Why Leap Seconds Are Being Replaced by a Leap Hour

What is a leap second, and why has it been used?

The clocks on phones, computers and other devices usually follow a shared time standard called Coordinated Universal Time, or UTC. It is based on highly precise atomic clocks. But the Earth does not rotate at a perfectly steady pace, so time measured by its rotation can gradually move away from UTC ¹⁴.

Timekeepers have used leap seconds to keep those two measures close. A leap second is a one-second adjustment to UTC, added occasionally when needed. The aim has been to keep UTC within one second of time based on Earth’s rotation, known as UT1 ¹.

This is not the same as changing every clock by hand. UTC is the underlying standard used to coordinate time. Devices and services may get time from systems that follow that standard, but the sources do not describe exactly how every device will handle the new arrangement.

Why are timekeepers phasing out leap seconds?

The adjustment is small, but it can be awkward for software. A minute that lasts 61 seconds, or one that lasts 59 seconds, is unusual. Computer systems that rely on regular minutes may not handle these exceptions smoothly. Reports say concerns about disruption to computers and infrastructure helped drive the decision ²³.

In place of frequent one-second corrections, the proposed approach allows a much wider gap between UTC and Earth-rotation time. The 2022 description of the change called this a “leap hour”: UTC could be allowed to differ from UT1 by up to an hour, or 3,600 seconds ¹³. That gives timekeepers more room before a correction is needed.

This does not mean ordinary clocks will suddenly jump forward by an hour. The point is to stop making the occasional one-second adjustment and accept a larger difference between the official time standard and time measured by Earth’s rotation. The reports say a leap hour itself is not expected soon; one account says it may be thousands of years before the first is needed ³.

When does the change take effect?

The reports describe a change to the rules for UTC, not an immediate change to the display on everyone’s clock. The Verge says the earlier plan to end leap seconds on or before 2035 was brought forward, with the new arrangement due to take effect on May 20, 2027 ². The Guardian likewise reports that the leap-hour approach is to begin the following year ³.

That date marks when the new allowance is intended to apply, not when a leap hour will be added. Under the reported rule, the difference between UTC and Earth-rotation time could grow much further before reaching the one-hour limit ¹³.

The sources do not explain all the technical steps that countries, clock services or software providers will take during the transition. They do establish the broad change: a larger permitted difference, and less need for irregular one-second adjustments.

How could this affect clocks people use?

For most people, the change is likely to be behind the scenes. The sources focus on the systems that distribute and use precise time, rather than a new setting people must choose on a phone or a household clock. They do not say that everyday clocks will display a different time as a result.

The practical reason for changing the rule is to make timekeeping easier for computer systems that may be affected by an exceptional-length minute. The Guardian reports that supporters see this as a way to reduce risk to systems and national infrastructure that depend on accurate time ³. The Verge also describes the change as an effort to prevent problems for technology infrastructure ².

The trade-off is that UTC will be allowed to move farther from time based on Earth’s rotation. That means the two measures will not be kept within one second, as the leap-second system aimed to do ¹. For people checking an everyday clock, the key point is that the decision concerns how global time standards are managed, not an announced change to the way ordinary clocks show minutes and hours.

What remains uncertain about the leap hour?

The reports provide the broad policy and the intended start date, but not a detailed account of how each timekeeping service will implement the change. They also do not give a firm date for any future leap hour. The Guardian says the first one may be thousands of years away, rather than presenting a scheduled event ³.

There is also a change in the Earth’s rotation that may affect the size and direction of the gap. The Verge says timekeepers are facing a possibility of having to subtract time rather than add it, which helped prompt the faster timetable ². The supplied reports do not provide enough detail to predict exactly when that would happen or how it would affect the first correction.

For now, the dependable takeaway is that timekeepers have chosen a less frequent, more flexible approach. The detailed consequences for individual systems will depend on how the new standard is put into practice.