ploring and implementing workarounds like ‘time smearing’ to manage the transition smoothly.
A leap second is essentially an extra second inserted into Coordinated Universal Time (UTC) to keep it closely aligned with UT1, which is the astronomical measure of Earth’s rotation. This adjustment ensures that the difference between the two never exceeds 0.9 seconds.
Last updated: July 22, 2026
The need arises because UTC is based on International Atomic Time (TAI), a super-stable timescale derived from a network of highly precise atomic clocks. TAI doesn’t account for the subtle, irregular slowdowns and speedups of our planet’s spin. UT1, on the other hand, is directly tied to Earth’s actual rotation, which isn’t perfectly constant.
Practically speaking, when a leap second is added, the minute containing it has 61 seconds instead of the usual 60. This typically happens at the end of June 30th or December 31st, and is announced months in advance by the International Earth Rotation and Reference Systems Service (IERS). Since 1972, twenty-seven leap seconds have been added, the most recent being in 2017.
The Earth’s rotation isn’t as perfectly steady as we might assume. While it broadly slows down over millennia due to tidal forces from the Moon, its day-to-day speed can fluctuate unpredictably. A complex interplay of influencs these variations factors, including atmospheric and oceanic currents, seismic activity, and even the melting and freezing of polar ice caps.
For instance, changes in ocean currents, like El Niño events, can subtly alter the distribution of mass on the planet, impacting its rotational speed. Similarly, major earthquakes can cause minor but measurable shifts. These natural phenomena mean that while atomic clocks tick with unwavering precision, the Earth’s ‘clock’ occasionally drifts.
The IERS meticulously monitors these rotational changes, publishing bulletins that inform global timekeepers when a leap second might be necessary. This system has allowed us to maintain a consistent civil time that doesn’t drift too far from the sun’s position in the sky, ensuring that noon remains roughly when the sun is highest.
While conceptually simple, adding a leap second has proven to be a surprisingly complex and costly effort for modern computer systems. Most software is designed assuming every minute has 60 seconds. Introducing a 61-second minute can throw critical systems out of sync, leading to outages, data corruption, and even safety hazards.
A notable incident occurred in 2012 when a leap second caused widespread outages on major websites like Reddit, LinkedIn, and Yelp, as well as critical infrastructure systems. Servers running Linux and Java applications were particularly affected, struggling to cope with the unexpected time adjustment. In 2015, another leap second caused problems for many systems, including parts of the Qantas airline check-in system.
For industries like finance, where microseconds can mean millions of dollars in trades, and for global navigation satellite systems (GNSS) like GPS, where precise timing is paramount, the unpredictable nature and implementation challenges of leap seconds are a constant source of anxiety. The need for precise synchronization across distributed systems makes these seemingly minor adjustments a major operational risk.
The global community has decided to put an end to these timekeeping headaches. In November 2022, the International Telecommunication Union (ITU), which oversees global time standards, formally adopted a resolution to suspend the insertion of leap seconds from 2035. This means that after the last potential insertion in December 2034, no further leap seconds will be added, at least for a century.
This decision, supported by many nations and scientific bodies like the U.S. National Institute of Standards and Technology (NIST), acknowledges that the benefits of maintaining strict alignment with UT1 no longer outweigh the operational risks. The primary goal is to simplify global timekeeping and eliminate a significant source of system instability for critical infrastructure, from telecommunications to financial markets.
What this means in practice is that after 2035, UTC will gradually drift further from UT1. While our daily lives won’t notice a one-second difference, over hundreds of years, the discrepancy could become noticeable, potentially requiring a larger, less frequent adjustment in the distant future. For now, the focus is on a smooth transition away from the current system.
Even before the ITU’s 2022 decision, many tech giants and critical infrastructure providers had already developed and implemented their own solutions to mitigate leap second risks. The most widely adopted method is known as ‘time smearing’ or ‘leap smearing’.
Instead of adding a single, disruptive second, time smearing involves gradually adjusting the clock over a longer period, typically 24 hours. For example, Google’s approach, implemented years ago, involves slightly slowing down their NTP (Network Time Protocol) servers’ clocks over the 24 hours preceding a scheduled leap second. This distributes the one-second adjustment over 86,400 regular seconds, making each second infinitesimally longer, a change imperceptible to human users and most software. Amazon Web Services (AWS) adopted a similar approach, known as ‘leap smear’, allowing their cloud infrastructure to handle the adjustment without abrupt changes.
