What Are Leap Seconds? Why the World Occasionally Adds (or May Subtract) a Second

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Why Leap Seconds Exist

Two Kinds of Time

To understand leap seconds, you need to understand that there are two fundamentally different ways to measure time:

1. Astronomical time (UT1): Based on Earth's rotation. One "day" is one complete rotation of Earth relative to distant stars (more precisely, relative to the quasar reference frame). UT1 is the time that matches the Sun's position in the sky.

2. Atomic time (TAI): Based on the oscillation of cesium-133 atoms. One "second" is exactly 9,192,631,770 oscillations of a cesium atom. TAI is the time measured by atomic clocks, which are extraordinarily precise and never vary.

These two time scales do not agree perfectly. The problem is that Earth's rotation is not constant.

Earth's Rotation Is Slowing

Earth's rotation is gradually decelerating due to tidal friction from the Moon. The Moon's gravitational pull creates tidal bulges in Earth's oceans, and the friction between these bulges and the ocean floor slowly transfers rotational energy to the Moon's orbital energy. The result: Earth's day is getting longer by about 1.7 milliseconds per century.

This means that over long periods, the astronomical day (UT1) is slightly longer than 86,400 atomic seconds (TAI). The discrepancy is small — about 0.5 to 1.0 seconds per year — but it accumulates. Without correction, atomic time and solar time would gradually drift apart.

The 0.9-Second Tolerance

UTC was designed as a compromise: it runs on atomic seconds (like TAI) but is kept within 0.9 seconds of UT1 (astronomical time). When UTC drifts more than 0.9 seconds away from UT1, a leap second is inserted to bring them back into alignment. The leap second effectively pauses UTC for one second, allowing UT1 to "catch up."

Who Decides When Leap Seconds Are Added

The International Earth Rotation and Reference Systems Service (IERS), based in Frankfurt, Germany, monitors Earth's rotation and determines when a leap second is needed. IERS announces leap seconds at least 8 weeks in advance, and they are always scheduled for the last second of June 30 or December 31 (UTC). The decision is based purely on astronomical measurements — there is no fixed schedule.

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How Leap Seconds Work

The Mechanics of a Positive Leap Second

When a positive leap second is added, the last minute of the day (June 30 or December 31) has 61 seconds instead of the usual 60. The sequence of seconds looks like this:

`

23:59:58

23:59:59

23:59:60 ← the leap second

00:00:00

`

The second labeled "23:59:60" is the leap second. It is a perfectly valid timestamp in UTC. After the leap second, UTC has effectively gained one second relative to TAI — reducing the gap between UTC and UT1.

Negative Leap Seconds

In theory, a negative leap second could be subtracted if Earth's rotation sped up enough that UT1 moved ahead of UTC by more than 0.9 seconds. This would remove the second "23:59:59," making the last minute of the day only 59 seconds long. However, no negative leap second has ever been used. Earth's rotation has not sped up sufficiently to require one — though it came close in 2020 and 2021, when Earth experienced its shortest days in decades due to short-term rotational fluctuations.

The Leap Second in Practice

When a leap second occurs:

• Atomic clocks worldwide add the extra second simultaneously.
• GPS satellites transmit a flag indicating the leap second, which GPS receivers use to adjust their internal clocks.
• NTP (Network Time Protocol) servers distribute the leap second announcement and adjust their time accordingly.
• Financial markets must handle the extra second in their trading systems and timestamps.
• Airlines and air traffic control must account for the extra second in their scheduling systems.

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All Leap Seconds Since 1972

Since the leap second system began on January 1, 1972, there have been 27 positive leap seconds. None have been negative.

| Date | TAI–UTC After Leap | |---|------|-------------------| | 1 | June 30, 1972 | 11 seconds | | 2 | December 31, 1972 | 12 seconds | | 3 | December 31, 1973 | 13 seconds | | 4 | December 31, 1974 | 14 seconds | | 5 | December 31, 1975 | 15 seconds | | 6 | December 31, 1976 | 16 seconds | | 7 | December 31, 1977 | 17 seconds | | 8 | December 31, 1978 | 18 seconds | | 9 | December 31, 1979 | 19 seconds | | 10 | June 30, 1981 | 20 seconds | | 11 | June 30, 1982 | 21 seconds | | 12 | June 30, 1983 | 22 seconds | | 13 | June 30, 1985 | 23 seconds | | 14 | December 31, 1987 | 24 seconds | | 15 | December 31, 1989 | 25 seconds | | 16 | December 31, 1990 | 26 seconds | | 17 | June 30, 1992 | 27 seconds | | 18 | June 30, 1993 | 28 seconds | | 19 | June 30, 1994 | 29 seconds | | 20 | December 31, 1995 | 30 seconds | | 21 | June 30, 1997 | 31 seconds | | 22 | December 31, 1998 | 32 seconds | | 23 | December 31, 2005 | 33 seconds | | 24 | December 31, 2008 | 34 seconds | | 25 | June 30, 2012 | 35 seconds | | 26 | June 30, 2015 | 36 seconds | | 27 | December 31, 2016 | 37 seconds |

Note: As of early 2026, the last leap second was added on December 31, 2016. The current TAI–UTC offset is 37 seconds. No leap second has been added in over 9 years — the longest gap since the system began. This is because Earth's rotation has been slightly faster than its long-term trend in recent years, reducing the need for leap seconds.

