A leap second: on December 31, 1973 the last minute of the day ran to 23:59:60 UTC, the 3rd since 1972 (13 seconds between UTC and atomic time after it). They keep atomic time in step with the Earth's slowing, uneven spin. None has been needed since 2016, and in 2022 the world's timekeepers voted to stop adding them by 2035.