At midnight on January 1, 2017, internal code at Cloudflare encountered an impossible paradox: the difference between two timestamps turned negative. The software assumed time would always move forward, yet the inclusion of a leap second at the end of 2016 caused the clock to read 23:59:60. Within minutes, DNS servers began to crash, causing 0.2 percent of queries to fail during peak hours. This single, extra second, added to keep human timekeeping in sync with our planet's erratic rotation, highlighted a growing vulnerability in modern infrastructure.

This extra second was the last to be added to date. While the General Conference on Weights and Measures (CGPM) initially resolved in 2022 to end the practice by 2035, the 28th conference, meeting in Versailles from October 13 to 15, is now considering a proposal to move that deadline up to May 20, 2027.

What this article covers:
  • The reasons behind the 27 leap seconds added over 54 years and the impact of the final addition.
  • How a single leap second in 2012 disrupted more than 400 flights in Australia.
  • Why the recent acceleration of Earth's rotation poses a new risk and how melting glaciers are delaying that impact.
  • What the proposed changes mean for the alignment between atomic clocks and solar time.

How do atomic clocks compare with the Earth's rotation?

Our daily clocks struggle to reconcile two different measures of time. The first is International Atomic Time (TAI), an average of atomic clocks worldwide that counts the vibrations of cesium atoms with near-perfect consistency. The second is Universal Time (UT1), which is based on the Earth's rotation. Because tidal forces, ocean currents, and core movements cause the Earth's rotation to speed up or slow down, the planet rarely completes a full rotation in exactly 24 hours. Coordinated Universal Time (UTC) follows the speed of atomic clocks but is kept within 0.9 seconds of UT1. When the gap approaches this threshold, the International Earth Rotation and Reference Systems Service (IERS) announces the addition of a leap second at the end of June or December.

This system began in 1972 with an initial difference of 10 seconds. Between then and the end of 2016, 27 leap seconds were added, bringing the total gap to 37 seconds. The IERS bulletin from July 6, 2026, confirmed that no additional second will be added at the end of this year.

What is the cost of adding a single second?

Even with months of advance warning, software systems have repeatedly failed to handle leap seconds gracefully. On June 30, 2012, a leap second exposed the absence of a specific instruction in the Linux kernel, causing high-resolution timers to lock up as processors entered an infinite loop of resetting. The outage crashed servers for the reservation system Amadeus Altea, forcing airlines like Qantas and Virgin Australia to switch to manual check-ins and delaying over 400 flights. The same error impacted major platforms including Reddit, Mozilla, and LinkedIn.

To mitigate these risks, large technology companies have begun "smearing" the leap second over a longer period rather than inserting it instantaneously. Engineering teams at Meta reported that they distributed the 2022 leap second across a 17-hour window. Other companies start the process a day early or hours in advance, which creates inconsistencies across different systems. The draft resolution before the CGPM notes that these disparate methods cause significant discrepancies and prevent systems from adhering to a uniform SI second.

What happens when the Earth accelerates?

Since 1972, every leap second has been positive, as the Earth's rotation was slower than atomic time. Recently, however, days have become shorter. According to the July 2026 report by the Consultative Committee for Time and Frequency (CCTF), the Earth's rotation has now reached a duration equal to an atomic day. If this acceleration continues, we may face the first-ever need for a negative leap second, where clocks jump from 23:59:58 directly to 00:00:00.

Geophysicist Duncan Agnew of the Scripps Institution of Oceanography, in a March 2024 study in Nature, modeled this shift. While satellite gravity measurements show that melting ice in Greenland and Antarctica is slowing the Earth down by shifting mass toward the equator, Agnew found that the Earth's liquid core has been decelerating since 1972, which effectively accelerates the planet's rotation. Without the melting ice, a negative leap second would have been required three years earlier. According to a March 2025 workshop held by the BIPM and IERS, there is a 30 percent chance of needing a negative leap second in the next decade and a 50 percent chance of needing a positive one. The draft resolution warns that preparing critical infrastructure for a negative second would require investments similar to those made for the Y2K bug.

Why move the implementation date to May 20, 2027?

The CCTF evaluated several options, including allowing a one-minute or one-hour tolerance between UTC and UT1. The proposal recommends a one-hour threshold, or 3,600 seconds, because many countries already maintain a gap between solar time and official time due to time zones and daylight saving measures. The acceleration of the timeline from 2035 to 2027 is driven specifically by the risk of a negative leap second. Groups including the International Telecommunication Union and the IEEE 1588 working group have requested the earlier date to prevent institutions from abandoning UTC entirely in favor of satellite-based time systems like GPS.

Astronomers and satellite ground stations needing UT1 data will henceforth retrieve it from satellite navigation messages and IERS web services. Per a 2023 ITU decision, five countries—Argentina, Canada, Russia, the United Kingdom, and the United States—have been granted until 2040 to phase out radio broadcasts that include the leap second. If the draft resolution is adopted, the difference between atomic time and UTC will be frozen at 37 seconds, honoring the Metre Convention signed on May 20, 1875.

Notes and Sources

  1. John Graham-Cumming, Cloudflare Blog (2017). "How and why the leap second affected Cloudflare DNS".
  2. CGPM (2022). Resolution 4, "On the use and future development of UTC".
  3. IERS Earth Orientation Center, Paris Observatory (2026). Bulletin C 72.
  4. Jonathan Corbet, LWN.net (2012). "Leaping seconds and looping servers".
  5. Charles Arthur, The Guardian (2012). "Leap second hits Qantas air bookings, while Reddit and Mozilla stutter".
  6. Oleg Obleukhov and Ahmad Byagowi, Meta Engineering (2022). "It's time to leave the leap second in the past".
  7. BIPM (2026). Draft Resolution C, "On the technical actions needed to ensure the continuity of UTC".
  8. CCTF (2026). "Report from the CCTF on Draft Resolution C".
  9. Duncan C. Agnew (2024). "A global timekeeping problem postponed by global warming". Nature 628, 333-336.