Patterns in the Data

Several observations from the table:

• Leap seconds were more frequent in the 1970s: Nine leap seconds were added in the first 8 years (1972–1979), averaging more than one per year.
• They became less frequent over time: Only two leap seconds were added between 1999 and 2016 (a 17-year period).
• The most recent gap is the longest: No leap second has been needed since December 31, 2016 — over 9 years and counting.
• Earth's rotation has been speeding up slightly: Since 2020, Earth has recorded several of its shortest days in decades. On June 29, 2022, Earth completed one rotation in 1.59 milliseconds less than 86,400 seconds. This means UT1 is actually running slightly ahead of UTC in the short term, reducing the need for positive leap seconds and raising the possibility of the first-ever negative leap second.

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When the Next Leap Second Might Happen

As of early 2026, IERS has not announced any upcoming leap second. The long gap since December 2016 is unprecedented and reflects Earth's recent rotational speedup.

Could a Negative Leap Second Happen?

If Earth's rotation continues to be faster than average, UT1 could eventually drift more than 0.9 seconds ahead of UTC, requiring a negative leap second. This would be a first. Some scientists predicted this could happen as early as 2025 or 2026, but the required threshold has not yet been reached. The situation is being closely monitored by IERS.

However, if the 2022 ITU resolution to abolish leap seconds by 2035 is implemented, neither positive nor negative leap seconds may ever be added again.

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The Controversy

The ITU Resolution to Abolish Leap Seconds by 2035

At the World Radiocommunication Conference in 2022, the International Telecommunication Union (ITU) passed a resolution to eliminate leap seconds by 2035. The resolution calls for UTC to no longer be kept within 0.9 seconds of UT1, instead allowing the gap to grow. A future mechanism (possibly a "leap minute" applied once every 50–100 years) would eventually correct the accumulated difference.

The resolution was driven by decades of complaints from the technology sector, but it was opposed by many astronomers and navigators.

The Technology Argument

Leap seconds are a nightmare for computing systems. The problem is that the second "23:59:60" does not exist in most programming languages, operating systems, or database formats. When a leap second occurs, systems must be specially configured to handle it — and many are not.

Key incidents include:

• July 2012 leap second: The addition of a leap second on June 30, 2012, caused outages at Reddit, Yelp, LinkedIn, FourSquare, Gawker, and other major websites. The Linux kernel's handling of the leap second triggered a race condition that caused CPUs to spike to 100%. Engineers at Google had to develop "leap smear" technology to avoid the problem (see below).
• July 2015 leap second: Airlines in Australia and South America experienced check-in system failures. The Amadeus airline reservation system was disrupted.
• January 2017 leap second: Cloudflare's DNS service experienced a partial outage due to a leap second bug in its Go code that caused negative time values in the random number generator.
• Financial systems: High-frequency trading systems that process millions of transactions per second cannot easily handle a timestamp of "23:59:60." Stock exchanges have had to implement special leap second procedures, sometimes shutting down briefly during the transition.

The Astronomer Argument

Astronomers, navigators, and satellite operators argue that leap seconds are necessary to keep UTC aligned with the sky. Without leap seconds:

• Celestial navigation would become less accurate, because the time used for navigation calculations would not match Earth's actual rotation.
• Telescope pointing would require corrections, because sidereal time calculations depend on UT1.
• Satellite tracking would become more complex, because Earth's orientation relative to the stars would diverge from UTC.
• Legal definitions of time in many countries reference Earth's rotation; changing this would require legislative updates.

The Compromise: Leap Minutes

The most likely replacement for leap seconds is the "leap minute" — a one-minute adjustment applied much less frequently (perhaps once every 50–100 years). At the current rate of drift (roughly 0.5–1.0 seconds per year), UTC would accumulate about 1 minute of drift from UT1 over approximately 60–120 years. A leap minute could then be applied to bring them back into alignment.

The advantage of leap minutes is that they would be rare enough to cause minimal disruption to computing systems, while still keeping UTC and UT1 from drifting too far apart. The disadvantage is that for decades at a time, UTC would not accurately represent the Sun's position — solar noon could drift by tens of seconds from clock noon.

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How Leap Seconds Affect Computing

The 2012 Reddit/LinkedIn Outage

The most famous leap second computing failure occurred on June 30, 2012. When the leap second was inserted at 23:59:60 UTC, the Linux kernel's hrtimers (high-resolution timers) subsystem handled the event by calling a function called clock_was_set(). This function triggered a futex (fast userspace mutex) deadlock in some multi-threaded applications, causing Java-based services to hang.

