Leap Seconds: Why the World's Clocks Kept Getting a One-Second Fix

Earth does not rotate at a fixed speed. It slows down, speeds up, and wobbles under the gravitational tug of the Moon, the shifting mass of its own oceans, and the drag of atmospheric pressure systems moving across its surface. On any given day, a full rotation takes somewhere between 86,399.999 and 86,400.001 seconds, a variation that sounds trivial until you are running a satellite navigation system, a financial exchange, or a power grid that depends on clocks agreeing to the microsecond.

Before 1955, this did not matter much. Clocks were set to astronomical time, which is to say they were set to the Earth itself. The second was defined as a fraction of the solar day, so if the Earth slowed down, the second stretched with it. Everybody's clocks were wrong together in the same direction, which is functionally the same as being right.

The caesium atomic clock, developed at the National Physical Laboratory in the United Kingdom in 1955, defined the second not by the Earth's rotation but by the frequency of radiation emitted when a caesium-133 atom transitions between two energy states: exactly 9,192,631,770 cycles. This number does not change. It does not respond to ocean tides or atmospheric drag. It ticks at the same rate on a Tuesday in January as it does on a Friday in August, and it will do so for longer than the Sun will burn.

The International System of Units adopted this definition in 1967. From that point, two kinds of time existed: International Atomic Time, which is the average of roughly 450 atomic clocks running in laboratories across the world, and Universal Time, which tracks the actual rotation of the Earth. The two immediately began to drift apart, because the Earth is not a caesium atom.

In 1972, the International Telecommunication Union introduced Coordinated Universal Time, UTC, as a compromise. UTC runs on atomic seconds but is kept within 0.9 seconds of Universal Time by inserting a leap second whenever the gap threatens to exceed that threshold. The International Earth Rotation and Reference Systems Service monitors the drift and announces each insertion at least eight weeks in advance. The extra second is added at 23:59:59 on either 30 June or 31 December, so clocks read 23:59:60 before rolling to midnight.

Between 1972 and 2016, 27 leap seconds were added. None have been added since, partly because the Earth has been rotating slightly faster than average over the past decade, a trend that geophysicists attribute to changes in the planet's core and to the redistribution of mass as glaciers melt. There was a period around 2023 when scientists were discussing whether a negative leap second, removing a second, might become necessary for the first time. That would have been considerably more disruptive than adding one.

The leap second was designed for an era when clocks were synchronised by radio signals and a one-second correction was a minor administrative event. It was not designed for the internet. When a leap second is inserted, every networked system in the world must decide how to handle a minute that contains 61 seconds. Some systems crash. Some loop. Some simply refuse to acknowledge that 23:59:60 exists and either freeze for a second or apply the correction twice, which is worse.

In 2012, a leap second caused Reddit, Foursquare, Yelp, LinkedIn and several airline booking systems to fail simultaneously. The Linux kernel had a known bug that caused servers to spike to 100 percent CPU usage at the moment of insertion. In 2017, a leap second disrupted Cloudflare's DNS infrastructure. Financial exchanges, which timestamp transactions to the microsecond, have to freeze trading or implement elaborate workarounds. The GPS system, operated by the United States Space Force, does not use leap seconds at all, it has been running 18 seconds ahead of UTC since 1980 and simply broadcasts the offset, which means every GPS receiver in the world already carries a small table of corrections just to stay in sync with civil time.

In November 2022, the General Conference on Weights and Measures voted to suspend the leap second by 2035. The specific mechanism is still being worked out. One option is to allow the gap between atomic time and Earth time to grow to a full minute before any correction is applied, a tolerance that would take roughly a century to reach at current drift rates. Another is to absorb the difference gradually over years rather than in a single abrupt second.

What the agreement settles is the principle: the atomic clock wins. Civil time will no longer be adjusted to match a planet whose rotation is, from the perspective of a caesium atom, erratic and unreliable. The Earth will keep wobbling. The clocks will keep ticking at exactly 9,192,631,770 cycles per second. The gap between them will grow, quietly, for a very long time before anyone has to do anything about it, and when they do, they have agreed it will not be a single lurching second dropped into the world's networks at midnight.