UTC also has this problem, to a lesser degree. The length of its second is determined atomically, but it’s also guaranteed to stay within 0.9 seconds of solar

UTC also has this problem, to a lesser degree. The length of its second is determined atomically, but it’s also guaranteed to stay within 0.9 seconds of solar time. Individual seconds are either skipped or repeated to maintain this guarantee in the face of solar clock drift. (cf. https://en.m.wikipedia.org/wiki/Leap_second https://en.m.wikipedia.org/wiki/Leap_second )