Your chart in a nutshell
Saju describes your birth moment with four pillars: year, month, day and hour. Each pillar has two characters, a heavenly stem (天干) and an earthly branch (地支), so there are eight in total, hence 八字, “eight characters.” The stem of your day pillar, your day master, stands for you in the chart, and it sets your Gangnam Saju beauty type.
Getting those eight characters right is pure astronomy and calendar math. A few minutes here or one hour of daylight saving there can change a pillar, so this is where we went all in.
01 · Solar terms to the minute
In saju the year doesn’t begin on January 1 or at Lunar New Year. It begins at 立春 (Start of Spring), the moment the Sun reaches 315° along the ecliptic, and each month begins at one of the twelve 節 (solar terms) that follow, every 30°. These are exact instants, not calendar dates: 立春 fell at 23:10 on February 3 in 2025 and at 05:02 on February 4 in 2026 (Korea time). So a baby born on the evening of February 3, 2026, is still a Snake-year baby (乙巳), not a Horse.
We calculate every solar term from 1899 to 2101 from NASA JPL’s DE440 planetary ephemeris, including ΔT, the slow drift in Earth’s rotation. For every year from 1900 to 2049 our moments agree with an independent almanac algorithm to within a few seconds, and in our spot checks they match the times published by the Korea Astronomy and Space Science Institute (KASI) to the minute.
02 · True solar time for your birthplace
The hour pillar follows the Sun, not the clock. Time zones are wide: Korean clocks are set for 135° E, but Seoul sits near 127° E, so on average the Sun reaches its highest point about 32 minutes “late.” On top of that comes the equation of time. Because Earth’s orbit is an ellipse and its axis is tilted, a sundial runs up to 16 minutes ahead of the clock in early November and up to 14 minutes behind it in mid-February.
Put together, true solar time in Seoul is 16 to 46 minutes behind the clock, depending on the date. Born in Seoul at 11:40 on February 11? That’s about 10:54 by the Sun: the 巳 (Snake) hour, not the 午 (Horse) hour the clock suggests. Elsewhere the gap can be far bigger. In Madrid the Sun runs about two hours behind the clock in summer.
We make this correction with the longitude of the city you choose and the equation of time for your birth date. If your city isn’t on our list and you pick a time zone instead, we use your clock time as it is and say so on your result.
03 · Daylight saving: the hidden hour
Many places have pushed their clocks forward in summer, some only for a few years. Korea did so in 1948–51, 1955–60 and 1987–88 (around the Seoul Olympics), China in 1986–91 and Japan in 1948–51, and much of North America and Europe still does every year. Korea even ran on UTC+8:30 as its standard time from 1954 to 1961. Birth records show the clock time, so we look up the exact rules for your birthplace and date in the IANA time zone database, the same one your phone uses, and take that hour out first.
Example: a baby born in Seoul at 12:30 on July 15, 1988, was born at 11:30 standard time, or about 10:52 by the Sun. The clock says 午 hour; the Sun says 巳.
04 · The 子 hour: when does a new day begin?
The 子 (Rat) hour runs from 23:00 to 01:00 solar time, straddling midnight, and the masters never fully agreed on which day its first half belongs to. 萬民英, author of 三命通會, is quoted in 袁樹珊’s 命理探源 assigning the part before midnight to the previous day, while the 20th-century master 韋千里 argues in 千里命稿 that the new day starts as soon as the 子 hour begins.
By default we follow the start-of-子 rule (子初換日): from 23:00 true solar time, your day pillar belongs to the next day. Born between 23:00 and 01:00? This is the most common reason another app shows a different day pillar, and with it a different beauty type, since apps that use the “late 子 hour” (夜子時) only change the day at midnight.
05 · Lunar birthdays: Korean or Chinese almanac?
A lunar month begins on the day of the new moon, but which day that is depends on the time zone. For about a century, Korea’s almanac has been calculated for UTC+9 and China’s for UTC+8, so when a new moon falls in the last hour before midnight in Beijing, Korea starts the month a day later. Lunar New Year itself fell on different days in 1988 (February 17 in China, February 18 in Korea) and in 1997 (February 7 and 8), and in 2012 and 2017 the two calendars even placed the leap month differently.
So we convert lunar birthdays with the Chinese almanac if you were born in mainland China, Taiwan, Hong Kong or Macau, and with the Korean almanac everywhere else, including Japan, which shares Korea’s UTC+9. If your birthday is in a leap month (閏月), check the leap-month box.
06 · Grounded in the classics
Every rule in a saju reading goes back to a handful of classical texts. These are the ones we lean on most:
- 滴天髓 (Di Tian Sui): the poetic core of the day-master method. Its verses on the ten heavenly stems (天干論) appear with your beauty type, and our archive also holds 任鐵樵’s commentary, 滴天髓闡微.
- 窮通寶鑑 (Qiongtong Baojian): what each day master needs in each month of the year. Its 調候 (climate-balancing) principle feeds into your lucky element.
- 三命通會 (Sanming Tonghui): the great Ming-dynasty encyclopedia compiled by 萬民英, which sets out the 納音 elements, the twelve life stages and charm stars (神煞) such as 天乙貴人, 桃花 and 驛馬.
- 子平真詮 (Ziping Zhenquan): 沈孝瞻’s Qing-dynasty classic on the ten gods, chart structures (格局) and the useful god (用神).
- 淵海子平 (Yuanhai Ziping): one of the earliest handbooks of the Ziping method, which puts the day master at the center of the chart.
07 · Our classical-text archive
To work from the original wording rather than second-hand summaries, we built our own digital archive. As of October 2026 it holds close to 400 texts and editions (355 classical, plus 37 modern commentaries) in more than 7,400 chapters, about 40 million characters in all. Most come from Chinese Wikisource (about 280 titles), and the rest from Suanzhun (算准网, 56), the Kanripo repository of premodern Chinese texts (46) and Taiji Shuguan (太极书馆, 11).
More than 90 of them are about 命理 (destiny reading) itself, and the key works come in several editions (三命通會 alone in five), so we can compare versions of the same passage. The rest cover the traditions saju grew up with: the I Ching, divination, physiognomy, date selection and feng shui.
What the math can and can’t do
We can tell you precisely which eight characters belong to your birth moment. What they mean comes from a centuries-old symbolic tradition, not from science. Enjoy your reading as a mirror for mood, style and self-care, never as medical, financial or legal advice, and never as a reason to choose or skip a procedure.