"Isn't Saju calculated with the lunar calendar?" is one of the most frequent questions any Saju service receives. Surprisingly, the answer is closer to "neither." The standard calendar of Saju is not the lunar or the Gregorian calendar but the solar-term calendar, called Jeolgi-ryeok (節氣曆) in Korean.

What the Solar-Term Calendar Is

The solar terms divide the year into 24 segments based on the sun's position along the ecliptic. Names you may have heard — Ipchun (start of spring), Haji (summer solstice), Dongji (winter solstice) — are all solar terms. Because they follow the sun, their dates are nearly fixed in the Gregorian calendar: Ipchun falls around February 4 every year, Dongji around December 22.

The year pillar and month pillar of a Saju chart are determined by these solar terms. In other words, the Saju year begins neither on January 1 nor on Lunar New Year, but at Ipchun. Months change not on the 1st, but on each term's entry day.

The Classic Confusion Ipchun Creates

The most common confusion caused by the solar-term calendar is the zodiac sign of people born early in the year. Someone born in late January is, by the wall calendar, born in the new year — but in Saju they are still before Ipchun, so they carry the previous year's stem and branch. Their zodiac animal shifts back by one year. If you have always known your sign by the Lunar New Year convention, a Saju service may show something different. That is not a bug; it is a difference of standards.

Entering a Lunar Birthday

Older generations often remember their birthday in the lunar calendar. What Saju needs is the actual day you were born, so a lunar birthday must be converted to the Gregorian date of the birth year. Hamhee K-SAJU offers a lunar input option that performs this conversion automatically. The one detail to double-check is the leap month: the same lunar date in a regular month versus a leap month differs by about a month in the solar calendar, so if you were born in a leap month, make sure to mark it.

Time-of-Birth Pitfalls

The hour pillar follows the twelve double-hour divisions (Ja-si, Chuk-si, and so on). Two historical variables can interfere here. One is standard-time changes: Korea used the UTC+8:30 meridian between 1954 and 1961, so clock time and solar time for births in that era differ by roughly 30 minutes. The other is daylight saving time, which Korea applied in parts of 1948–1960 and again in 1987–1988. If you were born near an hour-pillar boundary, these variables can flip your hour pillar.

The Short Version

The essentials of the Saju calendar boil down to four rules: the year starts at Ipchun, the month starts at each term's entry day, enter the day you were actually born (convert if lunar), and make the time as precise as you can. Follow these and the calendar will almost never skew your result. What to do when you don't know your birth time is covered in a separate guide.