The leap second: scheduled end in 2035

Scientific summary

Astronoo's article explores the leap second, an adjustment of Universal Time intended to compensate for the irregularities of Earth's rotation compared to atomic time. It describes the measurements of Universal Time (UT) and International Atomic Time (TAI), the causes of drifts (tides, earthquakes, movements of the inner core) and the role of the IERS. The article reviews the 27 seconds added since 1972 and discusses the implications of this practice for computer systems, as well as the decision to abolish the leap second from 2035.

Why is a second added to our clocks from time to time?

This article is devoted to the leap second, a subtle mechanism that synchronizes ultra-precise atomic time with the irregular rotation of the Earth. While cesium atomic clocks have defined an invariable second since 1967, Earth's rotation slows down and speeds up due to tides, movements of the inner core and climatic variations. This difference, although microscopic, accumulates and reaches several tens of seconds per century.

To maintain coherence between Universal Time (UT1) and Atomic Time (TAI), the International Earth Rotation and Reference Systems Service (IERS) decides to add or subtract a leap second when the difference exceeds 0.9 seconds. Since 1972, only additions have been made, totaling 27 seconds to date.

But this seemingly harmless correction disrupts computer systems and telecommunication networks. Therefore, in 2022, the General Conference on Weights and Measures decided to abolish the leap second by 2035, to allow smoother management of global time. The article traces the history, scientific and technical challenges of this fleeting second.

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Add or subtract a second?

Universal Time, a measure of Earth's rotation

Since 1972, Universal Time (UT) is the time synchronized with Earth's rotation. It is measured by precisely observing the passage of distant quasars at the meridian of the observation site with some corrections. With this method, astronomers obtain a precision of the order of a microsecond (10^-6 second). But Earth's rotation on its axis is not regular.

International Atomic Time, an invariable reference

International Atomic Time (TAI) is the time scale based on the definition of the second and is calculated from 349 cesium atomic clocks distributed around the world. This reference, used everywhere in real time, has an accuracy of 10^-10 second with cesium 133 (Cs133). This isochronous time for scientific use is called Terrestrial Time (TT).

The causes of drifts between the two time scales

Given the irregularity of Earth's rotation, Universal Time gradually drifts relative to atomic time. This drift is significant, on the order of 35 seconds since 1972. These cumulative micro-drifts are due to tidal effects of the Moon and Sun, seasonal variations of ice caps, earthquakes, tsunamis and various movements of Earth's inner core.

Adjustment by leap second

The drifts between the two clocks, UT and TAI, must be corrected, which is why since 1972, 27 seconds have been added to Universal Time. These seconds are called leap seconds or addition seconds.

Since the origin of TAI measurement, the International Earth Rotation and Reference Systems Service (IERS) located at the Paris Observatory has only added leap seconds. This leap second is triggered when the offset between the clocks reaches about 0.5 seconds.

Announcement and dissemination of the leap second

The announcement is made in advance via an information bulletin, the Bulletin C published every 6 months. The dissemination is coordinated by the International Telecommunication Union.

N.B. :
In 2015, in France, this leap second occurred with summer time on the morning of July 1. Clocks displayed 1h 59min 59s, then 1h 59min 60s, then 2h 00min 00s. The addition of this second, correlated with variations in our planet's rotation, is decided globally at the Paris Observatory within the SYRTE laboratory (Reference Systems Time-Space).

Alternatives to the leap second after 2035

From 2035, the leap second will be abandoned. Several options are being considered to manage the growing gap between UTC and UT1:

Let the gap grow

UTC will no longer be adjusted, and the gap with UT1 could reach 1 second around 2035-2040, then several seconds.
Advantage: Simplicity for computer systems.
Disadvantage: In the long term (in about 100 years), the gap could exceed 1 minute, causing problems for astronomical observations.

Adjust UTC by larger jumps

Instead of adding 1 second, one could add 1 minute every 50-100 years.
Advantage: Fewer disruptions for computing.
Disadvantage: A 1-minute jump would be even harder to handle than 1 second.

Use a hybrid time scale

Some propose creating a new time scale, combining the stability of TAI and Earth's rotation, but without leap seconds.
Example: Terrestrial Time (TT) is already used in astronomy, but it is not synchronized with Earth's rotation.

