Today just before 5pm California time is a special moment in timekeeping, 04:59:60 PDT. All sorts of software will subtly break because of the assumption that every minute has exactly 60 seconds or that time can’t stand still or repeat itself. Even my way of avoiding DST problems won’t help; UTC itself is changing.
Why do we need leap seconds? Because the earth’s rotation is slowing down about 1.7ms every 100 years. Leap seconds resync our daily earth time to the absolute measurement of atomic clocks. There have been 25 leap seconds since 1972.larger, in D3 code) is the offset between UTC and UT1. We decide to add a leap second when that offset gets too big. The detailed science behind this is fascinating; in the US the Naval Observatory manages the process.
I find it frightening how variable the earth’s rotation speed is. That anomaly around 1998–2005 makes me nervous. The speed changes both on daily and yearly scales in an unpredictable fashion. Among other things it depends on the size of the polar glaciers; global warming may well be speeding the earth up.