What Is a Leap Second?
Last updated September 25, 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.
Why Earth's Rotation Needs Correcting
Atomic clocks, which define the modern second, are extremely stable. The Earth's spin is not. Tides, the molten core, even large earthquakes shift the planet's rotation speed by tiny amounts, so a day measured by atomic time and a day measured by the Earth slowly drift apart.
When the gap gets close to a full second, the International Earth Rotation and Reference Systems Service (IERS) schedules a leap second, usually inserted at the end of June 30 or December 31 UTC.
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 most recent leap second was added at the end of December 2016. Since then, none has been needed, partly because the Earth has actually been spinning slightly faster in recent years, the opposite problem from what leap seconds were designed to fix.
They're Being Phased Out
In November 2022, the General Conference on Weights and Measures (CGPM), the body that sets international timekeeping standards, voted to stop using leap seconds entirely by 2035. The irregular scheduling had become a real operational headache for exactly the kind of computer systems that treat every day as a fixed 86,400 seconds.
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. None has been added since, and international standards bodies voted in 2022 to eliminate leap seconds entirely by 2035.
Related Tools & Terms
Source: International Earth Rotation and Reference Systems Service (IERS)