Leap Seconds Explained: Why UTC Gets an Extra Second, How Many There Have Been, and Plans to Change the Rule

By Web Asset House Studio · October 29, 2026

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How we wrote this: A plain-English summary of NIST's Leap Seconds FAQs and its leap second and UT1-UTC data page, and Resolution 4 of the 27th General Conference on Weights and Measures (CGPM, 2022), all linked below and checked October 29, 2026. The leap second count comes from NIST's table.

Most years, the clock ticks from 23:59:59 straight to midnight. But 27 times since 1972, the world's official time has paused for one extra second, 23:59:60. That's a leap second. Here's why it exists, how it's done and why it's on its way out.

Two kinds of time

NIST explains that there are two time scales that keep drifting apart:

UTC (Coordinated Universal Time) is an atomic time scale, based on atomic clocks that are more stable than Earth's rotation. It's the basis for civil time.
UT1 (astronomical time, or mean solar time) is based on the rotation of Earth, which is irregular.

A leap second is added to UTC to keep it in step with UT1, so that clock time stays matched to the position of the Sun and stars.

Why they drift apart

NIST gives two reasons:

History of the second. The atomic second was calibrated against an older time scale (Ephemeris Time) whose second was slightly shorter than the mean solar second, and that difference carried over.
Earth's rotation varies. It sometimes speeds up and sometimes slows down, but over long periods the trend is gradual slowing.

When a leap second is added

The International Earth Rotation and Reference Systems Service (IERS) measures Earth's rotation and publishes the UT1 minus UTC difference. Per NIST:

Leap seconds keep that difference under 0.9 seconds.
They're usually added when UTC is ahead of UT1 by about 0.4 seconds or more.
They have always come at the end of June or the end of December, on the last second of the UTC day.

The clock sequence for a positive leap second:

23:59:59
23:59:60
00:00:00  (start of the next day)

In the United States, the last second of the UTC day falls in the evening local time; for example, 6:59:60 p.m. Eastern Standard Time on December 31.

How many there have been

From NIST's table of leap seconds inserted into UTC:

The first was on June 30, 1972. When corrections started that year, UTC was also adjusted by an initial 10 seconds.
There have been 27 leap seconds in total, about one every year and a half on average.
The most recent was on December 31, 2016.
As a result, International Atomic Time (TAI) is now 37 seconds ahead of UTC (10 + 27).

NIST publishes the same data in a machine-readable file, leap-seconds.list.

Could a second ever be removed?

So far every leap second has been positive (a second added). NIST notes there's a provision for a negative leap second if Earth's rotation requires it, where the clock would skip from 23:59:58 directly to 00:00:00. The CGPM's 2022 resolution says recent observations of Earth's rotation indicate the possible need for the first negative leap second, something that has never been tested.

Why leap seconds cause trouble

NIST says keeping UTC within a second of astronomical time helps astronomers, but adding a second can cause problems for data logging, telecommunication systems and time distribution services. The 2022 CGPM resolution goes further: leap seconds create discontinuities that risk serious malfunctions in critical digital infrastructure, including satellite navigation (GNSS), telecommunications and energy transmission, and different operators handle them with different, uncoordinated methods.

The plan to change the rule

In Resolution 4 of 2022, the General Conference on Weights and Measures decided that the maximum allowed difference between UT1 and UTC will be increased in, or before, 2035. It asked for:

a new maximum value that keeps UTC continuous for at least a century,
a plan to implement it by or before 2035, and
a draft resolution for the 28th CGPM meeting in 2026.

A larger maximum would mean leap seconds are needed far less often, if at all, under the new value. Check the BIPM's site for the decisions taken at the 2026 meeting.

Leap seconds vs. leap years

NIST points out the difference: leap years follow a fixed rule (an extra day, February 29, because a year is about 365.2422 days; century years count only if divisible by 400, so 2000 was a leap year but 1900 was not). Leap seconds are added only when measurements show they're needed.

For more on how clocks and calendars are set, see our guides to daylight saving time starting in 2027 and equinoxes and solstices in 2027.