Earth's rotation is gradually slowing, so occasional leap seconds keep clocks matched to the planet.

The longer version

Tidal friction from the Moon and the Sun drags on the oceans and gradually lengthens the day by a tiny fraction of a second per year. Atomic clocks do not slow down, so the steady count and the turning planet slowly diverge. Events such as very large earthquakes also shift the rate slightly.

Leap seconds are inserted occasionally to close that gap, usually at the end of June or December. The irregularity makes them awkward for computers, which expect days of fixed length, and some systems handle them badly. Software bugs around the extra second have caused outages, and there is ongoing debate about letting the two drift apart instead.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.