Impact on modern technologies

The leap second poses major challenges for:

Computer systems

Databases: Some do not handle second 60 (e.g., MySQL before 2017).
Time protocols: NTP (Network Time Protocol) must be updated to broadcast second 60.
Web applications: In 2012, sites like Reddit, LinkedIn, and Cloudflare suffered outages.
Embedded systems: GPS, satellites, and power grids must synchronize their time with extreme precision.

Critical infrastructure

Power grids: Desynchronization could cause cascading failures.
Financial markets: Stock transactions depend on ultra-precise time (e.g., Nasdaq uses atomic clocks).
Telecommunications: 5G/6G networks require microsecond-level synchronization.

Current technical solutions

To circumvent the problem, some companies use:

Leap smear: Google spreads the leap second over 24 hours (e.g., +1 ms every 864 seconds).
Ignoring the second: Amazon and Microsoft synchronize their servers on TAI, without leap seconds.
Hybrid clocks: Combining GPS and local atomic clocks.

Table of leap seconds

History of additions since 1972

In 1958, the two clocks, Universal Time (UT1) and International Atomic Time (TAI) were declared synchronous. In 1972, 10 seconds separated them. This initial offset was kept, but since then, 27 leap seconds have been added to Universal Time to correct its drifts.

Between 2016 and 2026, the Earth has rotated steadily without any sudden changes, or more precisely, drifts in one direction have compensated for drifts in the other, and the offset has remained constant.

History of leap seconds (1972-2026)
Year	June 30	Dec 31	Year	June 30	Dec 31
1972	+1	+1	2000	0	0
1973	0	+1	2001	0	0
1974	0	0	2002	0	0
1975	+1	0	2003	0	0
1976	+1	0	2004	0	0
1977	+1	0	2005	0	+1
1978	0	+1	2006	0	0
1979	0	+1	2007	0	0
1980	0	0	2008	0	+1
1981	+1	0	2009	0	0
1982	+1	0	2010	0	0
1983	+1	0	2011	0	0
1984	0	0	2012	+1	0
1985	+1	0	2013	0	0
1986	0	0	2014	0	0
1987	0	+1	2015	+1	0
1988	0	0	2016	0	+1
1989	0	+1	2017	0	0
1990	0	+1	2018	0	0
1991	0	0	2019	0	0
1992	+1	0	2020	0	0
1993	+1	0	2021	0	0
1994	+1	0	2022	0	0
1995	0	+1	2023	0	0
1996	0	0	2024	0	0
1997	+1	0	2025	0	0
1998	0	+1	2026	0	0
1999	0	0

References

FAQ: Everything you need to know about the leap second

What is a leap second?

A leap second is an extra second added (or subtracted) to Coordinated Universal Time (UTC) to compensate for the offset between atomic time, which is perfectly regular, and Earth's rotation, which is irregular. It keeps the difference between UTC and UT1 below 0.9 seconds.

Why is Earth's rotation not regular?

Earth's rotation undergoes short- and long-term variations: tidal effects from the Moon and Sun gradually slow it down, while sudden events such as earthquakes or movements of the inner core can speed it up or slow it down. Seasonal variations of ice caps also affect rotation speed.

How is a leap second decided and announced?

The International Earth Rotation and Reference Systems Service (IERS) continuously monitors the offset between UT1 and TAI. When this offset exceeds 0.9 seconds, the IERS publishes a Bulletin C to announce the addition or removal of a leap second, usually scheduled for June 30 or December 31. The decision is made in consultation with the International Telecommunication Union.

How many leap seconds have been added since 1972?

Since the system was introduced in 1972, 27 leap seconds have been added (none have been removed). The last one occurred on December 31, 2016. Since 2017, the offset between UT1 and TAI has remained below the threshold, and no new leap second has been scheduled.

What problems are caused by the leap second?

Adding or removing a leap second disrupts computer systems and telecommunications networks that rely on continuous time. Protocols like NTP (Network Time Protocol) and databases must handle this anomaly, which can cause bugs, time shifts, and service interruptions, as happened in 2012 for some websites.

Why abolish it in 2035?

In 2022, the General Conference on Weights and Measures (CGPM) decided to abolish the leap second from 2035. The aim is to simplify global time management for digital technologies that suffer from this irregularity. From 2035, the difference between UTC and UT1 may reach a full second, then several seconds, without systematic correction.

How do companies handle the leap second today?

Several strategies are used:

Leap smear (Google): Spread the leap second over 24 hours.
Ignoring the second (Amazon, Microsoft): Synchronize servers on TAI.
Hybrid clocks: Combine GPS and local atomic clocks.