Leap Months: Where They Come From and Which Years Have One

A lunar month runs from one new moon to the next, about 29.5 days, so twelve of them come to roughly 354 days, about 11 days short of a year. Left alone, Lunar New Year would drift earlier and earlier until it landed in autumn. The leap month is how the calendar catches up.

How the leap month is chosen

The current rule works like this. Of the 24 solar terms in a year, 12 are "major" terms (zhongqi), such as Rain Water, the Spring Equinox and Grain Rain. Normally each lunar month contains one. When a month happens to contain none, it becomes the leap month and takes the name of the month before it, so a 5th month can be followed by a "leap 5th month". The solar terms are explained in the 24 solar terms.

How we counted: our calendar engine checked every year from 1900 to 2100 for a leap month.

The old rule of thumb is "seven leap months in nineteen years": 19 solar years and 235 lunar months are almost exactly the same length. Checking every 19-year window, we found 6 to 8 leap months, which fits.

Leap months, 2000–2060

Which months get doubled

Leap months cluster in summer and are rare in winter; between 1900 and 2100 there is no leap 1st month and no leap 12th month at all. The reason is that Earth is closest to the sun in the northern winter and moves fastest then, so the winter solar terms come closer together and a month rarely "misses" one.

Born in a leap month: when is your birthday?

Most years have no leap month. By custom, in those years you keep your birthday in the regular month of the same name: someone born in a leap 5th month celebrates on the same day of the 5th month. The lunar birthday converter lists every future year at once.

BaZi ignores leap months entirely. Its months change at the solar terms, so someone born in a leap month is charted from their Gregorian birth time like anyone else.

Common misconceptions