Leap Seconds Explained — Why Clocks Sometimes Get an Extra Second
On a handful of occasions since 1972, the world’s official clocks have counted to 23:59:60 before rolling over to midnight. That extra second — a leap second — exists because of a fundamental mismatch between how we define time and how the Earth actually rotates. And after decades of debate, the international community has decided to stop adding them.
Why Leap Seconds Exist
We define a second precisely: it’s 9,192,631,770 oscillations of a cesium-133 atom. Atomic clocks measure this with extraordinary accuracy, losing less than a second over millions of years. The time standard built on these clocks is called International Atomic Time (TAI).
But we also want our clocks to stay aligned with the Earth’s rotation — we want noon to mean the sun is roughly overhead. The problem is that the Earth’s rotation isn’t perfectly constant. Tidal forces from the Moon, changes in ocean currents, earthquakes, and even the redistribution of ice mass all cause tiny variations in the length of a day.
On average, the Earth’s rotation is gradually slowing. A day is currently about 2 milliseconds longer than the 86,400 SI seconds that define a day in atomic time. That doesn’t sound like much, but it accumulates: without correction, atomic time and solar time would drift apart by about a second every 18 months.
How Leap Seconds Work
Coordinated Universal Time (UTC) is the compromise. It ticks at the same rate as atomic time (TAI) but periodically adds (or theoretically subtracts, though this has never happened) a leap second to stay within 0.9 seconds of the Earth’s rotation.
The decision to add a leap second is made by the International Earth Rotation and Reference Systems Service (IERS), based in Paris. They monitor the difference between UT1 (a measure of Earth’s rotational angle) and UTC. When the gap approaches 0.9 seconds, they announce a leap second, typically to be inserted at the end of June 30 or December 31.
During a positive leap second, the UTC clock counts:
23:59:58
23:59:59
23:59:60 ← the leap second
00:00:00This has happened 27 times since the system was introduced in 1972. The most recent leap second was added on December 31, 2016.
The Technological Headaches
Leap seconds are a nightmare for technology systems. Software that assumes every minute has 60 seconds and every day has 86,400 seconds can break in subtle and sometimes spectacular ways when a leap second occurs.
Notable incidents include:
2012: A Linux kernel bug triggered by a leap second caused widespread crashes. Reddit, Mozilla, Foursquare, and many other services experienced outages. The bug caused the kernel’s clock to jump, triggering a cascade of CPU-intensive futex operations that effectively locked up servers.
2015: Multiple internet services experienced glitches during the June 30 leap second, including issues with the Amadeus airline booking system that affected travelers worldwide.
2017: Cloudflare experienced a leap-second-related bug that caused its DNS servers to produce negative time values, leading to service disruptions.
Strategies for Handling Leap Seconds
Different systems handle leap seconds in different ways:
Stepping: The clock literally counts 23:59:60. This is the “correct” approach according to UTC, but many software systems can’t handle a 61-second minute and may crash or produce errors.
Smearing: Instead of inserting a full second at once, the clock is gradually adjusted over a period of hours. Google pioneered “leap smearing,” spreading the extra second across a 24-hour window so that each second is slightly longer than normal. The clock never shows 23:59:60, and naive software isn’t confused. However, during the smear window, the system’s time doesn’t match UTC precisely.
Ignoring: Unix time simply ignores leap seconds. The Unix timestamp counts SI seconds without any leap second adjustment, meaning it technically disagreed with UTC by 27 seconds as of 2016. The operating system handles the discrepancy through NTP (Network Time Protocol) corrections.
GPS time: The GPS system uses its own timescale that doesn’t include leap seconds. GPS time has been running since January 6, 1980, and is currently ahead of UTC by 18 seconds.
The Decision to Abolish
In November 2022, the General Conference on Weights and Measures (CGPM) voted to abolish leap seconds by 2035. The resolution allows UTC to gradually drift away from Earth’s rotation, with a larger correction to be applied sometime before 2135 if the accumulated difference becomes too large.
The reasons for abolition are pragmatic:
- The technological costs and risks of leap seconds have grown enormously as software systems multiply.
- The number of people who need UTC to precisely track the Earth’s rotation is tiny compared to the number affected by the disruptions.
- Modern navigation and astronomy systems already use their own timescales independent of leap seconds.
The accumulated drift is extremely small in human terms. Even without any leap seconds, it would take roughly 1,000 years for UTC to drift by about 10 minutes from solar time. For context, your local solar noon already differs from clock noon by up to 30 minutes depending on your position within your time zone.
What Happens Next
Between now and 2035, the details of the transition need to be worked out. No new leap seconds have been added since 2016, partly because the Earth’s rotation has recently been speeding up slightly (for reasons not fully understood). If this trend continues, the transition may be seamless — the accumulated difference might remain small enough to require no special handling.
The abolition of leap seconds marks the end of a 50-year experiment in trying to keep atomic precision and astronomical observation in lockstep. In the end, the atomic clocks won: our definition of time is now fundamentally about the physics of atoms, not the spin of the planet. The Earth will continue to rotate at its own pace, and our clocks will carry on at theirs, with the tiny gap between them growing so slowly that no human will ever notice.