Reddit, LinkedIn, Yelp, and other sites running on Java and Linux experienced outages lasting from minutes to hours. The fix required either patching the kernel or restarting affected services.

Google's Leap Smear

After experiencing problems with the 2005 and 2008 leap seconds, Google developed a technique called "leap smear." Instead of inserting a discrete 23:59:60 second, Google's NTP servers gradually adjust their time over a 24-hour period around the leap second — adding a tiny fraction of a second to each clock tick. By the end of the smear period, the total adjustment equals exactly one second.

For example, over 24 hours (86,400 seconds), adding 1/86,400 of a second per tick accumulates to exactly 1 second. Systems using Google's NTP servers never see a "23:59:60" timestamp and never experience a discontinuity. Amazon, Microsoft, and Meta have since adopted similar leap smear approaches.

The POSIX Problem

The POSIX standard for Unix time defines each day as exactly 86,400 seconds. This definition cannot accommodate leap seconds. On a POSIX system, when a leap second occurs, the timestamp "23:59:60" is mapped to "23:59:59" (repeating the previous second) or to the next day's "00:00:00" (skipping the leap second). Either way, the timestamp is ambiguous or inaccurate. This fundamental incompatibility between POSIX time and UTC is the root cause of most leap second computing problems.

What Developers Should Do

1. Use leap smear NTP servers (Google, AWS, Meta) to avoid discrete leap second jumps.

2. Never assume 86,400 seconds per day in code that requires precision.

3. Use monotonic clocks for measuring elapsed time, rather than wall-clock time.

4. Store timestamps in UTC and use the IANA time zone database for conversions.

5. Test leap second handling before each scheduled leap second (when announced).

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What Replaces Leap Seconds

The Current Plan

The 2022 ITU resolution sets a target date of 2035 for the last leap second. After that, UTC will be allowed to drift from UT1. A new maximum tolerance (likely larger than 0.9 seconds) will be established, and a less frequent correction mechanism will be introduced.

Candidate Replacement Mechanisms

What Happens After 2035

The specific replacement mechanism has not yet been decided. The ITU resolution called for further study and a decision by the 2027 World Radiocommunication Conference. Whatever is chosen will need to balance the needs of astronomers (who want UTC close to UT1) and technologists (who want UTC to be a continuous, uninterrupted time scale).

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FAQ

What is a leap second?

A leap second is a one-second adjustment added to UTC to keep it within 0.9 seconds of UT1 (astronomical time based on Earth's rotation). Leap seconds are inserted on June 30 or December 31 when needed, as determined by the International Earth Rotation and Reference Systems Service (IERS).

How many leap seconds have been added?

As of early 2026, 27 leap seconds have been added since the system began in 1972. The most recent was on December 31, 2016. No negative leap second has ever been subtracted.

Why does Earth's rotation need correction?

Earth's rotation is gradually slowing due to tidal friction from the Moon. This means the astronomical day is getting slightly longer over time. Atomic clocks, which define the SI second, run at a constant rate. Without leap second corrections, UTC (based on atomic time) would gradually drift away from UT1 (based on Earth's rotation), and solar noon would no longer match clock noon.

Has a negative leap second ever been used?

No. All 27 leap seconds have been positive (added). Earth's long-term rotational deceleration has always outweighed short-term speedups. However, in 2020–2022, Earth experienced its shortest days in decades, and some scientists predicted a negative leap second might be needed. As of 2026, this has not occurred.

Why do leap seconds cause computer problems?

Most programming languages, operating systems, and time formats do not support the timestamp "23:59:60." When a leap second occurs, systems must handle a 61-second minute, which can trigger bugs, race conditions, and outages. Major incidents include the 2012 Reddit/LinkedIn outage and the 2017 Cloudflare DNS failure.

Will leap seconds be abolished?

Yes, in principle. The ITU voted in 2022 to eliminate leap seconds by 2035. The specific replacement mechanism has not been decided but will likely involve a much less frequent correction (such as a "leap minute" every 50–100 years). The 2027 World Radiocommunication Conference is expected to finalize the details.

What is "leap smear"?

Leap smear is a technique used by Google, Amazon, Meta, and other tech companies to handle leap seconds without a discrete one-second jump. Instead of inserting a "23:59:60" timestamp, leap smear gradually adjusts the clock rate over 24 hours so that the total adjustment equals one second. This avoids the discontinuity that causes most computing problems.

How much does Earth's rotation slow per year?

Earth's rotation slows by about 1.7 milliseconds per century due to tidal friction. This translates to roughly 0.5 to 1.0 seconds of drift between UT1 and TAI per year. However, short-term fluctuations (caused by atmospheric circulation, ocean currents, and glacial melting) can make Earth's rotation temporarily speed up or slow down by several milliseconds per day.