The leap second is being retired, and the decision changes the definition of civil time for the first time in half a century. Here is what a leap second is, why it was invented, and why timekeepers decided it was doing more harm than good.

Two different clocks

There have always been two ways to say what time it is, and they do not agree.

The older one is astronomical: a day is one rotation of the Earth, and a second is a fixed fraction of that. The newer one is atomic: since 1967 the second has been defined by a specific number of oscillations of a caesium atom, which is vastly more stable than anything the planet does.

The problem is that the Earth is not a good clock. Its rotation is gradually slowing, chiefly because of tidal friction from the Moon, and it also wobbles unpredictably as mass shifts between the oceans, atmosphere and core. Atomic time runs steadily; the planet does not.

Left alone, the two would drift apart. Over very long periods noon by the clock would stop being noon by the Sun.

The fix, and why it misfired

The solution adopted in 1972 was the leap second: when atomic time and astronomical time drift apart by close to a second, an extra second is inserted into the last minute of a chosen day. That minute has 61 seconds. More than two dozen have been added since.

For most of human activity this is invisible. For computers it is not, and the reason is specific rather than general incompetence. A great deal of software assumes that time moves forward at a constant rate and never repeats. A clock reading 23:59:60, or appearing to go backwards while a correction is applied, breaks that assumption. Leap seconds have been associated with outages at large internet companies, airlines and financial exchanges.

Worse, from an engineering standpoint, they are unpredictable. Because they depend on the Earth’s rotation, they cannot be scheduled far ahead; they are announced roughly six months in advance by the service that monitors the planet’s spin. Systems cannot be built to anticipate them.

Who objected

Abolition was not unanimous, and the objections were real rather than sentimental.

Astronomers and satellite navigation specialists need to know the Earth’s actual orientation to point instruments and fix positions. Dropping leap seconds does not remove that need; it means the correction has to be tracked and applied separately by those who require it, rather than being built into civil time for everyone.

Some states also argued that time standards are a matter of sovereignty rather than engineering convenience, and that a definition in use worldwide should not be changed to suit the architecture of particular computer systems.

And there is a genuine long-run question nobody has answered. Without leap seconds, clock time and solar time will drift apart indefinitely — slowly, by roughly a minute per century at present rates, but without limit. At some distant point a correction will be needed, and a larger one is harder than a small one. The decision defers that problem rather than solving it.

Does it affect you

Almost certainly not in any way you will notice. Nobody’s watch changes, no clock needs resetting, and the practical effect falls on the people who run time servers, navigation systems and telescopes. It is a rare case of a change to something fundamental that is designed to be invisible — which is roughly the opposite of how the leap second behaved.

Sources

• Al Jazeera explainer, 15 October 2026
• International definition of the SI second (1967) and the introduction of leap seconds (1972)