Lunar Birthdays and Leap Months

Saju is calculated by solar terms, not by the lunar calendar

Many people think saju is read by the lunar calendar, but the four pillars do not use a lunar calendar. The year changes at Ipchun (입춘, the Start of Spring), and the month changes at the moment one of the twelve solar terms (절기) arrives, such as Ipchun, Gyeongchip (경칩) and Cheongmyeong (청명). Solar terms are set by the position of the sun, so in effect they are tied to the solar calendar. Days run through the sixty-pair cycle (60 gapja) one slot per day without a break, and hours are divided by the sun's time. No pillar involves the lunar new moon day or the length of a lunar month.

So a lunar birthday only needs to be converted to a solar date before the saju calculation. Halmeoni does the same. When you enter a lunar date, it first finds the solar date of the same day, and from there calculates exactly as if you had entered the solar date. If it is the same day, the saju is the same whether you enter it as lunar or solar.

The same goes for the zodiac animal. People often say the animal changes at Seollal (Lunar New Year), but in saju it changes at Ipchun. In 2026 Ipchun falls on 4 February and Seollal on 17 February. 10 February 2026, which falls between them, is still December 23, 2025 by the lunar calendar, but by saju it is already the 丙午 병오 year and the 庚寅 경인 month. If you set the animal by the lunar date, the year comes out wrong on days like this.

Why do leap months exist?

A lunar month is 29 or 30 days, following the cycle of the moon's phases. Twelve months add up to around 354 days, about eleven days shorter than a solar year. Left like this, the lunar months and the seasons would drift far apart within a few years, so once every few years an extra month is added to bring them back into line. This extra month is the leap month (윤달). A leap month takes the name of the month right before it, so one placed after the 4th month is called the leap 4th month (윤4월).

By the Korean lunar calendar of the Korea Astronomy and Space Science Institute, 55 of the 150 years from 1900 to 2049 have a leap month, about seven in every 19 years. The table below shows the leap months from 1970 to 2035.

The leap month table is built from the same Korean lunar table that Halmeoni uses when converting a date of birth. By the Chinese lunar calendar, the leap months of 2012 and 2017 fall in different months. If your result differs from a manseryeok that uses the Chinese lunar calendar, this may be the reason.

How to enter a leap-month birthday

• Next to the date of birth heading, choose "Lunar" out of "Solar · Lunar."
• Choose the year, month and day exactly as the lunar date. For the leap 4th month, choose 4 as the month. Some lunar months have 30 days, so the day can be chosen up to 30.
• Tick the "Leap month" box that appears below the date. If you do not tick it, it is calculated as the regular month with the same number.

If that month has no leap month in that year, you will see the notice, "There is no leap month for that month in that year." For example, the only leap month of 2020 is the leap 4th month, so ticking the leap month box for lunar May 2020 shows this notice. If you choose day 30 in a month that has only 29 days, Halmeoni tells you, "That day doesn't exist in that lunar month." Halmeoni does not swap a nonexistent date for the nearest day and calculate; it asks you to enter it again, because asking is better than quietly calculating with a wrong date.

There is a folk belief that because a leap month is a "bonus month squeezed in," people born in it have a different fortune, but the saju calculation has no such rule. After a leap-month birthday is converted to a solar date, the month is set by solar terms just like any other day.

If you are not sure whether the birthday your parents told you is lunar or solar, or whether it is in a leap month, it is best to check first. A single leap-month tick can move the solar date by nearly a month, and the month pillar and day pillar of the saju change with it.

Example: day 15 of the 4th month and of the leap 4th month, 2020

In 2020, a leap 4th month came after the lunar 4th month. The leap 4th month began on 23 May 2020 (solar) and lasted 29 days. Even for the same "day 15 of the lunar 4th month," the result differs depending on whether the leap month is ticked. The birth time is treated as unknown and the birthplace as Seoul.

The two dates are 30 days apart in the solar calendar. The year pillar is the 庚子 경자 year for both, but day 15 of the regular 4th month comes after Ipha (입하, the Start of Summer), so it is the 辛巳 신사 month, while day 15 of the leap 4th month is the day after Mangjong (망종, 5 June), so it is the 壬午 임오 month. The day pillar also changes from 庚戌 경술 to 庚辰 경진. Thirty days is half of the sixty-pair cycle, so the top character (the Day Master) is the same 庚 and only the bottom character moves to the opposite position. Even with the same Day Master, the season of birth has moved from late spring to midsummer, so the way of looking at the strength of the Day Master and the favourable element changes.

Range and limits of the conversion table

To convert lunar dates to solar, Halmeoni uses the Korean lunar table published by the Korea Astronomy and Space Science Institute. This table runs to lunar 2049; dates after that are converted with the Chinese lunar table of a public almanac library (lunar-javascript). A date of birth can be entered from 1900 to 2100. Lunar dates from 2050 on follow the Chinese lunar calendar, which can differ from the Korean one by a day, so please double-check dates around then.

Korean and Chinese lunar calendars differ because of the time standard. A lunar month begins on the day of the new moon (合朔 합삭, when the sun and moon line up), and the Chinese calendar counts days in UTC+8 while the Korean calendar counts them in UTC+9. If the new moon falls between 0:00 and 1:00 Korean time, that day is the first day of the month in Korea, but in China the day before is, so the lunar dates of that month are off by one day. Between 1912 and 2050 there are about 60 such months, and in some years the first day shifted so much that even the month containing the leap month changed. If a Korean lunar birthday is entered into a manseryeok based on the Chinese lunar calendar, someone born in such a month gets a solar date that is a day off, and the day pillar changes with it. This is why Halmeoni uses the Korean lunar table.

If you know your solar birthday exactly, entering it as solar is the surest way. If you know it only as lunar, enter it as lunar, and check whether the solar date shown together on the saju card as "Lunar year ○, month ○, day ○ (solar …)" matches the day your family knows.