What Is a Leap Second — and Why Are They Being Retired?

01What is a leap second balancing?

The modern SI second is not defined as 1/86,400 of whatever rotation Earth completed today. The International Bureau of Weights and Measures defines it by fixing the caesium-133 transition frequency at exactly 9,192,631,770 hertz. Atomic clocks realize that stable unit, and many clocks around the world contribute to International Atomic Time, TAI.

UT1 answers another question: what is Earth's measured angle of rotation? Astronomers derive it from observations of the turning planet. A UT1 second is not the civil unit changing size; UT1 is the rotation-based time scale whose pace wanders relative to atomic time.

UTC combines the needs. It runs at the atomic rate and differs from TAI by a whole number of seconds. Under the present procedure, the International Earth Rotation and Reference Systems Service monitors UT1−UTC. When that difference is predicted to approach 0.9 second, it can announce a leap second for the end of June or December.

Since 1 January 2017, TAI has been 37 seconds ahead of UTC. Ten seconds were built into the relationship when the current system began in 1972, then 27 positive leaps increased the offset. The history of our 24-hour day explains the older units; the leap second is a modern negotiation between their atomic realization and a moving Earth.

02A long slowdown can contain shorter speedups

Lunar tidal friction transfers angular momentum and tends to lengthen Earth's day over long spans. That broad trend does not make each new day longer than the one before. Atmosphere, oceans, groundwater, ice, earthquakes and motion inside Earth exchange or redistribute angular momentum over shorter periods. The length of day moves by milliseconds around 86,400 SI seconds.

Recent observations produced stretches in which Earth rotated slightly faster relative to atomic time. A positive leap second repeats an extra label:

23:59:59 → 23:59:60 → 00:00:00

If rotation pulled far enough the other way, a negative leap second would omit a label:

23:59:58 → 00:00:00

That subtraction has never been used. A 2024 Nature study modeled changes involving Earth's core and melting polar ice and projected that, if the current UTC rule continued, a negative discontinuity could be needed around 2029. That is a model result, not an appointment. Rotation cannot be forecast exactly years ahead, and international rules are changing.

The latest IERS Bulletin C available in August 2026 says no leap second will occur at the end of December 2026. The last insertion remains 31 December 2016. “Earth is spinning faster” is therefore a short-term comparison, not a reversal of every long-term physical trend.

03One unusual timestamp can break ordinary assumptions

Most human observers barely notice an added second. Software notices everything—especially when its author assumed every minute contains exactly 60 labelled seconds and time always increases smoothly.

A parser may reject `23:59:60` as invalid. A scheduler can run twice or not at all when a clock repeats or steps. Two timestamps subtracted in the wrong order can produce a negative duration that downstream code never expected. At the 2016 leap, Cloudflare traced a DNS disruption to code that allowed such a negative value to reach a function that required a nonnegative one.

Large operators therefore do not all implement the event in the same way. Some step the clock at the boundary. Some repeat a timestamp. Others use a leap smear, changing the clock rate slightly across several hours. Google Cloud documents a 24-hour smear, twelve hours on each side of the leap; Meta has described another window and curve.

Smearing avoids one abrupt timestamp, but incompatible smears mean two systems can disagree during the window. The 2022 international resolution specifically noted that uncoordinated methods threaten dependable synchronization across navigation, telecommunications and energy infrastructure. The leap is only one second. The disagreement about representing it is the larger engineering problem.

04Why leap seconds are being retired

In 2022, the General Conference on Weights and Measures decided that the permitted value of |UT1−UTC| would be increased in or before 2035. A larger tolerance lets UTC continue without frequent one-second steps for at least a century. The 2023 World Radiocommunication Conference endorsed that direction.

This is often summarized as “abolishing leap seconds by 2035,” but the adopted wording matters. The 2022 decision did not delete past leap seconds, redefine the SI second or instantly sever civil time from Earth. It instructed the responsible bodies to choose a larger maximum difference and an implementation plan. Astronomers and navigators who need Earth's precise rotation can still use UT1 and published offsets rather than assuming UTC is the rotation angle.

As of August 2026, a draft resolution prepared for the 28th CGPM proposes making continuous UTC effective on 20 May 2027 with a one-hour maximum difference. It is a proposal awaiting the meeting's decision, not yet the final rule. The durable fact is the adopted direction: end the recurring one-second discontinuities no later than 2035 by allowing UTC and UT1 to separate further.

The trade is clear. Civil systems gain a continuous time scale. Solar time and UTC slowly diverge beyond the old 0.9-second leash, and a much later generation must decide how to handle a larger accumulated difference. Timekeepers are exchanging frequent tiny interruptions for a rarer, larger policy question.

05Precision still depends on naming the time scale

Removing future leaps will not make “the time” one universal number. TAI, UTC, UT1, GPS time and Terrestrial Time answer different operational questions. A scientific timestamp still needs its scale, and software still needs rules for the 37-second historical UTC–TAI offset and all earlier changes.

Leap days solve another mismatch entirely. They keep a calendar year near the seasons because the year is not exactly 365 days. A leap second deals with the variable rotation of one day relative to atomic seconds. Removing second insertions does not change Gregorian leap-year rules, February 29 or the length of a calendar month.

The broader lesson is humbling: the most precise clocks humans have built still need a policy for living on a planet that does not keep their rate. Measurement is exact. Coordination is negotiated.

06Common questions