. ¹²³

Will my phone or household clock change?

The sources do not say that people will need to change the time on everyday clocks. The decision changes how official time can differ from Earth’s rotation; it does not announce a new daily time or a routine clock adjustment for the public. ¹³

The change matters most for technical systems that use precise time. The sources explain the aim of reducing risks, but do not describe exactly how every computer, network, or national time service will make the transition. ²³

What we don't know yet

The sources do not provide detailed instructions for implementing the change across computer systems and national time services.
They do not specify exactly when a future leap-hour correction would occur.
The timing will depend on how the difference between UTC and Earth’s rotation changes.

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Questions people ask

Will leap seconds change the time on my phone?

The change concerns official timekeeping and the permitted difference between UTC and Earth’s rotation. The sources do not say that people will need to adjust their phone clocks. ¹³

When are leap seconds being phased out?

The Verge reports that the change is planned to take effect on May 20, 2027. It says this moves up the earlier target of ending leap seconds on or before 2035. ²

What does a leap hour mean?

It means that UTC may be allowed to differ from time based on Earth’s rotation by as much as an hour before a correction is made. The Guardian says the first leap hour is not expected for thousands of years. ¹³

Why can leap seconds cause computer problems?

A leap second makes a minute longer than the usual 60 seconds, and systems may not handle that irregularity consistently. The Guardian also notes that timekeepers may need to remove a second, making a minute shorter. ³

Does Earth’s rotation affect official time?

Yes. UTC has been kept close to time based on Earth’s rotation, which varies. Leap seconds helped keep the measures aligned; the replacement allows a larger difference. ¹⁴

Sources

Scientists just voted to redefine how the world keeps time — Scientific American
Time is a-changin’: say goodbye to the leap second — The Verge, 2026-10-16
How an accelerating Earth led experts to adopt the leap hour — the Guardian, 2026-10-15
Leap second — Wikipedia

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