Leap Seconds Explained

Why we add an extra second to our clocks to keep atomic time aligned with Earth's rotation

What Is a Leap Second?

A leap second is exactly what its name suggests: a single extra second added to the world's civil time scale. It appears on the clock as 23:59:60, a second that does not exist in normal timekeeping. When a positive leap second occurs, the affected minute grows from 60 seconds to 61 seconds, and UTC briefly counts 23:59:59, 23:59:60, then 00:00:00.

UTC (Coordinated Universal Time) is the time standard that governs civil clocks worldwide. It is built on International Atomic Time (TAI), which is generated by roughly 450 atomic clocks in laboratories across the globe. Atomic time is exceptionally stable — so stable that the best atomic clocks would not lose a second in hundreds of millions of years. But there is a catch: TAI completely ignores Earth's rotation, and Earth's rotation is not constant. Leap seconds exist to reconcile these two competing realities.

Why Earth's Rotation Drifts

For most of human history, a "day" was simply the time it took Earth to complete one turn on its axis. It felt constant. In reality, Earth's rotation is not a perfect, uniform spin. It slows down and speeds up for a range of physical reasons:

• Tidal friction from the Moon — the largest effect — transfers angular momentum away from the planet, gradually lengthening the day.
• As the Moon's tidal bulge slows Earth, the Moon drifts away from us by about 3.8 cm every year.
• Movements in Earth's molten core can speed the spin up slightly for decades at a time.
• Large earthquakes can shift mass toward the core, making the planet spin marginally faster.
• Glacial rebound, atmospheric winds, and ocean currents all nudge the rotation rate by fractions of a millisecond.

On average, the day has been lengthening by about 1.7 to 2 milliseconds per century since ancient eclipse records began. That sounds trivial, but it accumulates: over centuries, the gap between atomic time and astronomical time grows into whole seconds. In some periods the planet spins faster than average — 2020 produced some of the shortest days on record — which is why a negative leap second is occasionally discussed.

How Leap Seconds Are Inserted

Leap seconds are inserted at the end of June or the end of December, at 23:59:60 UTC, when a positive leap second is required. The decision is made by the International Earth Rotation and Reference Systems Service (IERS), which announces it about six months in advance in its Bulletin C.

A positive leap second sequence looks like this: 23:59:59 → 23:59:60 → 00:00:00. A negative leap second — never yet needed — would run 23:59:58 → 00:00:00 instead.

Insertions are limited to June 30 and December 31 precisely because these months can absorb the extra second without disturbing the calendar, and the IERS only schedules one when the difference between UTC and astronomical time (UT1) approaches 0.9 seconds.

A Brief History of Leap Seconds

The leap second system began in 1972, when UTC was synchronized with atomic time. To account for the difference that had already accumulated since 1958, UTC was initially set 10 seconds behind TAI. From that point, leap seconds have been added whenever the gap between UTC and UT1 edged close to the 0.9-second limit.

The first leap second was inserted on 30 June 1972. The most recent — the 27th — was added on 31 December 2016. No leap second has been scheduled since.

The Debate and the 2022 Decision

Leap seconds are beloved by metrologists and reviled by engineers. For most of us, an extra second is imperceptible. For computing and telecommunications systems that depend on precise timestamps, a 23:59:60 can cause havoc. In 2012, a leap second took down websites and disrupted airline systems, with fallout reaching Reddit, Mozilla, and Linux kernels. Financial networks, GPS receivers, and navigation systems all had to build workarounds, such as "leap smearing," where the extra second is spread across a full day in tiny fractions.

After decades of debate, the world's nations voted in November 2022 at the 27th General Conference on Weights and Measures to abolish the leap second by 2035. The resolution allows the difference between UTC and astronomical time to grow to a maximum of one minute and calls for UTC to be redefined on a purely atomic basis.

What Happens Next

In practical terms, the leap second is already on hold. Since the 2016 insertion, the IERS has repeatedly confirmed in its Bulletin C that no leap second is required, and the latest bulletins indicate none will be inserted in 2026 or the near term — a de facto moratorium expected to run through the transition in the 2030s.

Once UTC is detached from Earth's rotation, the rotational day will gradually pull ahead of the atomic clock. At the current drift rate, it will take on the order of a century for the difference to reach a minute — the maximum allowed under the new rules. Clocks will keep atomic time while the Sun drifts relative to the clock at a barely perceptible pace, a discrepancy that could be corrected centuries from now with a single adjustment rather than a continuous stream of leap seconds.

For navigation and astronomy, UT1 is still essential, and systems such as GPS will continue to broadcast the difference between the two time scales. For the rest of us, the change should be invisible — your phone, computer, and watch will simply keep counting perfectly regular atomic seconds. If you want to compare time scales right now, see our time tools or read more about how GMT differs from UTC.

Frequently Asked Questions

What is a leap second?

A leap second is a one-second adjustment inserted into UTC to keep atomic time aligned with Earth's rotation. Since 1972, 27 leap seconds have been added, the last on 31 December 2016.

When are leap seconds added?

Leap seconds are inserted at 23:59:60 UTC on June 30 or December 31. The International Earth Rotation and Reference Systems Service (IERS) announces them about six months in advance in its Bulletin C.

Has a negative leap second ever happened?

No. All 27 leap seconds inserted since 1972 have been positive. A negative leap second has never been needed, though Earth's rotation sometimes speeds up slightly.

Why is the leap second being abolished?

Leap seconds are disruptive to computing and telecommunications, which struggle with the extra 23:59:60 second. In November 2022, the world voted to abolish leap seconds by 2035 and redefine UTC on a purely atomic basis.