When Computers Set the Clock: Why the World Gave Up the Leap Second

Two Clocks That Never Agree and the Choice Between Them Here is

what is actually happening. The world keeps time with two very different clocks. One is the Earth itself, spinning once a day but never quite evenly, slowed by tidal friction from the Moon and nudged by movements in its core, oceans and atmosphere.

Time measured this way is called UT1. The other is the atomic clock, which counts oscillations of caesium-133 atoms with almost perfect regularity. Since 1967, the official second has been defined by that atom, not by the Earth.

Coordinated Universal Time (UTC), the time your phone shows, is atomic time with occasional corrections to stay close to the Earth's rotation. Since 1972, whenever the gap approached 0.9 seconds, timekeepers added a leap second.

Twenty-seven have been added so far, the last on 31 December 2016.

Why One Second Became a Problem To a

person, a leap second is invisible. To a computer, it is a minute with 61 seconds, labelled 23:59:60. Much software assumes every minute has exactly 60 seconds. In 2012, a leap second triggered failures in Linux systems that disrupted sites such as Reddit and LinkedIn and affected Qantas's airline check-in systems.

Google responded by spreading the extra second over a full day, a technique called leap smearing, so its servers never saw the jump. The risk could get worse. The Earth's rotation has lately been running slightly fast, which raises the prospect of a negative leap second: a minute with only 59 seconds.

That has never happened and few systems have ever been tested for it.

The Decision and What It Means On 15 October 2026, the

CGPM resolved that from 20 May 2027 UTC may drift up to an hour from UT1 before any correction. Given that only 27 seconds have built up over more than five decades, a leap hour is likely to be far in the future. The analogy has a limit.

Calling this choosing computers over the sky is not quite right: astronomers and navigators who need Earth-rotation time can still use UT1 directly. What changes is which clock civil time follows. The world has decided that the time running on its networks should be smooth and predictable, even if it slowly drifts from the Sun.

Where India Stands

India's time comes from CSIR-National Physical Laboratory, which maintains Indian Standard Time. Its draft Legal Metrology (Indian Standard Time) Rules, 2025, under the slogan One Nation, One Time, aim to make synchronisation with IST mandatory for digital systems. The CGPM decision makes that task easier: IST, like UTC, will no longer need occasional one-second jumps.