It is exactly the length of a civil day in TAI (International Atomic Time). It also happens to match almost all civil days in UTC-derived timezones (with the ex

It is exactly the length of a civil day in TAI (International Atomic Time). It also happens to match almost all civil days in UTC-derived timezones (with the exception of leap seconds, daylight savings, and timezones switching to a different UTC offset) It doesn't match solar days, but most people don't actually use solar days.