What Are Leap Seconds?

Why Earth's Rotation Is Irregular

For most of human history, the length of a day was the ultimate reference for time. One rotation of the Earth defined a day, and a second was simply a fixed fraction of that day. The problem is that the Earth does not spin at a perfectly constant rate. Tidal friction from the Moon gradually slows the planet's rotation, lengthening the day by roughly 1.7 milliseconds per century over long timescales.

On shorter timescales the rotation wobbles in less predictable ways. Earthquakes, the melting and redistribution of polar ice, the motion of molten material in the Earth's core, and seasonal shifts in atmospheric and ocean circulation all nudge the spin rate up or down. These effects can make a day a few milliseconds longer or shorter than the nominal 86,400 seconds, and they cannot be forecast far in advance.

A common misconception is that the Earth is steadily slowing and that leap seconds are therefore always added. In recent years the planet has actually been spinning slightly faster than average, which is why scientists have begun seriously discussing the possibility of a negative leap second, an event that has never occurred.

UT1 Versus UTC

To understand leap seconds you need two different timescales. UT1, or Universal Time, is astronomical time tied directly to the actual rotation of the Earth as measured against distant celestial objects. It represents where the planet really is in its daily spin, and it drifts as the rotation speeds up or slows down.

UTC, or Coordinated Universal Time, is the basis of civil time worldwide. Its ticks come from atomic clocks, which count the resonance of cesium atoms with extraordinary precision and do not care about the Earth at all. Atomic time is so stable that it would happily drift away from the position of the Sun in the sky if left alone.

Leap seconds are the bridge between these two systems. The rule is that UTC must never differ from UT1 by more than 0.9 seconds. When the gap approaches that limit because atomic time has gotten ahead of the slowing Earth, a leap second is inserted into UTC to let the rotation catch up, keeping atomic precision and astronomical reality loosely in step.

How and Why Leap Seconds Are Inserted

A leap second is added at the end of a chosen day, expressed in UTC. The clock reads 23:59:59, then ticks to an extra second written as 23:59:60, and only then rolls over to 00:00:00 of the next day. The preferred dates are June 30 and December 31, with March 31 and September 30 as secondary options.

The decision to insert one is made several months ahead, based on careful measurements of the Earth's orientation. Because the rotation cannot be predicted reliably more than a few months out, leap seconds cannot be scheduled years in advance the way leap years can. This unpredictability is precisely what makes them awkward for long-term planning.

Every leap second so far has been positive, meaning a second was added. The mechanism technically allows a negative leap second, in which 23:59:58 would be followed directly by 00:00:00, skipping a second. With the Earth's recent faster spinning, the prospect of a first-ever negative leap second has moved from theoretical curiosity to a real engineering concern.

Who Decides: The IERS

The authority responsible for leap seconds is the International Earth Rotation and Reference Systems Service, usually abbreviated IERS. Headquartered in Germany and drawing on observatories and timing laboratories around the world, the IERS continuously monitors the planet's orientation in space using techniques such as very long baseline interferometry that observe distant quasars.

When the data show that the UT1 minus UTC difference is approaching the 0.9 second threshold, the IERS issues a formal notice called Bulletin C. This bulletin announces whether a leap second will be introduced at the next opportunity, giving operating system vendors, network operators, and timekeeping services months of advance warning to prepare.

It is worth noting that the IERS does not invent the rules; it applies a standard maintained by international bodies governing radio and time signals. The service is the measurer and announcer, while the broader scientific and standards community sets the policy that the IERS carries out.

A Short History Since 1972

The modern leap second system began on January 1, 1972, when UTC was redefined to tick at the atomic rate while staying close to astronomical time. To start everything off in alignment, that transition included an initial adjustment, and the first true leap second was added at the end of June 1972.

Since then, more than two dozen leap seconds have been inserted, all of them positive. They came frequently in the 1970s and 1980s, sometimes nearly every year, then grew rarer as the Earth's rotation behavior shifted. The most recent leap second was added at the end of December 2016, and as of this writing none has been needed since.

Each leap second is a small reminder that human timekeeping rests on a compromise between the elegance of atomic physics and the messy reality of a spinning rock in space. For half a century that compromise was managed second by second, but the costs eventually prompted a rethink.

Computer Problems, Leap Smear, and the 2035 Retirement

Computers expect every minute to have exactly sixty seconds, so the appearance of a 61-second minute can break software in surprising ways. Past leap seconds have triggered crashes and outages in airline booking systems, social networks, and Linux servers, often because programs encountered the impossible-looking timestamp 23:59:60 or because time appeared to stand still or jump. The fact that leap seconds arrive on irregular notice makes them hard to test for.

To avoid the discontinuity, many large operators use a technique called leap smear. Instead of inserting one abrupt second, they spread the adjustment across many hours by running their clocks very slightly slow or fast, so the extra second is absorbed smoothly and no minute ever has to contain a 60th tick. Different providers smear over different windows, however, which means clocks can briefly disagree across organizations.

Frustrated by these recurring risks, the international metrology community voted in 2022 to retire leap seconds. Under that decision the practice will be suspended by 2035, after which UTC will be allowed to drift further from astronomical time before any large correction is considered, perhaps decades later. The change trades a tiny, precise alignment with the Sun for far greater stability and predictability in the systems that now depend on accurate time.