Born in a lunar leap month

A lunar month follows the Moon from one new moon to the next, so it lasts 29 or 30 days. Twelve of them fall short of a solar year, and left alone the lunar calendar would slowly drift away from the seasons. So every few years one extra month is added, giving that year thirteen months. The extra one is the leap month.

We compute our lunar tables astronomically and fix the month containing the winter solstice as the 11th month. When thirteen new moons begin between one winter solstice and the next, the first month that contains no major solar term becomes the leap month. Major solar terms are the twelve of the 24 solar terms, such as Rain Water and the Spring Equinox, that give each lunar month its number. A leap month follows the ordinary month of the same number, so leap month 4 comes straight after month 4.

Between 1900 and 2050 our table has 56 leap months, and the gap from one leap year to the next is two or three years in every case. Which month repeats varies: 2023 had a leap month 2, 2025 a leap month 6, and 2028 has a leap month 5. We checked every lunar month from 1900 to 2050 against the Korea Astronomy and Space Science Institute (KASI), and every leap month fell in the same place.

In the Four Pillars, the month pillar isn't set by the lunar month number. It turns over at the Solar Term Boundary of the twelve month-opening solar terms, such as the Start of Spring, Awakening of Insects and Clear and Bright. These are a different set from the major solar terms above. The lunar calendar tracks the Moon and the solar terms track the Sun, so their boundaries don't line up. In a year when Lunar New Year came before the Start of Spring, a baby born in lunar month 1 but before the Start of Spring still has the previous year's 丑 (Chou) month pillar.

A leap month goes through the same steps. We first convert the lunar date to its solar date, and from there the calculation is identical to a solar entry. Once the solar date is fixed, the leap month flag plays no further part, so the same birth entered as a leap-month lunar date or as a solar date gives the same Natal Chart. Nothing in your reading changes because the month was a leap month.

That doesn't make the leap month box optional. An ordinary month and the leap month with the same number are different months, so the choice moves the solar date by about a month. Lunar 2023, month 2, day 1 is February 20, 2023 in the ordinary month and March 22, 2023 in the leap month. Thirty days apart, the day pillars differ, and with the Awakening of Insects boundary in between, the month pillars split too: 甲寅 (Jia Yin) and 乙卯 (Yi Mao).

The month pillar can even change inside a single leap month. A leap month has no major solar term, but it can contain one of the twelve boundaries. Leap month 2 of 2023 ran for 29 days from March 22, and the Clear and Bright boundary fell on April 5. A birth in that leap month before the boundary has the 乙卯 (Yi Mao) month pillar; a birth after it has 丙辰 (Bing Chen).

If you're not sure whether your lunar birthday was in a leap month, check family records or a lunar calendar for that year first. If a birth record gives the solar date, entering that is the simpler route.

1. Under Calendar, choose Lunar.
2. Enter your lunar birth date as it is. For day 10 of leap month 4, enter month 4, day 10.
3. If that lunar year has a leap month, you'll see "This lunar year has a leap month" followed by its number.
4. If you were born in that leap month, tick "It was a leap month".

Tick it for a year without that leap month and we'll say so and ask you to clear the option. 1990, for example, has a leap month 5, so ticking the box for a month-4 birthday brings up that message. Enter a day the month doesn't have, such as day 30 of lunar 2023 month 1, which ends on day 29, and we'll tell you how many days it has. Once calculated, "See how this was calculated" on your result shows the conversion, for example "Lunar Feb 1, 2023 (leap) → Solar Mar 22, 2023".

Enter a lunar year on the input screen and we'll show which month, if any, repeats that year. The table below lists the leap months from 1984 to 2033, taken from our Korean lunar table. A year that isn't listed has no leap month, so there's nothing to tick.

We accept lunar dates for births from 1900 to 2100 in South Korea, North Korea, China, Taiwan, Hong Kong, Macao, Singapore and Malaysia. Dates from 2051 on lie beyond the range KASI publishes, so they come from our astronomical calculation alone. A birth in Greater China, Singapore or Malaysia goes through the Chinese calendar of that place instead. It now and then starts a month a day apart from the Korean one, and its leap months for 2012 and 2017 in Table 2 are 4 and 6. The input screen shows the leap month for your birthplace's calendar. If you were born outside Korea, see also our guide to births outside Korea.