The extra second that almost nobody sees
A leap second is a one-second correction added to Coordinated Universal Time (UTC) so civil time stays close to the rotation of Earth. During a positive leap second, the final minute of a UTC day has 61 seconds and the clock can read 23:59:60 before moving to 00:00:00. That looks like a typo, but it is a real UTC label.
The reason is that two different ideas of time are being kept in conversation. Atomic clocks are extremely steady; Earth is not. Our planet speeds up and slows down slightly because of tides, atmosphere, oceans, its core, and other geophysical effects. UTC is based on International Atomic Time (TAI), but it has historically been adjusted so it does not drift too far from UT1, the time scale tied to Earth rotation.
Why leap seconds exist
The modern leap-second system began in 1972. By rule, when the difference between UT1 and UTC is predicted to approach 0.9 seconds, the International Earth Rotation and Reference Systems Service (IERS) can announce a leap second. Leap seconds have always been placed at the end of June or December in UTC.
Since 1972 there have been 27 positive leap seconds. The most recent one was on December 31, 2016. As of the latest official announcement, no leap second is scheduled for the end of June 2026.
What actually happens at midnight
| Situation | UTC sequence | What it means |
|---|---|---|
| Normal day | 23:59:58, 23:59:59, 00:00:00 | The last minute has 60 seconds. |
| Positive leap second | 23:59:58, 23:59:59, 23:59:60, 00:00:00 | One extra second is inserted. |
| Possible negative leap second | 23:59:57, 23:59:58, 00:00:00 | One second would be skipped; this has never happened. |
A negative leap second is allowed in principle, but it has never been used. That matters because the recent behavior of Earth rotation raised the possibility of needing one someday, and many systems have never been tested for that case.
Why computers dislike leap seconds
Humans can tolerate an odd clock label once every few years. Software is less forgiving. Many systems assume every minute has exactly 60 seconds. Some timestamp formats cannot represent 23:59:60 at all. Logs can show duplicated times, event ordering can become ambiguous, and distributed systems can disagree if one server steps the clock while another smears the second gradually over several hours.
NIST describes several ways time services handle a leap second, including repeating a second or smearing the adjustment. Both approaches can work, but mixing them across databases, APIs, monitoring tools, and financial or industrial systems creates practical risk. This is also why Unix timestamps can be confusing around leap seconds: many common systems count seconds from 1970 without giving the leap-second label its own clean representation.
Why the leap second is being retired
In 2022, the General Conference on Weights and Measures (CGPM) adopted a resolution to increase the allowed maximum difference between UT1 and UTC in or before 2035. In plain language, the world plans to stop inserting regular leap seconds and let atomic civil time drift a little farther from Earth-rotation time.
The change does not mean sunrise will suddenly move on your clock. The drift is tiny for daily life. It does mean astronomy, satellite work, navigation, and other precision fields will need to rely more explicitly on published UT1-UTC corrections when Earth rotation matters. For everyone else, time systems become simpler and less fragile.
Before and after the reform
| Question | Current leap-second system | After the planned UTC reform |
|---|---|---|
| Goal | Keep UTC within about 0.9 seconds of UT1. | Keep UTC continuous for much longer periods. |
| Adjustment | Occasional one-second jumps. | No regular leap seconds for at least a long time. |
| Computer impact | Special handling needed near leap seconds. | Fewer discontinuities in civil time. |
| Astronomy impact | UTC stays very close to Earth rotation. | UT1 corrections become more important for precise work. |
The useful mental model
A leap second is not like February 29. Leap years follow a predictable calendar rule; leap seconds are measurement-based decisions about Earth rotation. They are announced only after observing how far atomic time and Earth-rotation time are separating.
The story is interesting because it shows a tension inside modern timekeeping. We want clocks that are stable enough for computers, satellites, and markets, but we also inherited a civil day whose meaning comes from the turning Earth. The leap second was the compromise. By 2035, the compromise is expected to change.