The Extra Second at 23:59:60
A leap second is a one-second adjustment to Coordinated Universal Time (UTC). It keeps UTC within 0.9 seconds of UT1, the astronomical time measured by the planet's spin.
Think of it like a calendar leap day, but for clocks. Instead of adding an extra day in February, timekeepers add a whole second to the last minute of June or the final month of the year. The clock reads 23:59:60 before rolling over to 00:00:00 the next day.
Why Leap Seconds Exist: A Planet That Slows
The world's spin is not constant. It slows down gradually, mainly because of tidal friction from the Moon. The effect is tiny, about 1.7 milliseconds per century, but it adds up.
Before atomic clocks, this didn't matter. Time was measured by the stars (UT1). But in 1955, the first caesium atomic clock gave us a second defined by exactly 9,192,631,770 cycles of radiation from a caesium atom. Atomic time (TAI) is uniform. Earth time (UT1) is not.
By the 1960s, the gap between atomic time and astronomical time was already noticeable. If left uncorrected, noon on an atomic clock would eventually happen at midnight. The leap second was the compromise: keep the atomic precision of TAI, but occasionally add or remove a second to stay aligned with the sun.
How Leap Seconds Work: 23:59:60 UTC
A positive leap second inserts an extra second at 23:59:60 UTC on either 30 June or the last day of the year. The sequence looks like this:
- 23:59:59
- 23:59:60 (the leap second)
- 00:00:00 (next day)
A negative leap second would skip a second: 23:59:58, then 00:00:00. None have ever been needed.
The International Earth Rotation and Reference Systems Service (IERS) decides when. They announce the decision about six months ahead in a document called Bulletin C. That gives network operators, financial exchanges, and broadcasters time to prepare.
The History of Leap Seconds: 1972 to Today
The current leap-second system started on 1 January 1972. Before that, UTC used smaller, more frequent adjustments called "rate offsets."
The first leap second was added on 30 June 1972. Since then, 27 have been added. All have been positive. The most recent was on 31 December 2016.
The rate of leap seconds has slowed. In the 1970s and 1980s, they were needed almost every year. Since 2000, the gap has stretched to several years. This is because the planet's spin has temporarily sped up slightly, not because the long-term slowing stopped.
Complete List of Leap Seconds (1972-2016)
| Date (UTC) | Leap second |
|---|---|
| 30 June 1972 | +1 |
| 31 December 1972 | +1 |
| 31 December 1973 | +1 |
| 31 December 1974 | +1 |
| 31 December 1975 | +1 |
| 31 December 1976 | +1 |
| 31 December 1977 | +1 |
| 31 December 1978 | +1 |
| 31 December 1979 | +1 |
| 30 June 1981 | +1 |
| 30 June 1982 | +1 |
| 30 June 1983 | +1 |
| 30 June 1985 | +1 |
| 31 December 1987 | +1 |
| 31 December 1989 | +1 |
| 31 December 1990 | +1 |
| 30 June 1992 | +1 |
| 30 June 1993 | +1 |
| 30 June 1994 | +1 |
| 31 December 1995 | +1 |
| 30 June 1997 | +1 |
| 31 December 1998 | +1 |
| 31 December 2005 | +1 |
| 31 December 2008 | +1 |
| 30 June 2012 | +1 |
| 30 June 2015 | +1 |
| 31 December 2016 | +1 |
Total: 27 leap seconds.
Positive vs Negative Leap Seconds
A positive leap second adds time. A negative one would remove it. All 27 have been positive because the planet's spin is, on average, slower than the atomic clock.
A negative leap second would require the world to turn faster than the atomic day. That has not happened yet. If it did, clocks would jump from 23:59:58 directly to 00:00:00, skipping 23:59:59 entirely.
Who Decides When to Add a Leap Second?
Three organisations are involved.
The Bureau International des Poids et Mesures (BIPM) computes UTC from about 450 atomic clocks worldwide. The IERS monitors the planet's spin and decides when a leap second is needed. The International Telecommunication Union (ITU) governs the time-signal broadcasts that distribute UTC.
The IERS publishes Bulletin C in January and July each year. If a leap second is needed, the bulletin announces it roughly six months before the insertion date. If no leap second is needed, the bulletin says so.
The 2035 Plan to Abolish Leap Seconds
In 2022, the General Conference on Weights and Measures (CGPM) passed Resolution 4. It called for leap seconds to be discontinued by 2035 at the latest.
The reason is practical. Leap seconds cause problems for computer systems. A single extra second can confuse time-synchronisation protocols, break database transactions, and produce ambiguous timestamps. The 2012 and 2015 leap seconds caused widespread outages at companies like Reddit, Mozilla, and LinkedIn.
Under the new plan, UTC would stop being adjusted to match UT1. The difference between the two would be allowed to grow, possibly to one minute or more. A replacement mechanism, perhaps a "leap minute" or no adjustment at all, is still under discussion. Russia, whose GLONASS satellite system depends on leap seconds, has opposed abolition.
As of 2026, no final decision has been made. The 2035 deadline is a target, not a guarantee.
What Happens During a Leap Second? System Impacts
A leap second causes a problem for Unix time. Unix time counts seconds since 1 January 1970, ignoring leap seconds. During a leap second, the Unix timestamp does not advance. The second 23:59:60 maps to the same Unix timestamp as 00:00:00 the next day.
This "stutter" can confuse software that expects timestamps to be strictly increasing. Log files may have duplicate timestamps. Scheduled tasks may run twice. Some systems handle it by freezing time for one second. Others crash.
The 2015 leap second brought down parts of Twitter, Instagram, and Netflix. The 2012 one disrupted Reddit and caused problems for Amazon's AWS.
Leap Second Smearing: How Google and Amazon Handle It
Google and Amazon use a technique called "leap second smearing." Instead of inserting the whole second at once, they spread it across several hours before midnight.
Google's "leap smear" adds a few milliseconds to each second over the 20 hours leading up to the leap second. The total adjustment equals one second, but no single moment has a duplicate timestamp. Amazon uses a similar "leap second smear" for AWS services.
The smear technique prevents the stutter. Software sees a smooth, continuous increase in time. The cost is that during the smear window, the clock is slightly off from true UTC. For most applications, that is better than a crash.
UTC vs UT1: The Astronomical Time Behind Leap Seconds
UT1 is the astronomical time based on the planet's spin. It is measured by radio telescopes observing distant quasars. The difference between UTC and UT1 is the reason leap seconds exist.
UTC is kept within ±0.9 seconds of UT1. When the difference approaches 0.6 seconds, the IERS schedules a leap second to bring it back. This is why we have leap seconds: to stop UTC from drifting away from the sun.
Other universal-time variants exist, UT0 and UT2, but they are corrections for polar motion and seasonal spin changes. UT1 is the one that matters for leap seconds.
For more on the relationship between the two time scales, see TAI vs UTC and Who Maintains UTC.