Why Timekeepers Want to Eliminate Leap Seconds

A leap second lasts no longer than any other second, but its consequences can reach satellite networks, financial systems, cloud platforms, telecommunications infrastructure, and scientific instruments.

Since leap seconds were introduced in 1972, 27 have been added to Coordinated Universal Time (UTC). Each adjustment kept atomic clocks broadly aligned with Earth’s irregular rotation. International timekeeping bodies now support changing the system so that leap seconds will not be added indefinitely.

The debate concerns two kinds of precision. Earth scientists and astronomers want civil time to remain close to the planet’s rotation. Computer systems need a continuous, predictable timescale that does not suddenly contain an unusual timestamp such as 23:59:60.

What Is a Leap Second?

UTC Combines Atomic Time and Earth Rotation

UTC is the international reference time used for civil timekeeping. National timing laboratories distribute it, and computers, telecommunications networks, navigation systems, financial institutions, and public services rely on it.

UTC is based on atomic time, which comes from highly stable atomic clocks. These clocks measure a defined atomic frequency and provide a far more uniform rate than Earth’s rotation.

Earth does not rotate at a perfectly constant speed. UTC therefore combines two goals:

• Atomic clocks provide a stable measure of elapsed time.
• Earth-rotation measurements keep civil time close to astronomical time.

The International Bureau of Weights and Measures (BIPM) calculates UTC from data supplied by timing laboratories worldwide. The International Earth Rotation and Reference Systems Service (IERS) monitors Earth’s rotation and announces when a leap second is needed. Source 1

How the Adjustment Works

A positive leap second traditionally appears at the end of June or December:

23:59:59
23:59:60
00:00:00

The label 23:59:60 represents an additional second. It does not change the length of an ordinary second; it inserts one extra step into UTC so that it does not move too far from Earth-rotation time.

Leap seconds are not added on a fixed schedule. The IERS examines Earth-orientation measurements and announces an adjustment when necessary. The last leap second occurred at the end of December 2016. No negative leap second has been used, although the rules allow one in principle. Source 2

Leap Seconds Are Not Leap Years

A leap year corrects the calendar’s relationship with Earth’s orbit around the Sun. A leap second addresses a different problem: keeping clock time close to Earth’s rotation as that rotation varies.

Why Have There Been 27 Leap Seconds?

Earth’s rotation changes slightly because of tidal friction, movements in the atmosphere and oceans, changes in ice and groundwater, interactions between the mantle and core, earthquakes, and other geophysical events.

These changes are too small to affect ordinary schedules, but they matter to precision timekeeping. Atomic clocks maintain an exceptionally stable rate while Earth’s rotational rate fluctuates.

The 27 leap seconds added between 1972 and 2016 were not signs of a sudden planetary slowdown. Each represented the gradual accumulation of a small difference between atomic time and Earth-rotation time.

Without leap seconds, atomic time would continue smoothly while Earth-rotation time followed the planet’s changing motion. UTC would gradually move away from the position of the Sun in the sky. The divergence would be slow, but it would create a growing offset that future users would need to calculate.

Why Do Timekeepers Want to Eliminate Leap Seconds?

Leap Seconds Are Difficult for Computer Systems

Many software systems assume that time follows a regular sequence in which every minute contains 60 seconds. A leap second challenges those assumptions. Problems can include:

• Software rejecting 23:59:60.
• Two events receiving the same timestamp.
• A clock repeating a second.
• Logs disagreeing about event order.
• Scheduled jobs running twice or not at all.
• Databases and monitoring tools handling the event inconsistently.

The risk comes less from the one-second adjustment than from the number of interconnected systems that must interpret it consistently.

Systems can insert the second directly, repeat the previous timestamp, spread the correction over a longer period, or ignore the event and correct the clock afterward. Each method has different consequences.

Distributed Networks Need Consistent Timestamps

Distributed systems use timestamps to order events, reconcile data, investigate incidents, validate security records, and coordinate operations. A leap second can complicate financial records, cloud logs, replicated databases, security trails, billing systems, telecommunications, industrial monitoring, and software deployment.

If one server inserts the leap second, another smears it, and a third ignores it, their clocks can temporarily disagree even when all three use valid time sources.

Some Platforms Use Leap Smearing

Leap smearing distributes the adjustment over a longer period instead of creating a single exceptional second. A service gradually slows or speeds its clock so that the total correction equals one second.

This avoids 23:59:60, but different organizations may smear the adjustment over different periods or use different rates. Their clocks can then disagree temporarily.

What Would Eliminating Leap Seconds Mean?

