What Is a Leap Second? Why We Add Extra Seconds to UTC
A leap second is an extra second occasionally added to UTC to keep it aligned with the Earth's irregular rotation. Here is how they work, why they exist, and why they will be abolished by 2035.
A Leap Second in 60 Seconds
A leap second is an extra second occasionally inserted into Coordinated Universal Time (UTC) to keep the world's atomic clocks in approximate alignment with the Earth's slightly irregular rotation. When a leap second is added, the last minute of the day has 61 seconds instead of 60. The clock counts: ...23:59:59, 23:59:60, 00:00:00 instead of jumping straight from :59 to :00. There have been 27 leap seconds added since the system was introduced in 1972. There has never been a NEGATIVE leap second (one removed), though there has been technical preparation for one. As of 2026, leap seconds are scheduled to be abolished by 2035.
Why Leap Seconds Exist
There are two different ways of measuring time that the world cares about. International Atomic Time (TAI) is measured by a network of over 400 atomic clocks around the world, ticking at perfectly regular intervals — 9,192,631,770 cycles of cesium radiation per second. Universal Time (UT1) is based on the Earth's rotation — essentially solar time. The problem is that the Earth's rotation is slightly irregular and very slowly slowing down due to tidal friction with the Moon. Atomic time and rotation time drift apart by about a second every 1–2 years. To keep UTC (the time everyone actually uses) within 0.9 seconds of UT1, leap seconds are inserted occasionally to bring them back into alignment.
Why the Earth Is Slowing Down
The Earth's rotation is gradually slowing because of tidal friction caused by the Moon. As the Moon orbits Earth, its gravity creates tides in the oceans (and even small tides in the solid Earth itself). This tidal bulge is pulled forward by Earth's rotation, but the Moon pulls it back, creating a friction that very slowly transfers Earth's rotational energy to the Moon's orbital energy. The result: Earth's day gets longer by about 1.7 milliseconds per century, and the Moon slowly moves further away. None of this is noticeable in a human lifetime, but it adds up over geological time. Hundreds of millions of years ago, an Earth day was only about 22 hours long.
When Leap Seconds Are Added
Leap seconds are scheduled by the International Earth Rotation and Reference Systems Service (IERS), based in Paris. They are typically inserted at the end of June 30 or December 31 (UTC), giving system administrators around the world time to prepare. The IERS announces a leap second about six months in advance. There is no fixed schedule — they are added as needed when UT1 has drifted close to 0.9 seconds away from UTC. There were periods (1972–1979) when leap seconds were added almost annually. There have also been long gaps — the most recent leap second was added on December 31, 2016, and none have been added since.
How a Leap Second Looks on a Clock
When a leap second is added, the official UTC clock shows: 23:59:58, 23:59:59, 23:59:60, 00:00:00. That extra "23:59:60" is the leap second — a moment that exists for one full second. Most consumer digital clocks ignore leap seconds entirely; they just briefly stay at 23:59:59 for an extra second, or jump from :59 to :00 with the OS handling the smear behind the scenes. Computer systems handle leap seconds in different ways: some pause for a second, some "smear" the extra second across an hour or a day (Google's approach), some stop the clock entirely for one second. The variations have caused real bugs in the past.
The Famous 2012 Leap Second Bug
On June 30, 2012, a leap second was added at the end of the day. Several major systems crashed or malfunctioned. Mozilla's Firefox sync service went down. Reddit had hours of outage. LinkedIn and Yelp had problems. Australia's Qantas airline reported flight delays because their reservation system's leap-second handling was buggy. The cause was that Linux's kernel had a race condition in its leap-second code that caused certain processes to spin at 100% CPU. The bug was patched, but the incident illustrated how widespread the dependency on accurate UTC time really is, and how subtle the failure modes can be when you mess with the clock.
Why Leap Seconds Are Being Abolished
In November 2022, the General Conference on Weights and Measures voted to abolish leap seconds by 2035. The decision came after decades of debate. The pro-abolition argument: leap seconds cause real software bugs, are unpredictable, and the underlying problem (Earth's rotation drift) is so small that humans will not notice if UTC and UT1 drift apart by a few seconds or even a few minutes over centuries. The pro-leap-second argument: astronomers and some legacy systems care about keeping clock time aligned with solar time. The abolitionists won. After 2035, UTC will stop adding leap seconds and will simply continue ticking with atomic time. Eventually, hundreds or thousands of years from now, the difference between UTC and solar noon will be noticeable — but that is a problem for future generations.
The Negative Leap Second Possibility
A leap second can technically be either positive (extra second added) or negative (a second removed). Every leap second added so far has been positive, because the Earth has been slowing down. But in recent years, the Earth has actually been spinning slightly FASTER, which raises the possibility of a negative leap second — the first ever. Many systems handle the negative leap second case poorly because they have never been tested against it. There has been industry concern about the potential for chaos if a negative leap second is ever needed. The 2035 abolition will eliminate this risk entirely.
How Leap Seconds Affect Unix Time
Unix time — the seconds-since-1970 timestamp used by computers everywhere — was defined to assume every day has exactly 86,400 seconds, with no leap seconds. As a result, the exact moment of a leap second technically does not exist in Unix time. Two events one second apart that span a leap second can have the SAME Unix timestamp, depending on how the system handles it. For 99.99% of applications, this is invisible. For high-precision scientific applications, financial transaction logs, or distributed systems trying to maintain global ordering, it can cause subtle bugs. Most production systems use techniques like Google's "leap smear" to spread the extra second across many hours, avoiding the discontinuity.
Why Most People Will Never Notice
For ordinary daily life, leap seconds are invisible. Your watch, your phone, your computer's display, your alarm clock — none of these care about leap seconds at the second-by-second level. The differences only matter to systems that need to coordinate at sub-second precision: GPS satellites, financial trading systems, scientific timekeeping, distributed databases, and astronomical observations. The general public has never been impacted directly by a leap second. Even the famous 2012 outages were noticed by tech enthusiasts but barely registered for most users. So in a sense, abolishing leap seconds in 2035 will be a non-event for most people — and that is the point.
How Clockzilla Handles Leap Seconds
Clockzilla's sync engine uses NTP-referenced time sources that handle leap seconds at their own level. Our display does not show or count individual leap seconds. The accuracy of our time display is bounded by network latency at around 50–500 milliseconds, which is much larger than any single leap second offset. So while leap seconds are interesting to know about, they are not a practical limitation for using Clockzilla. After 2035, when leap seconds are abolished, neither we nor most other consumer time tools will need to change anything.
The Bottom Line
A leap second is an extra second occasionally added to UTC to keep it aligned with the Earth's slowly slowing rotation. There have been 27 of them since 1972. They cause subtle bugs in software systems and are scheduled to be abolished by 2035. After that, UTC will simply continue with atomic time, and Earth's rotation will be allowed to slowly drift away from official time. For everyday users, leap seconds are invisible. For programmers, they are a small but persistent gotcha. And for astronomers and scientists, they are the reason civil time has stayed aligned with the stars for the last 50 years.
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