This proactive strategy has proven highly effective in preventing outages and ensuring continuous operation during leap second events. As of July 2026, many organizations are reviewing their time synchronization protocols and considering implementing such smearing techniques to prepare for the 2035 retirement, even if it’s no longer for an active leap second insertion, but rather to ensure their systems are resilient to any future time anomalies.
for more on network time protocol configuration.
With leap seconds paused indefinitely from 2035, the world will operate with a more stable, atomic-based UTC. However, the Earth’s rotation will continue its unpredictable dance. This means that over centuries, the difference between UTC and UT1 will grow, potentially to several minutes.
Scientists and timekeeping experts are already discussing future strategies. One proposal suggests introducing a much larger ‘leap minute’ or ‘leap hour’ far less frequently—perhaps only once every few centuries—when the accumulated difference becomes significant. This would offer the stability of an atomic timescale for daily operations while still acknowledging the astronomical reality of our planet’s rotation.
Another area of focus is on improving the precision of UT1 measurements and predictions, allowing for even more solid planning for any distant future adjustments. The goal is a truly global, unified time standard that minimizes disruption while remaining scientifically sound. The 2035 decision is a pragmatic step towards a more resilient global time infrastructure, setting the stage for these longer-term considerations.
It’s easy to confuse leap seconds with other time-related adjustments. One common misconception is equating them with leap years. A leap year, like 2024, adds an extra day (February 29th) to the calendar to keep it synchronized with the Earth’s orbit around the sun. Leap seconds, however, deal with the Earth’s rotation on its axis, not its orbital period.
Another misunderstanding is that leap seconds are always negative, meaning time is skipped. In reality, all 27 leap seconds added since 1972 have been positive, meaning an extra second was added. While theoretically a negative leap second (skipping a second) is possible if the Earth were to significantly speed up, it has never occurred. The Earth’s overall trend is slowing, making positive adjustments more likely.
Finally, some believe the retirement means Earth’s rotation will no longer be monitored. This is incorrect. The IERS will continue to track Earth’s rotation to high precision, as this data is crucial for satellite navigation, geophysics, and other scientific endeavors, even if civil time no longer directly reflects those variations.
Even with the upcoming retirement of leap seconds, maintaining accurate time synchronization remains crucial for individuals and organizations. For most personal devices, operating systems automatically handle time adjustments using widely trusted NTP servers, often implementing smearing techniques themselves. Ensure your devices are set to synchronize time automatically.
For businesses, especially those in finance, data centers, or critical infrastructure, a solid time synchronization strategy is non-negotiable. As of July 2026, it’s vital to:
This proactive approach helps ensure smooth operation, regardless of the subtle shifts in global timekeeping. What Is Zulu Time in 2026 and Why Pilots Swear By It for Safety to dig deeper into what is Zulu Time and its importance in precise synchronization.
The International Telecommunication Union (ITU) has decided to suspend leap second insertions from 2035. This means that any potential leap seconds could be added up to December 31, 2034, but none will be added thereafter for at least a century, effectively retiring the practice.
The primary reason for the Earth’s long-term slowdown is tidal friction caused by the Moon. The Moon’s gravitational pull creates bulges in Earth’s oceans, and the friction generated as these bulges move across the ocean floor acts as a brake on the planet’s rotation.
GPS systems rely on highly precise time synchronization. Currently, GPS time operates without leap seconds, maintaining a constant offset from TAI. The retirement of leap seconds from UTC will simplify the relationship between UTC and GPS time, making synchronization less complex in the long run.
No, the average person won’t notice the absence of leap seconds. The one-second adjustments are too small and infrequent to impact daily activities. The changes are primarily relevant for highly precise scientific, financial, and technological systems that demand absolute time accuracy.
The IERS is the international body responsible for maintaining global time standards and Earth reference frames. It monitors Earth’s rotation, defines UT1, and previously issued official announcements for when a leap second was to be inserted into UTC.
Theoretically, yes. If the Earth’s rotation were to speed up significantly, it might necessitate a negative leap second, where a second is removed from the clock. However, this has never occurred, and the long-term trend of Earth’s rotation is slowing down.
Information current as of July 2026.
Source: Britannica.
