What Is a Leap Second?
Last updated October 18, 2026
One-line definition: A leap second is a one-second correction occasionally added to UTC to keep clock time aligned with the Earth's actual, slightly irregular rotation. The last one was on December 31, 2016, and they are being phased out. Computer timestamps like those in the epoch time converter ignore them.
Why Earth's Rotation Needs Correcting
Atomic clocks, which define the modern second, are extremely stable. The Earth's spin is not. Tidal friction slows it, and movement of the planet's core, mantle, and crust nudges it in both directions. Over the long run the average day is getting longer by about 1.7 milliseconds per century, so a day measured by atomic time and a day measured by the Earth slowly drift apart.
When the gap approaches a full second, the International Earth Rotation and Reference Systems Service (IERS) announces a leap second, typically about six months ahead. The preferred dates are the end of June or the end of December UTC, and every one so far has landed on one of those.
What a Leap Second Looks Like
Instead of rolling from 23:59:59 straight to midnight, UTC clocks show an extra second, 23:59:60, and that day has 86,401 seconds. Twenty-seven leap seconds have been added since the system started on June 30, 1972, and all of them were additions.
Together they mean atomic time (TAI) is now 37 seconds ahead of UTC, a gap that has not changed since the 2016 leap second. A “negative” leap second, which would remove a second, has been possible on paper but has never happened.
Not the Same Thing as a Leap Year
A leap year adds a full day every four years to keep the calendar in sync with Earth's orbit around the sun. A leap second adds one second, on an irregular schedule announced roughly six months ahead, to keep the clock in sync with Earth's spin on its own axis. Different orbit, different fix, different name for a reason.
The Part That Trips Up Developers: Unix Time Ignores It
Unix time counts every day as exactly 86,400 seconds, no exceptions. So when a real leap second happens, Unix time doesn't gain a second to match it. Systems either repeat a second, skip one, or “smear” the extra second across the surrounding hours, depending on how that particular system handles it.
The POSIX standard defines it that way on purpose: each day is exactly 86,400 seconds, so Unix timestamps repeat during a leap second. That keeps the arithmetic simple, but it means Unix time isn't a literal count of elapsed seconds.
When Leap Seconds Break Things
Software that has never seen “23:59:60” can misbehave. After the June 30, 2012 leap second, outages were reported at Reddit, Mozilla, Qantas, and various sites running Linux. In 2015, leap-second bugs in Linux reportedly hit an airline reservation system used by Qantas and Virgin Australia.
To avoid the problem, some operators “smear” the extra second across many hours instead of adding it all at once. Google's smear spread it over about 20 hours around the leap second; Amazon used a different pattern in 2015.
They're Being Phased Out
In 2022 the General Conference on Weights and Measures (CGPM) passed Resolution 4, which says the maximum allowed gap between UT1 (Earth-rotation time) and UTC, now 0.9 seconds, will be increased in or before 2035. The goal is to keep UTC continuous for at least a century, which in practice ends leap seconds.
Meanwhile, the Earth has sped up. 2020 had the 28 shortest days since 1960, and the shortest day on record was June 29, 2022, about 1.59 milliseconds under 24 hours. In July 2026 the IERS confirmed that no leap second will be added at the end of December 2026. As of October 2026, the exact new limit was due to be settled at the 28th CGPM, and reports point to a one-hour limit from May 2027. Check the BIPM for the final decision.
Frequently Asked Questions
What is a leap second?
A one-second adjustment occasionally added to UTC to keep it aligned with the Earth's actual rotation, which drifts slightly out of sync with the perfectly steady atomic clocks that define the second.
How is a leap second different from a leap year?
A leap year fixes the calendar's drift against Earth's orbit around the sun by adding a day. A leap second fixes clock time's drift against Earth's spin on its axis by adding a second. They correct two unrelated kinds of drift.
Does Unix time account for leap seconds?
No. Unix time treats every day as exactly 86,400 seconds and does not count leap seconds, which is why systems have to repeat, skip, or smear a second when a real one occurs.
When was the last leap second?
December 31, 2016, which brought the total to 27 since 1972. In July 2026 the IERS announced that none will be added at the end of December 2026.
Will leap seconds continue forever?
No. In 2022 the CGPM resolved to raise the maximum allowed difference between Earth-rotation time and UTC in or before 2035, with the aim of keeping UTC continuous for at least a century. That effectively ends leap seconds. Check the BIPM for the final details.
Has there ever been a negative leap second?
No. All 27 leap seconds so far added a second. A negative one, which would remove a second, has been possible in theory, and the faster Earth rotation in recent years made it more plausible before the policy change.
Related Tools & Terms
Sources: IERS, BIPM: CGPM Resolution 4 (2022), Leap second (Wikipedia)