The day is getting shorter

A day is 86,400 seconds long. That is the definition used by the calendar, not by the Earth. Since 1967 the second has been defined by the oscillation of a caesium-133 atom, and an atomic clock does not care whether the planet turns fast or slow. The planet’s actual rotation does care: tidal friction from the Moon has been dragging it slower by roughly 1.8 milliseconds per century, steadily, across geological time.

To keep the two together, the world invented the leap second in 1972. When the gap between atomic time and true rotation exceeds 0.9 seconds, the International Earth Rotation Service inserts an extra second at the end of June or the end of December. There have been 27 leap seconds since, all of them additions. The most recent was 31 December 2016.

Then the run stopped. Nearly a decade has passed without a leap second, not because the system was abandoned, but because the Earth started turning faster.

Louis Essen and J. V. L. Parry standing either side of the world's first caesium atomic clock, a long rack of apparatus on a laboratory bench at the UK National Physical Laboratory, 1955. — Fig. 01 — 1-bit halftone · Louis Essen and J. V. L. Parry beside the first caesium clock, NPL, 1955 National Physical Laboratory · public domain

We no longer measure time. We measure the gap between two ways of measuring time.

Why the planet sped up

Earth’s rotation is not a smooth motion. The liquid core trades angular momentum with the mantle, the atmosphere shifts direction with the seasons, melting ice moves mass from the poles towards the equator. Each one nudges the rotation rate by a few tenths of a millisecond.

The speed-up since 2020 most likely comes from the core. But melting ice in Greenland and Antarctica pushes the other way, slowing the planet down, and so postpones the day we have to subtract a second. The climate is now intervening in how we number the hours.

Fig. 02 — day length departure from 86,400 s milliseconds · annual mean

Source: IERS finals2000A series (Bulletin A), LOD column, final UT1 rows only. Arithmetic means by calendar year, rounded to 0.01 ms. Retrieved 7 August 2026; 2026 is incomplete and is not plotted.

departure from 86,400 s

+3.04

1973 · a leap second almost every year

−1 MS 0 +3.5 MS

step 01 / 1973

In the early seventies each day ran about three milliseconds longer than 86,400 seconds. The gap piled up fast, so a leap second had to be inserted almost every year.

step 02 / 2003

Thirty years on, the surplus had shrunk to a quarter of a millisecond a day. Plenty of people assumed the leap second was about to become a historical curiosity.

step 03 / 2016

Then the planet slowed again. Through the 2010s the surplus climbed back to nearly one and a half milliseconds, and the 27th leap second was inserted on 31 December 2016. It is still the most recent one.

step 04 / 2022

Six years later the curve crossed zero. Since 2020 the annual mean has sat below 86,400 seconds, and 5 July 2024 was the shortest day ever measured — 1.66 milliseconds short.

In 2022 the General Conference on Weights and Measures voted to end the leap second. The resolution allows the gap between atomic time and Earth’s rotation to grow, with a deadline of 2035 to propose a replacement. The reason is pragmatic: computing infrastructure handles leap seconds badly. Reddit went down over one in 2012. Qantas had to check passengers in by hand.

But before 2035 there is still a possibility that has never occurred. If Earth holds its current pace, the world will have to subtract a second, some time late this decade. No major operating system has ever been tested on 23:59:58 rolling straight into 00:00:00.

That is the strangest part of the story. We measured the planet so precisely that we stopped using it as the standard, and now we have to write software to handle it not turning the way the documentation says.