The International Earth Rotation and Reference Systems Service reported that 9 July 2025 lasted roughly 1.3–1.36 milliseconds less than the customary 86,400 seconds, making it the shortest day so far this year. Similar time deficits are projected for 22 July and 5 August, continuing a run of accelerated spins that began in 2020 and eclipsed only by the 5 July 2024 record of –1.66 milliseconds. Researchers say the speed-up reflects a confluence of factors, including the Moon’s current distance from the equator, seasonal weakening of upper-air winds and unexplained changes in the planet’s liquid outer core. “The cause of this acceleration is not explained,” noted Leonid Zotov of Moscow State University, while geophysicist Duncan Agnew of UC San Diego added that momentum lost by a slowing core is transferred to the crust, imperceptibly quickening surface rotation. Although a millisecond is too small for humans to notice, the discrepancy matters for satellite navigation, high-frequency trading and other systems that rely on atomic time. Since 1972, timekeepers have inserted 27 leap seconds to compensate for Earth’s normally slower rotation, but none have been needed since 2016. National Institute of Standards and Technology physicist Judah Levine cautions that a sustained acceleration could force the first-ever negative leap second around 2029. IERS is monitoring conditions and says any adjustment will be announced well in advance.