Leap second

From: https://fanf.dreamwidth.org/133823.html

Leap seconds exist because the Earth takes (very roughly) about a millisecond more than 24 * 60 * 60 seconds to rotate each day; when we have accumulated enough extra milliseconds, a leap second is inserted into UTC to keep it in sync with the Earth. At the moment the Earth is rotating faster than in recent decades: these shorter days, with a lower length-of-day, means the milliseconds accumulate more slowly, and we get fewer leap seconds.

The absence of leap seconds has the advantage that leap second bugs don’t get tickled, but it has the disadvantage that timekeeping code might rot and new bugs or regressions can be introduced without anyone noticing. Even worse is the risk of the length of day getting shorter which could in theory mean we might need a negative leap second. There has never been a negative leap second, and if there is one, everyone who deals with NTP or kernel timekeeping code expects that it will be an appalling shitshow.