The “time patch”—the leap second—which has been used for more than half a century to bridge the gap between atomic clock time and Earth’s rotational time, has been abolished. According to a resolution passed by the General Conference on Weights and Measures on the 15th, starting May 20, 2027, leap seconds will no longer be artificially added or subtracted, and Coordinated Universal Time (UTC) will be adopted continuously.

  On April 12, 2026, visitors view exhibits at the International Watch Museum in La Chaux-de-Fonds, Switzerland. Photo by Lian Yi, Xinhua News Agency

  To determine time, there are two commonly used time systems in the world: Universal Time (UT), which is based on the Earth’s rotation, and International Atomic Time (TAI), which is based on the period of atomic oscillation. The Earth’s rotational rate is not constant, whereas the length of a second in International Atomic Time is highly stable. As time passes, a discrepancy gradually develops between the times indicated by the two systems. To reconcile this discrepancy, relevant international organizations established Coordinated Universal Time (UTC), which is currently the time system widely adopted worldwide.

  Coordinated Universal Time (UTC) is based on the duration of the second as defined by International Atomic Time (ATON) and is kept as close as possible to Universal Time (UT). At the 1972 General Conference on Weights and Measures, it was decided that when the observed time difference reaches 0.9 seconds, one second would be added to or subtracted from Coordinated Universal Time; this adjustment is known as a leap second. To date, 27 positive leap seconds—adding a second to the time—have been implemented globally, with the most recent adjustment occurring on December 31, 2016.

  However, as digital infrastructure has become widespread, the leap second mechanism has gradually become a hidden risk for information systems. Christian Bizoual, chief scientist at the International Union of Geodesy and Geophysics’ Geodetic Coordination Center, said that when the leap second mechanism was first introduced, human society did not yet require extremely high-precision time. Today, however, many applications require time accuracy better than one second—financial systems being a prime example. Consequently, forcibly adjusting Coordinated Universal Time (UTC) by one second can cause significant problems. In the past, every leap second implementation has resulted in various errors. Today, even the slightest time discrepancy can have serious consequences.

  On March 2, 2026, a trader works on the trading floor of the New York Stock Exchange in the United States. Photo by Liu Yanan, Xinhua News Agency

  In addition, recent observations have revealed an abnormal acceleration in the Earth’s rotation. According to the existing rules, if this acceleration continues, a negative leap second—that is, skipping a second entirely—may be required in the future. The International Bureau of Weights and Measures (BIPM), in collaboration with the International Union of Geodesy and Geophysics (IUGG), will hold a technical workshop in 2025 to assess the risk of a negative leap second. Experts believe that the probability of a negative leap second occurring will rise rapidly in the coming years, reaching 30% by 2035. This is a scenario that all parties hope to avoid as much as possible, as a negative leap second is highly likely to cause widespread system failures.

  A resolution adopted by the General Conference on Weights and Measures established a new standard that increases the maximum permissible deviation between Coordinated Universal Time (UTC) and Universal Time (UT) to 3,600 seconds—or one hour—thereby ensuring the continued stable operation of UTC for centuries to come.