Most people know about the leap year — but there's also a leap second: a single extra second occasionally added to the world's clocks to keep them perfectly aligned with the spinning Earth. Here's what it is, why it exists, and why it's on its way out.
For how world time works, see GMT vs UTC explained or the live world clock.
What is a leap second?
A leap second is one extra second added to UTC (Coordinated Universal Time) — the global time standard. When one is inserted, that day has 86,401 seconds instead of the usual 86,400, and clocks tick 23:59:60 before rolling to 00:00:00.
Why do we need them?
There are two ways to measure time, and they don't quite agree:
- Atomic time — measured by atomic clocks, incredibly precise and steady.
- Astronomical time — based on Earth's rotation, which is slightly irregular and, on average, very gradually slowing.
Because Earth's spin isn't perfectly constant, atomic time slowly pulls ahead of "Sun time." A leap second nudges UTC back into line so our clocks stay matched to the actual day/night cycle.
How often do they happen?
Leap seconds aren't on a schedule — they're announced by the International Earth Rotation and Reference Systems Service (IERS) when needed, usually on 30 June or 31 December. Since the system began in 1972, 27 leap seconds have been added (as of the mid-2020s). None have ever been removed, though that remains theoretically possible.
Why leap seconds cause problems
For most people they're invisible — but for computers and networks they're a headache. A clock showing "60 seconds" can crash software that assumes a minute only ever has 0–59 seconds. Past leap seconds have caused outages at major tech companies, which is why many now "smear" the second gradually across a day instead.
The end of the leap second
Because of these problems, in 2022 the global metrology community voted to abolish the leap second by 2035. After that, UTC will be allowed to drift further from astronomical time before any (much larger, rarer) correction is considered.
Leap second vs leap year
| Leap year | Leap second | |
|---|---|---|
| What's added | One day (29 Feb) | One second |
| How often | Every ~4 years | Irregular (~every few years) |
| Why | Earth's orbit (~365.24 days) | Earth's rotation irregularity |
| Predictable? | Yes (fixed rule) | No (announced as needed) |
Quick answers
What is a leap second? An extra second occasionally added to UTC to keep atomic clocks aligned with Earth's slightly irregular rotation. That day has 86,401 seconds.
Why do leap seconds exist? Atomic time is perfectly steady, but Earth's rotation isn't — leap seconds nudge UTC back in line with the actual day.
How many leap seconds have there been? 27 since 1972, all additions, usually on 30 June or 31 December.
Are leap seconds being abolished? Yes — a 2022 international vote agreed to phase them out by 2035.