Lunar or Solar Birthday? Getting the Date Right for Saju
Maker's note: 기준일 2026-10-09. The AZET team (AZET 운영팀) put this guide together as operators of the product family. Seowoon's Korean-language site, seowoongwan.com, is operated by 에이치컴퍼니(사업자등록번호 316-47-00948, 통신판매업 신고 2023-부산강서구-1073). The menu name mentioned matches the site as of October 9, 2026. Saju is offered for fun and reference only and is not a forecast or advice.
Every Korean saju form has a small toggle that puzzles first-time users from abroad: 양력 or 음력, solar or lunar. Choose wrongly and the chart you get belongs to a different day, sometimes a different month. For most people born outside East Asia the answer is simple. For anyone with Korean parents or grandparents, it is worth a phone call. This post explains what the toggle does and how to set it.
Two calendars in one country
Korea officially adopted the Gregorian calendar at the end of the nineteenth century, and it governs all civil life: school years, contracts, passports. The traditional lunisolar calendar never disappeared, though. It still sets the two biggest holidays, Seollal and Chuseok, and for much of the twentieth century it was how many families remembered birthdays.
That creates a familiar Korean situation: a grandmother whose "birthday" moves around the Gregorian calendar every year, because the family celebrates it on a lunar date. It also creates official records that do not match family memory, since births in rural areas were sometimes registered late or under the lunar date written as if it were solar.
Younger Koreans almost always use solar birthdays. The lunar habit is concentrated among people born before roughly the 1970s, and it varies by family.
How the traditional calendar works
The traditional calendar is lunisolar. Months follow the moon: each begins at a new moon and lasts 29 or 30 days. Twelve such months come to about 354 days, eleven short of a solar year. To stop the months drifting through the seasons, an extra month is inserted roughly seven times in every nineteen years.
That extra month is the yundal (윤달), the leap month. It repeats the number of the month before it, so a year can have a fourth month followed by a leap fourth month.
| Feature | Solar (Gregorian) | Traditional lunisolar |
|---|---|---|
| Month length | 28 to 31 days | 29 or 30 days |
| Year length | 365 or 366 days | About 354, or about 384 with a leap month |
| Leap adjustment | One day | One whole month |
| New year | January 1 | Late January to mid February |
| Used today for | Civil life | Holidays, some elders' birthdays |
So a lunar date needs three pieces of information to be unambiguous: the year, the month and day, and whether the month was a leap month. Forms that offer 음력 usually have a second option for 윤달 for exactly this reason.
The twist: saju months are solar
Here is the part that surprises people. Although saju is old and the lunar calendar is old, a saju chart is not built from lunar months.
The month pillar is determined by the twenty-four solar terms, jeolgi (절기), which mark the sun's position through the year. Twelve of them act as month boundaries for saju. The first is ipchun, "start of spring," around February 4, which also turns the year pillar. The next boundary falls around March 6, the next around April 5, and so on. These dates shift by a day or so from year to year, and the exact moment of the changeover is calculated to the minute.
Two practical consequences:
- The lunar date is only a way of identifying the day. A calculator converts a lunar birthday to its solar equivalent and then works from solar terms. If you already know the solar date, lunar adds nothing.
- Births near a boundary are sensitive. Someone born on February 3 or 4, or within a day of any monthly boundary, may land in either month pillar depending on the hour. Good calculators handle this; it is still worth knowing why two tools might disagree for such a date.
The day pillar, meanwhile, comes from a continuous sixty-day count that ignores months entirely, and the hour pillar from local time. Neither depends on the lunar calendar.
How to find out which date you have
If your birth was recorded in a country that uses only the Gregorian calendar, choose solar and move on.
If you have Korean family, especially if you or a parent were born in Korea some decades ago, check:
- Ask directly. "Is my birthday lunar or solar?" is a normal question in a Korean family and elders usually know at once.
- Look for the tell. If the family celebrates a birthday on a different Gregorian date each year, it is lunar.
- Compare documents. If the date on an identity document differs from the one the family celebrates, ask which is the actual day of birth. Late registration was common in earlier generations.
- Ask about a leap month. It is rare, but if the birth fell in one, the conversion changes by about a month.
If nobody knows, run the chart both ways and see whether the two results differ in ways that matter to you. Often the main difference is the month pillar.
Keeping perspective
It can feel as if the whole reading hangs on this toggle. The date does determine the chart, so set it correctly if you can. But remember what the chart is: a traditional way of describing a birth moment, enjoyed as culture. An uncertain birth date makes for an uncertain chart, and that is all. It has no bearing on anything real in your life.
On Seowoon
Seowoon (서운관), the Korean saju service in the AZET family, asks for a birth date and the solar or lunar setting like any Korean saju service. At seowoongwan.com, 도사님 정통사주 is the general reading to start with. The site is in Korean; look for 양력 for solar and 음력 for lunar when you fill in the date, and use browser translation for everything else.
FAQ
Should I pick solar or lunar? Pick the calendar your known birth date is written in. For most people born outside East Asia, that is solar.
Is saju based on the lunar calendar? Not for the month pillar. Months in saju are set by the twenty-four solar terms. A lunar date is converted to solar first.
What is a leap month? An extra lunar month inserted about seven times in nineteen years to keep the calendar aligned with the seasons.
My grandmother's birthday changes every year. Why? The family is using her lunar birthday, which falls on a different Gregorian date each year.
What if I cannot find out? Try both settings. A reading is for fun and reference, so an uncertain date is not a serious problem.
Learn more
Seowoon is listed with the other AZET services at https://azet.io, where the free early-access waitlist for azet, our AI assistant in progress, is also open. Asking an elder which calendar your birthday is in tends to start a good family story, whatever you do with the chart afterwards.