Eliminating leap seconds would not end Earth-orientation measurements. The IERS and scientific institutions would continue monitoring the planet’s rotation, and astronomical time would continue to exist.

The proposed change concerns UTC. Instead of periodically adding a second to keep UTC close to Earth rotation, international timekeeping would allow the difference to grow.

In 2022, the General Conference on Weights and Measures adopted a resolution supporting changes to UTC so that the maximum difference between UTC and Earth-rotation time could increase by or before 2035. The resolution called for further technical work and international agreement; it did not establish an immediate worldwide clock change. Source 3

The change would not alter the definition of the second, make seconds longer or shorter, cause public clocks to jump by minutes, immediately change time zones, or end astronomical measurement. It would change how civil time relates to Earth’s rotation.

Arguments for Keeping Leap Seconds

Leap seconds preserve a close link between clocks and the sky. That relationship remains useful for astronomy, geodesy, navigation, and Earth science. It also preserves the traditional idea that midday should remain near the time when the Sun is highest.

Supporters argue that civil time should not become completely detached from the natural cycle that originally defined the day. Removing leap seconds would not make astronomy impossible, but it would require scientists to manage a larger and changing difference between UTC and Earth-rotation time.

Arguments for Eliminating Leap Seconds

Digital infrastructure needs predictable timestamps. A simpler UTC could remove an exceptional event that requires special testing, updates, monitoring, and incident procedures.

Eliminating leap seconds could reduce emergency patches, vendor-specific workarounds, conflicting smear policies, timestamp ambiguity, testing requirements, and operational confusion. It would not eliminate synchronization problems caused by network delays, faulty clocks, incorrect configurations, or inconsistent time sources.

Modern systems also distinguish between wall-clock time and monotonic time. Wall-clock time represents civil time and can be adjusted; monotonic time measures elapsed duration and should move forward consistently. A future UTC without leap seconds would simplify wall-clock handling while leaving monotonic clocks available for timeouts, performance measurements, and durations.

Who Would Notice the Change?

Most people would notice no immediate difference. Phones, computers, watches, and public clocks would continue displaying familiar civil time.

Technology companies and network operators could face fewer leap-second incidents but would still need to support historical data, legacy systems, and updated standards. Astronomers and Earth scientists would continue using Earth-rotation observations and specialized time scales, while managing a larger offset from UTC.

Navigation and satellite systems would require clear conversion rules and consistent standards. Financial markets, power grids, manufacturing systems, logistics networks, and compliance platforms would benefit from fewer exceptional events, not faster transactions.

What Happens Next?

International organizations must turn the policy decision into technical rules covering:

• The definition and dissemination of UTC.
• Conversion between UTC and Earth-rotation time.
• Time signals from national laboratories.
• Satellite and telecommunications systems.
• Software timestamp formats.
• Historical data and archival records.
• Scientific and navigation applications.

System owners should audit leap-second handling, test timestamps containing 23:59:60, document whether systems insert, repeat, smear, or ignore leap seconds, separate wall-clock time from monotonic time, check dependencies on operating systems and time services, and monitor announcements from timing authorities and infrastructure vendors.

Conclusion

Leap seconds have kept UTC close to Earth’s irregular rotation 27 times since 1972. They preserve an important connection between atomic clocks and the astronomical day, but they also create complexity for computer systems built around continuous, predictable time.

Eliminating leap seconds would not remove the difference between atomic time and Earth’s rotation. It would change how that difference is managed. UTC would become easier for machines to handle, while astronomers and Earth scientists would manage a larger offset from rotational time.

The central choice is whether civil time should prioritize precision relative to the sky or precision for interconnected machines. The future of global timekeeping depends on deciding how those two forms of precision should coexist.

FAQ

What is a leap second?

A leap second is a one-second adjustment to UTC that keeps atomic time broadly aligned with Earth’s irregular rotation.

Why have there been 27 leap seconds?

Earth’s rotation varies because of tidal forces, atmospheric and oceanic movements, changes inside Earth, earthquakes, and other geophysical effects.

Why are leap seconds difficult for computers?

Many systems expect timestamps to follow a regular sequence in which every minute contains 60 seconds. The value 23:59:60 can break software that does not support it.

Will eliminating leap seconds change the length of a second?

No. The definition of the second would remain unchanged. The change would affect how UTC is aligned with Earth’s rotation.

Will ordinary people notice?

Most people would not notice an immediate change. The main effects would concern infrastructure operators, scientific institutions, navigation systems, and organizations requiring precise synchronization.

When could leap seconds be eliminated?

International discussions aim to develop a new UTC framework by or before 2035. The exact implementation depends on further agreements, technical standards, and decisions by international and national timing authorities.