Leap second facts for kids
Imagine your clock needs a tiny adjustment to stay perfectly in sync with the Earth! A leap second is like adding one extra second to our official time, called Coordinated Universal Time (UTC). We do this because the Earth's spin isn't perfectly steady.
We have super-accurate atomic clocks that keep time very precisely (this is called International Atomic Time or TAI). But the Earth's rotation, which gives us our days, can speed up or slow down a little. This means the time from atomic clocks can get ahead of the time based on the sun (called Universal Time or UT1).
To keep these two types of time close, we sometimes add a leap second. This makes sure that UTC, which most countries use for everyday time, stays aligned with the Earth's actual rotation. Leap seconds started in 1972. So far, 27 leap seconds have been added, with the last one on December 31, 2016. All of them have added a second, making a day a tiny bit longer.
Because the Earth's spin changes in unpredictable ways, we don't know exactly when a leap second will be needed. Experts at the International Earth Rotation and Reference Systems Service (IERS) decide about six months ahead of time if one is needed. This keeps our clocks from being off by more than 0.9 seconds from the Earth's spin.
However, these extra seconds can cause problems, especially for computers and systems that need very exact timing. Because of these challenges, a big decision was made in November 2022 by the General Conference on Weights and Measures. They decided to stop adding leap seconds by 2035.
Contents
How We Started Measuring Time
From Sun-Based Time to Atomic Clocks
For a very long time, people measured time using the sun. A day was simply how long it took for the sun to appear in the same spot in the sky. This was called the mean solar day. Early scholars, like al-Biruni around the year 1000, divided this day into smaller and smaller parts, eventually creating the idea of a 'second' as a tiny fraction of the day.
However, scientists later discovered that the Earth's rotation isn't perfectly steady. It speeds up and slows down a little. This meant that a 'second' based on the Earth's spin wasn't always the same length!
To get super-accurate time, scientists developed atomic clocks. These clocks measure time by counting the vibrations of atoms, like caesium-133. In 1967, the 'second' was officially redefined based on these atomic vibrations. This new 'atomic second' is incredibly precise and always the same length.
But here's the catch: because the Earth's spin changes, the atomic clocks started to get ahead of the time based on the sun. By 1961, the average solar day was already a tiny bit longer than 86,400 atomic seconds.
To fix this, a system was created in 1972. Before that, they tried to adjust the speed of atomic clocks, which was complicated. The new system introduced the idea of adding a whole second when needed. This way, the official time (UTC) could use the super-accurate atomic seconds but still stay close to the Earth's actual rotation.
When leap seconds were introduced in 1972, the atomic time (TAI) was already 10 seconds ahead of the old sun-based time. Since then, 27 more leap seconds have been added to UTC. This means that as of 2025, TAI is 37 seconds ahead of UTC (10 initial seconds + 27 leap seconds).
Why Earth's Spin Changes
What Makes Our Days Longer or Shorter?
The Earth's rotation isn't perfectly smooth. It speeds up and slows down in ways that are hard to predict far in advance. This is why experts only announce leap seconds about six months before they happen.
Many things affect how fast the Earth spins. One big reason for it slowing down over very long periods is tidal friction. This is the pull from the Moon and Sun on Earth's oceans, which acts like a brake.
Other things can also change the Earth's speed. For example, movements deep inside the Earth, like changes in its crust and core, can shift its weight around. Imagine a spinning ice skater: if they pull their arms in, they spin faster; if they push them out, they slow down. The Earth works similarly. When its mass moves, its spin rate changes.
Sometimes these changes make the Earth spin faster, shortening the day. For instance, the melting of ice caps can cause water to move towards the equator, which can slow the Earth's rotation. Even big events like the 2004 Indian Ocean earthquake were thought to have made the day a tiny bit shorter!
It's important to remember that leap seconds don't just mean the Earth is slowing down. They show the total difference that has built up between super-accurate atomic time and the Earth's actual rotation. In recent years, the Earth has actually been spinning a bit faster. For example, in 2020, we had some of the shortest days recorded since 1960. This even made scientists wonder if we might need a 'negative' leap second, where we'd remove a second instead of adding one! The shortest day ever recorded was on June 29, 2022.
How Leap Seconds Are Added
When and Where Leap Seconds Happen
The job of deciding when to add a leap second belongs to the International Earth Rotation and Reference Systems Service (IERS). They usually decide to add a leap second when the difference between UTC (our official time) and UT1 (time based on Earth's spin) gets close to 0.6 seconds. This ensures the difference never goes over 0.9 seconds.
Leap seconds are usually added at the very end of June or December. Sometimes they could be added at the end of March or September, but as of September 2025, all of them have been on June 30th or December 31st. The IERS announces these decisions about six months in advance in a special publication called 'Bulletin C'.
When a positive leap second is added, it means that a minute will have 61 seconds instead of 60! So, the clock would go from 23:59:59 to 23:59:60, and then to 00:00:00 of the next day. Imagine seeing 23:59:60 on a clock!
Unlike leap days, which happen at different local times around the world, a leap second happens at the exact same moment everywhere. For example, the leap second on December 31, 2005, at 23:59:60 UTC, happened at different local times depending on your time zone.
