A leap second is a one-second adjustment added to Coordinated Universal Time (UTC) to keep clocks aligned with Earth's gradually slowing rotation. Atomic clocks measure time with extreme precision, but since Earth's rotation speed varies slightly, solar time can drift away from atomic time. Adding leap seconds ensures that our clocks stay in sync with the natural day-night cycle, which is essential for activities like navigation, astronomy, and everyday timekeeping.
What exactly is a leap second and why do we add it?
A leap second is an extra second inserted into the timekeeping system to correct the difference between atomic time and Earth's rotation. Atomic clocks generate International Atomic Time (TAI) based on consistent atomic vibrations, but Earth's rotation isn’t perfectly steady — it slows down and speeds up due to tidal forces and geological changes. This causes solar time, which tracks the Sun’s position, to drift compared to atomic time.
To keep civil time (UTC) synchronized with solar time, leap seconds are added as needed. Without these corrections, clock noon could eventually occur far from when the Sun is highest in the sky. Typically added at the end of June or December, leap seconds keep UTC within 0.9 seconds of mean solar time, preserving the link between clock time and the natural day-night cycle.
How do experts decide when to insert a leap second?
The International Earth Rotation and Reference Systems Service (IERS) monitors Earth's rotation and decides when to add leap seconds. They track the difference between atomic time and Universal Time (UT1), which reflects Earth’s actual rotation.
When this difference nears 0.6 seconds and is expected to exceed 0.9 seconds, the IERS schedules a leap second to keep the gap under that limit. These announcements are made several months in advance to give timekeeping organizations and systems time to prepare.
Leap seconds usually occur on June 30 or December 31 at 23:59:60 UTC, meaning the last second of the day is repeated. Because Earth's rotation speed varies unpredictably, leap seconds are added irregularly rather than on a fixed schedule.
What happens when a leap second is added—how does it affect clocks and digital systems?
When a leap second is added, clocks that display seconds show 23:59:59, then 23:59:60, and then roll over to 00:00:00 of the next day. This extra second keeps clock time aligned with Earth's rotation.
Most mechanical and simple digital clocks handle this smoothly. However, some computer systems and networks can have trouble because they expect every minute to have exactly 60 seconds. A leap second creates a 61-second minute, which can cause software to crash or behave unpredictably if it isn’t designed to handle it.
In the past, leap seconds have caused slowdowns or crashes in large-scale services like social media platforms and financial networks because their software relies on continuous, monotonic time and may not anticipate repeated timestamps.
To avoid these problems, some systems apply a "leap second smear," spreading the extra second gradually over a longer period by slightly slowing their clocks instead of adding a discrete second.
Why do leap seconds sometimes cause problems, and what are common myths about them?
Leap seconds cause problems mainly because they introduce an irregular event that many software systems don't expect. Most programming assumes each minute has 60 seconds. When a minute has 61 seconds, systems can misinterpret timestamps, leading to errors or crashes.
A common myth is that leap seconds occur frequently or predictably. Actually, they are added only when Earth's rotation slows enough to require it, so they happen irregularly, sometimes years apart. Another misconception is that leap seconds are obsolete or unnecessary. Although some argue for their removal to simplify timekeeping, leap seconds still play a key role in keeping civil time aligned with solar time.
Some also believe leap seconds are just a technological nuisance without practical benefit. In reality, maintaining the link between clock time and the natural day matters for astronomy, navigation, and legal definitions of time.
Handling leap seconds requires careful software and infrastructure design to prevent disruptions despite their small size.
How do different countries and tech systems handle leap seconds?
Most countries use UTC with leap seconds added as announced by the IERS, keeping civil time aligned with Earth's rotation. However, how systems handle leap seconds varies.
Some technology platforms, including major internet companies, ignore leap seconds or use a smearing technique that spreads the extra second over a longer interval. This approach reduces software crashes by avoiding sudden time jumps.
Other systems use time protocols like the Network Time Protocol (NTP) that explicitly support leap seconds by broadcasting announcements so devices can adjust properly.
There is ongoing international debate about whether to keep leap seconds or switch to a continuous atomic timescale without them. Removing leap seconds would simplify technology but cause clocks to drift gradually from solar time by minutes or hours over decades.
For now, leap seconds remain part of global timekeeping, but implementation varies by system, country, or technology.
Where can I find official information and updates about upcoming leap seconds?
The International Earth Rotation and Reference Systems Service (IERS) is the main source for official leap second announcements. They publish bulletins on Earth’s rotation and leap second scheduling on their website.
Other timekeeping organizations like the U.S. Naval Observatory and the International Bureau of Weights and Measures (BIPM) also provide leap second information.
Many software and hardware systems rely on Network Time Protocol (NTP) to receive leap second alerts automatically.
If you manage systems sensitive to time, monitor these official sources regularly and ensure your devices or software handle leap seconds properly—either natively or through methods like smearing or pausing updates during the leap second.
Conclusion
Leap seconds explain why our clocks sometimes include an extra second and why that matters beyond just marking time. Whether you work in technology, study astronomy, or are simply curious, understanding leap seconds shows how precise atomic time and Earth’s natural rotation are balanced. Staying informed and preparing systems for leap seconds helps minimize disruptions while keeping clock time aligned with the solar day.
Frequently Asked Questions
How often are leap seconds added?
Leap seconds are added irregularly, only when Earth's rotation slows enough to cause a significant difference from atomic time. They don't follow a fixed schedule and can be several years apart.
Can leap seconds cause my computer to crash?
If software isn’t designed to handle a minute with 61 seconds, a leap second can cause errors or crashes. Many modern systems manage leap seconds safely, but older or poorly designed software may still be vulnerable.
Why don't we just stop adding leap seconds?
Stopping leap seconds would simplify timekeeping but cause clocks to drift away from the solar day over time. After decades, noon on the clock could no longer match when the Sun is highest, which would affect navigation, astronomy, and cultural practices tied to the day-night cycle.