Earth is spinning faster, so timekeepers have scrapped the leap second

The odd extra second that clocks around the world occasionally squeezed in just before midnight is finished. The General Conference on Weights and Measures, the international body that keeps the world’s units of measurement in step, has voted to turn Coordinated Universal Time (UTC) into a continuous scale, with no leap seconds added or taken away. UTC is the time your phone, your bank and every server on the internet quietly agree on, so this is a change to the clock almost everything runs on, even though nobody will ever see it happen.

The reason is a small planetary surprise. Leap seconds were invented because Earth’s spin was gradually slowing, so the atomic clocks kept running ahead and every so often had to wait a second for the planet to catch up. In recent years Earth has sped up instead. Experts on Earth’s rotation who advised the conference estimated that the chance of a negative leap second, a second deleted from the clock, would climb fast and reach about one in three within roughly a decade. No negative leap second has ever been applied, and the resolution warns that critical infrastructure is “largely unprepared” for one, with preparation costs it compares to the millennium bug.

Even the familiar positive kind has a long record of breaking things. A normal minute ends at 23:59:59 and rolls straight to midnight. A leap second inserts 23:59:60, a timestamp that a great deal of software assumes cannot exist. When a leap second arrived at the end of June 2012, Reddit, Mozilla, Qantas and various sites running Linux reported problems. On New Year’s Day 2017, a number inside Cloudflare’s DNS software went negative and the code panicked. At the peak, about 0.2% of DNS queries were affected. Cloudflare’s engineers summed up the cause in one line: “The root cause of the bug that affected our DNS service was the belief that time cannot go backwards.”

The biggest tech companies stopped trusting the extra second years ago. Google spread it across about 20 hours of very slightly longer seconds, a trick called smearing. Meta, whose engineers once described every leap second as “a major source of pain for people who manage hardware infrastructures,” smears it over 17 hours. That workaround created its own mess, and the conference names it in the resolution: each provider smears in its own way, so different systems disagree about the time during the change. The conference also feared that some organizations would give up on UTC and switch to GPS time, which has never used leap seconds.

For everyone else, the change is meant to be invisible. “The nice fact is that we have nothing to do!” Patrizia Tavella, director of the time department at the International Bureau of Weights and Measures (BIPM), told Wired. “Just continue to accumulate second[s] and ‘normal’ minutes of 60 seconds.” Phones will keep syncing as they do now, and the gap between UTC and pure atomic time, which has stood at 37 seconds since the last adjustment, should stay frozen for centuries.

The price is a gentle drift away from the Sun. Clock time will no longer track Earth’s turning to within a second. The conference now allows the two to separate by up to one hour, enough, it says, to keep UTC continuous for several centuries. That sounds drastic until you notice what the resolution itself points out: time zones already leave many places more than an hour away from their local solar time, and in many countries daylight-saving time moves clocks by an hour every spring and autumn. Astronomers and satellite operators who need Earth’s real orientation will read the difference from the data that navigation satellites and Earth-rotation services already broadcast.

The fix does not tidy up everything. Leap-second code stays dormant inside billions of devices, and network time operators are already debating what software will do when the official list of leap seconds, which ships with time-zone data on many Linux systems and carries an expiry date at the end of this year, is never renewed. Under an international radio agreement, stations that broadcast time signals have a transition period until 2035, and no later than 2040, where old equipment cannot carry the bigger gap sooner. Nobody has scheduled a leap hour either. “We do not speak of leap hour yet,” Tavella told Wired, which leaves that problem to people several centuries from now.

The vote came at the 28th meeting of the conference in Versailles, which ended on October 15, and continuous UTC takes effect on May 20, 2027, the anniversary of the 1875 Metre Convention and World Metrology Day. No leap second is scheduled before then, so the one added on December 31, 2016, the 27th since the system began in 1972, will most likely stay the last. From then on, the clocks will simply keep counting, and the planet will be left to keep its own time.

Tags: atomic clocks, BIPM, General Conference on Weights and Measures, leap second, timekeeping, UTC