— — —

Solar

Lunisolar

Months follow the moon (29.5 days) but an extra month is slipped in periodically so the year still stays aligned with the seasons.

Lunar

Pure-lunar — 12 months of 29 or 30 days, with no intercalation. The year is ~11 days shorter than solar, so the months drift through the seasons.

A few facts

Why so many?

Calendars track the sun (year), the moon (month), or both (lunisolar). Different cultures prioritised different cycles, and religious observances anchored the system to a particular astronomical event.

The "Anno Domini" trick

Year 1 of the Gregorian (and Julian) calendar was retconned by Dionysius Exiguus around 525 CE. Year 0 doesn't exist — Gregorian goes from 1 BC straight to AD 1.

The leap-second problem

Civil time has occasional leap seconds inserted to keep clocks aligned with Earth's slowing rotation. The CLDR calendar conversions ignore them; for sub-second-accurate work, use TAI.

The Julian → Gregorian skip

October 4, 1582 was followed by October 15. Ten days were dropped to undo the Julian calendar's slow drift; Britain and the colonies didn't switch until 1752.