One of the first points of confusion for anyone new to saju is the calendar question. 'Doesn't saju use the lunar calendar?' is perhaps the most frequently asked question. The short answer is no — saju uses the solar term calendar (절기력, jeolgiryeok), not the lunar calendar. Understanding this distinction is the very first step toward an accurate saju reading.
The lunar calendar is based on the Moon's cycle. It counts one month as the period from one new moon to the next (roughly 29.5 days), and traditional holidays like Seollal (Lunar New Year) and Chuseok follow it. The solar term calendar, by contrast, is based on the Sun's position — specifically, where the Earth sits along the ecliptic as it orbits the Sun. The 24 solar terms (Ipchun, Usu, Gyeongchip, and so on) mark the divisions of this calendar.
In saju, the boundary of each month is determined by the 'jeol' (節) subset of the 24 solar terms. For example, the Tiger Month (寅月) begins at Ipchun (Start of Spring), and the Rabbit Month (卯月) begins at Gyeongchip (Awakening of Insects). These solar term dates align closely with the Gregorian calendar but can diverge significantly from the lunar calendar. Lunar New Year's Day (the first day of the first lunar month) and Ipchun (around February 3–5 in the Gregorian calendar) are not the same date.
This difference matters in practice because someone born near a solar term boundary may have a different Month Pillar depending on whether the lunar or solar term calendar is applied. For example, if you were born on February 3 in the Gregorian calendar, the lunar calendar might already consider it the new year — but if Ipchun falls on February 4 that year, saju still places you in the Ox Month (丑月) of the previous year. Since a different Month Pillar changes the entire chart interpretation, this distinction is crucial.
Similarly, the Year Pillar boundary is not January 1 or Lunar New Year's Day — it is Ipchun. If you were born before Ipchun, your saju uses the previous year's Year Pillar, even though both the Gregorian and lunar calendars may have already rolled over into the new year. Using the wrong calendar to determine your birth data can produce an entirely incorrect chart.
DESTIGER handles the solar term conversion automatically. When you enter your Gregorian birth date and time, the system calculates the precise solar term boundaries for that date to determine the correct Year and Month Pillars. If you were born near a solar term boundary, entering your birth time as accurately as possible will ensure the best results. If you are ever unsure which Month Pillar applies to you, DESTIGER's analysis will clarify it instantly.
Confusion caused by the solar term boundary is remarkably common. Someone born on February 3 in a given year might receive two entirely different chart readings from different websites — one placing them in the previous year's Ox Month, and another in the new year's Tiger Month. The discrepancy arises because different systems handle the Ipchun boundary with varying levels of precision. Even a few hours can make the difference, which is why knowing your exact birth time is so critical for anyone born in late January or early February.
A deeper understanding of the 24 solar terms strengthens your saju foundations. The 24 terms are divided into two categories: Jeol (節, node terms) and Gi (氣, energy terms). Only the 12 Jeol terms — Ipchun, Gyeongchip, Cheongmyeong, Ipha, Mangjong, Soseo, Ipchu, Baengno, Hallo, Ipdong, Daeseol, and Sohan — serve as month boundaries in saju. The remaining 12 Gi terms (Usu, Chunbun, Gogu, and so on) mark the midpoints of each month but do not affect pillar assignments.
The Hour Pillar introduces yet another calendar consideration. A saju day begins at the Rat Hour (Ja-si), which starts at 11:00 PM. Whether the period from 11:00 PM to midnight belongs to the current day or the next day is a matter of scholarly debate known as the Ya-Ja-Si (Night Rat Hour) controversy. If you were born between 11:00 PM and midnight, this distinction can alter your Day Pillar and Hour Pillar simultaneously, making accurate birth time even more important.
There is an easy way to observe the difference between the lunar and solar term calendars in everyday life. Compare the date of Seollal (Lunar New Year) with the date of Ipchun in any given year. Some years, Seollal falls before Ipchun; other years, it falls after. In a year where Seollal precedes Ipchun, someone born between those two dates would celebrate the new year by the lunar calendar but still belong to the previous year in saju terms. This gap directly affects the Year Pillar assignment.
Another fascinating detail is that the exact timing of each solar term shifts slightly from year to year. Ipchun might fall on February 3 one year and February 5 the next. Even on the same calendar date, the precise hour and minute differ. Accurate saju analysis therefore requires calculating the solar term transitions down to the hour for the specific year in question. DESTIGER's perpetual calendar engine performs this calculation automatically, ensuring your chart is built on a precise astronomical foundation.