How Chinese Lunar Leap Months Work

01

Twelve lunar months are shorter than a solar year

A lunar month begins at an astronomical new moon and lasts 29 or 30 days. Twelve average lunar months total about 354 days, while a tropical year is about 365.24 days. Without adjustment, the numbered months would move through the seasons.

Instead of adding a single leap day, the Chinese calendar periodically adds an entire month. Historically, seven leap months in nineteen years is a useful summary of the long-term pattern. It is not a repeating list of fixed Gregorian dates.

02

A principal solar term decides which month repeats

The twenty-four solar terms divide the Sun’s apparent annual path into 15-degree steps. Every other term is a principal solar term, often called zhōngqì (中气), so principal terms are spaced 30 degrees apart.

Under the modern Chinese calendar rules, when an interval between one eleventh month and the next contains thirteen lunar months, the first lunar month without a principal solar term becomes the leap month. It repeats the number of the month before it and adds rùn (闰): a leap second month is 闰二月, not “month thirteen.”

Scope: This is a practical account of the current Chinese calendar standard. Historical calendars used different rules at different times, so dates from earlier periods require specialist sources.

03

What a leap month looks like in a real year

In 2023, a regular second month was followed by a leap second month, then the third month. Each row below is the first day of that lunar month, converted to the Gregorian calendar.

1. 01
   Regular second month二月
   February 20, 2023Contains a principal solar term
2. 02
   Leap second month闰二月
   March 22, 2023No principal solar term
3. 03
   Third month三月
   April 20, 2023The numbered sequence continues

The leap month does not add a second version of every date each year. It creates duplicate month-and-day labels only in a year that repeats that particular month, so “the fifteenth day of the second month” and “the fifteenth day of the leap second month” are distinct dates.

04

Three misconceptions worth clearing up

“It happens every three years.”

Leap months occur roughly every two or three years, but they do not follow a fixed every-third-year rule. Their placement comes from astronomical month and solar-term boundaries.

“The same month always repeats.”

No. The month lacking a principal solar term determines the leap month, so the repeated month changes.

“It is the same as February 29.”

Both reconcile calendar cycles, but a Chinese leap month contains 29 or 30 days and repeats a lunar month number; the Gregorian leap year adds one day.

05

For birthdays, record whether the month was a leap month

If a family date falls in a leap month, keep the rùn marker with the month. Leaving it out can point to a different Gregorian date. How a lunar birthday is observed in years without that leap month varies by family and region; there is no single universal rule. Ask the person or family rather than letting a converter decide the custom.

OrientLore’s converter supports Chinese calendar years 1900 through 2100. It lists regular and leap months separately and validates their actual lengths. It can convert a specified date; it cannot choose a family’s birthday practice or settle historical dates outside its documented range.