Both leap years and leap seconds exist for the same underlying reason: the Earth's rotation and orbit don't divide neatly into whole days, hours, minutes, or seconds, so occasional corrections keep our timekeeping systems aligned with astronomical reality.

Leap years: correcting the calendar

A year — one full orbit of the Earth around the Sun — actually takes about 365.2422 days, not a clean 365. Without correction, a fixed 365-day calendar would drift out of sync with the seasons by about a day every four years, eventually putting winter holidays in the middle of summer over enough centuries. Adding a leap day (February 29) roughly every four years brings the calendar back in line. The rule has a refinement: years divisible by 100 are not leap years unless they're also divisible by 400, which is why the year 2000 was a leap year but 1900 was not.

Leap seconds: correcting atomic clocks against the Earth's spin

The Earth's rotation is not perfectly constant — tidal forces and geological activity cause it to speed up and slow down very slightly over time. Since 1972, an occasional leap second has been added to Coordinated Universal Time (UTC) to keep it aligned with the Earth's actual rotation, as measured by atomic clocks that are otherwise far more precise and stable than the planet itself.

Are leap seconds going away?

In 2022, international standards bodies agreed to discontinue the use of leap seconds by 2035 at the latest, largely because the sudden one-second jump causes real problems for computer systems, financial trading platforms, and satellite navigation, which rely on perfectly continuous timekeeping. Future corrections, if needed, are expected to be handled differently, likely with a larger adjustment made much less frequently.

Why this matters day to day

For most people, leap years and leap seconds are invisible background maintenance. But they're a good reminder that our clocks and calendars are human inventions layered on top of an imperfectly cooperative planet — and that even "exact" time requires the occasional correction.