Calendar Cycles and Leap Rules

A Gregorian year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So 1600 and 2000...

The Athenian astronomer Meton observed that 19 solar years are almost exactly 235 lunar months, so moon phases recur on nearly...

Because the Egyptian civil year had exactly 365 days, the heliacal rising of Sirius, Sopdet in Egyptian, drifted through it by a...

The sexagenary cycle pairs ten heavenly stems with twelve earthly branches to produce 60 combinations, used to name days since...

The Calendar Round is the period after which a given combination of the 260-day Tzolk'in and 365-day Haab' recurs, 18,980 days or...

The arithmetical, or tabular, Islamic calendar adds a day to Dhu al-Hijjah in 11 years of every 30, giving years of 354 or 355...

In the Julian calendar the pattern of weekdays and dates repeats every 28 years, the product of the 7-day week and the 4-year...

Leap seconds are inserted into Coordinated Universal Time to keep it within 0.9 seconds of time based on the Earth's rotation...

Eclipses recur after 223 synodic months, about 18 years and 11 days, because the Sun, Moon and lunar nodes return to nearly the...

The earliest Greek intercalation rule inserted three leap months every eight years, since 99 lunar months nearly match eight...

Callippus of Cyzicus improved the Metonic cycle by taking four 19 year cycles, 76 years, and dropping one day, giving 940 months...