On July 6, 2026, the International Earth Rotation and Reference Systems Service (IERS) published its latest Bulletin C. The announcement was brief: no leap second will be inserted at the end of December 2025. For software engineers and system administrators, this was good news. It means UTC stays continuous through the first half of 2026 without any disruption.
But the broader question remains. Earth's rotation is gradually slowing, and the gap between atomic time and solar time keeps growing. At some point, another leap second will be needed. The question is when, and whether the practice will continue at all.
What is a leap second?
UTC (Coordinated Universal Time) is based on atomic clocks, which measure time using the vibration of cesium atoms. Atomic time is incredibly precise and does not drift. But Earth's rotation is not perfectly constant. The planet slows down slightly over centuries due to tidal friction from the moon, and shorter-term variations come from atmospheric pressure, ocean currents, and core dynamics.
UT1 is the time scale based on Earth's actual rotation. Without correction, UT1 would drift away from UTC. The IERS monitors the difference between the two and inserts a leap second when the gap approaches 0.9 seconds. This keeps UTC within 0.9 seconds of UT1.
A positive leap second adds a 23:59:60 to the last minute of June or December. A negative leap second (which has never been used) would skip 23:59:59. The IERS announces leap seconds about 6 months in advance through Bulletin C.
The history of leap seconds
Leap seconds were introduced in 1972, when UTC was redefined to match atomic time with periodic adjustments for Earth's rotation. Between 1972 and 1999, 22 leap seconds were added, roughly one every 18 months.
Then something unexpected happened. Starting around 2000, Earth's rotation sped up slightly. No leap seconds were needed between 1999 and 2005. The last leap second was added on December 31, 2016. As of July 2026, nearly 10 years have passed without a leap second, the longest gap since the system began.
Some scientists attribute the slowdown in leap second frequency to changes in Earth's core and mantle dynamics. Others point to melting ice caps redistributing water mass and slightly accelerating the planet's rotation. The exact causes are still debated.
Why leap seconds cause problems
For most people, a leap second is invisible. But for computer systems, it is a serious headache. Unix timestamps assume every day has exactly 86,400 seconds. A leap second creates a 61-second minute, which breaks that assumption.
- In 2012, a leap second caused outages at Reddit, Mozilla, LinkedIn, and other major sites when Java virtual machines and Linux kernels struggled with the extra second.
- In 2015 and 2016, cloud providers pre-emptively patched systems, but smaller services still reported issues with NTP synchronization and database timestamps.
- Google developed a 'smeared leap second' technique that stretches the extra second across a 24-hour period, avoiding the discontinuity. Other companies have adopted similar approaches.
The core problem is that there is no standard way to represent a leap second in most programming languages. JavaScript's Date object, for example, does not support 23:59:60. Python's datetime module ignores leap seconds entirely. This means every system handles them differently, which leads to bugs.
The debate: should we abolish leap seconds?
The International Telecommunication Union (ITU-R) has been debating whether to abolish leap seconds since 2005. The argument for abolition is straightforward: computer systems struggle with leap seconds, the disruptions are costly, and the 0.9-second tolerance is not meaningful for most applications.
The argument against abolition is that decoupling UTC from Earth's rotation would eventually make clock noon diverge from solar noon. After a few hundred years, noon on the clock might happen at 1:00 PM solar time. Astronomers and navigators rely on UTC staying close to UT1.
In 2022, the General Conference on Weights and Measures (CGPM) passed a resolution to allow UTC to drift from UT1 by more than 0.9 seconds, with a target implementation date of 2035. This effectively means leap seconds would be replaced by a larger, less frequent adjustment, perhaps a leap minute every 50 to 100 years.
What happens next
The 2035 target is not a hard deadline. The ITU-R and CGPM still need to agree on the specific mechanism for replacing leap seconds. Several proposals are on the table, including a one-time adjustment of 60 seconds and a gradual smear over decades.
In the meantime, the IERS continues to monitor Earth's rotation and announce leap seconds as needed. The next possible insertion date is June 30 or December 31, 2026. Based on current Earth rotation data, a leap second in 2026 is possible but not confirmed. The IERS typically announces decisions 6 months in advance.
If you work with time-sensitive systems, the best practice is to use smear techniques (like Google's) or to rely on a time service that handles leap seconds for you. Our Unix Timestamp Converter can help you understand how timestamps work, but it does not account for leap seconds, since JavaScript's Date object does not support them.