So far, we've only ever added seconds. A 'negative' leap second would mean removing a second, so the clock would jump from 23:59:58 straight to 00:00:00. This has not happened yet, but recent changes in Earth's rotation have made it a possibility before leap seconds are stopped completely.
| Year | 30 Jun | 31 Dec |
|---|---|---|
| 1972 | +1 | +1 |
| 1973 | 0 | +1 |
| 1974 | 0 | +1 |
| 1975 | 0 | +1 |
| 1976 | 0 | +1 |
| 1977 | 0 | +1 |
| 1978 | 0 | +1 |
| 1979 | 0 | +1 |
| 1980 | 0 | 0 |
| 1981 | +1 | 0 |
| 1982 | +1 | 0 |
| 1983 | +1 | 0 |
| 1984 | 0 | 0 |
| 1985 | +1 | 0 |
| 1986 | 0 | 0 |
| 1987 | 0 | +1 |
| 1988 | 0 | 0 |
| 1989 | 0 | +1 |
| 1990 | 0 | +1 |
| 1991 | 0 | 0 |
| 1992 | +1 | 0 |
| 1993 | +1 | 0 |
| 1994 | +1 | 0 |
| 1995 | 0 | +1 |
| 1996 | 0 | 0 |
| 1997 | +1 | 0 |
| 1998 | 0 | +1 |
| 1999 | 0 | 0 |
| 2000 | 0 | 0 |
| 2001 | 0 | 0 |
| 2002 | 0 | 0 |
| 2003 | 0 | 0 |
| 2004 | 0 | 0 |
| 2005 | 0 | +1 |
| 2006 | 0 | 0 |
| 2007 | 0 | 0 |
| 2008 | 0 | +1 |
| 2009 | 0 | 0 |
| 2010 | 0 | 0 |
| 2011 | 0 | 0 |
| 2012 | +1 | 0 |
| 2013 | 0 | 0 |
| 2014 | 0 | 0 |
| 2015 | +1 | 0 |
| 2016 | 0 | +1 |
| 2017 | 0 | 0 |
| 2018 | 0 | 0 |
| 2019 | 0 | 0 |
| 2020 | 0 | 0 |
| 2021 | 0 | 0 |
| 2022 | 0 | 0 |
| 2023 | 0 | 0 |
| 2024 | 0 | 0 |
| 2025 | 0 | 0 |
| Year | 30 Jun | 31 Dec |
| Total | 11 | 16 |
| 27 | ||
| Current TAI − UTC | ||
| 37 | ||
Challenges with Leap Seconds
Why Computers Don't Like Leap Seconds
While leap seconds help keep our clocks aligned with the Earth, they create big headaches for computers and technology. Here's why:
- Confusing Calculations: Imagine trying to calculate how much time passed between two events. If a leap second was added in between, your calculation could be off by a second unless you knew about it. This makes planning for future events tricky, as we only know about leap seconds six months ahead.
- Unexpected Changes: Many computer systems expect every minute to have exactly 60 seconds. When a leap second adds a 61st second, it can confuse programs. Some systems might repeat a second, others might freeze, and some might even show the wrong time for a short period.
- Different Ways of Handling: There's no single way for all computers to handle a leap second. Some systems 'smear' the extra second over a longer period, making each second slightly longer. This means different computers might show slightly different times, which can be a problem for things that need perfect synchronization.
- Software Bugs: Many computer programs and devices were not designed to handle an extra second. This has led to many software errors, causing websites to crash, airline reservation systems to have issues, and even problems with navigation systems. For example, some systems have shown the wrong date or time after a leap second.
- Binary Time Issues: Computers often count time using a simple number of seconds since a starting point. When a leap second is added, this counter doesn't always know how to deal with it, leading to confusion about which second is which. This can cause problems in systems that rely on very precise timestamps.
The Future of Time: Saying Goodbye to Leap Seconds
Why Change is Happening
Because of all the problems leap seconds cause, especially for computers and modern technology, there have been many discussions about stopping them. Systems that need super-accurate time, like those used in high-frequency trading or for controlling important processes, find these unpredictable changes very difficult.
For years, international groups like the International Telecommunication Union (ITU) and the General Conference on Weights and Measures (CGPM) debated what to do. Many countries and scientists argued that the benefits of leap seconds no longer outweighed the problems they caused. They pointed out that other time scales, like International Atomic Time (TAI) and Global Positioning System (GPS) time, already exist and don't use leap seconds. Computers could use these and convert to UTC when needed.
What Happens Next?
Finally, a big decision was made! In November 2022, the General Conference on Weights and Measures decided to stop adding leap seconds by or before 2035. This means that after 2035, the difference between atomic time and the Earth's rotation will be allowed to grow. It won't be reset by adding a second.
The ITU World Radiocommunication Conference in 2023 confirmed this decision. They agreed that the maximum difference between UT1 (Earth's rotation time) and UTC (atomic time) will be increased by 2035.
One idea for the future is to let the difference grow until it reaches a full minute. This might take 50 to 100 years. Then, instead of a sudden jump, that minute could be 'smeared' out, meaning the last minute of the day would slowly take two minutes to pass, avoiding any sudden changes. This change is a big step in how we keep time around the world!
Smart Solutions for Timekeeping
Even before the decision to stop leap seconds, many organizations found clever ways to deal with them.
One common solution is for computer systems to use International Atomic Time (TAI) for all their internal operations. TAI is perfectly smooth and never has leap seconds. Then, when they need to show time to people, they convert it to UTC using a table of past leap seconds. This way, their internal systems run smoothly without interruptions.
Big tech companies like Google and Amazon developed their own methods called 'leap smear'. Instead of adding a full second all at once, they would slightly extend each second over a 24-hour period around the leap second. This made the change gradual and less disruptive for their systems.
These solutions show how important it is for technology to have a smooth, predictable time scale. The decision to stop leap seconds by 2035 will make timekeeping much simpler for everyone, especially for our increasingly connected world.
See Also
- Clock drift, phenomenon where a clock gains or loses time compared to another clock
- DUT1, which describes the difference between coordinated universal time (UTC) and universal time (UT1)
- Dynamical time scale
- Leap year, a year containing one extra day or month
