How we calculate a saju chart

The short version

Code does the calendar math, so the same birth details always give the same chart. AI does the writing: Claude turns the calculated chart into Horan’s voice, following rules we wrote, and no person reviews a reading before you see it. People chose the calendar conventions below and wrote those rules. Horan (a tiger) and Kachi (a magpie) are fictional characters; their story is on the legend page.

What the code calculates, and what the AI writes

The chart engine (version horan-chart-1.0.0/lunar-typescript-1.8.6) is built on lunar-typescript, an open-source Chinese calendar library. From a birth date, and optionally a birth time and time zone, it works out:

• the four pillars (year, month, day and hour), each one heavenly stem and one earthly branch;
• the hidden stems inside each branch;
• the ten gods: how every other character relates to your day stem, the Day Master;
• the five-element count and the five ten-god families;
• the twelve stages of the day stem, and the two void branches of the day pillar;
• the twelve saju months of 2027 and the moment each one begins.

Who writes the words

The free chart on the home page runs the math in your browser. Its short lines are prepared in advance and picked by code from your chart; nothing is sent to an AI.

The paid reading recalculates the chart on our server and sends the calculated facts to Claude, which writes the chapters in Horan’s voice under rules we wrote. Code then checks the text against our banned-phrase rules. Kachi’s folklore lines are written by people and picked by code. No person reads a reading before you do.

The calendar rules we use

Four Pillars calculators disagree mostly because they pick different conventions. Each rule below is one common choice among several. We say which one we use; we don’t present it as the only right one.

• Year: the saju year begins at the exact moment of Ipchun (입춘, 立春, “start of spring”, around February 4), not on January 1 and not at Lunar New Year. The Encyclopedia of Korean Culture’s entry on saju notes that in fortune reading the year is counted from Ipchun, so someone born after Lunar New Year but before Ipchun takes the previous year’s stem and branch (사주).
• Month: each saju month begins at the exact moment of one of the twelve jie (節) solar terms, starting with Ipchun.
• Time zone: a birth time becomes an exact moment through the IANA time zone database for the birthplace, including daylight saving time and historical clock changes. A clock time that never happened (skipped by a daylight-saving change) is rejected, and one that happened twice asks which one you mean.
• Day: the day pillar changes at local midnight. The engine also supports the other common rule, where the day turns at 23:00, the start of the Zi hour (子時); the chart form on this site uses midnight. For a birth between 23:00 and midnight, the hour pillar takes the next day’s stem (the “late Zi” convention), and the chart says so.
• Solar time: the engine can move clock time to apparent solar time from the birthplace longitude, with the NOAA equation-of-time approximation. The chart form on this site uses local clock time.
• No birth time: the date gives three pillars and the Day Master, with no hour pillar, and the day pillar uses your local date. If your birthday falls within about a day of Ipchun or another month boundary, the year or month pillar stays blank until you add a birth time and time zone, instead of being guessed.
• Hidden stems: the three-stem table used in Chinese BaZi (人元), main stem first, shown below. Many Japanese and Korean tables add a stem to 子, 卯, 午, 酉 and 亥, so those five branches can look different elsewhere.
• Ten gods: named from each character’s element and polarity relative to the day stem: 比肩 Friend, 劫財 Rob Wealth, 食神 Eating God, 傷官 Hurting Officer, 偏財 Indirect Wealth, 正財 Direct Wealth, 偏官 Seven Killings, 正官 Direct Officer, 偏印 Indirect Resource, 正印 Direct Resource (Japanese books often write 印綬).
• Twelve stages: yang stems run forward from their 長生 (Growth) branch and yin stems run backward, with 戊 following 丙 and 己 following 丁 (“fire and earth share a root”). Some schools pair earth with water instead, or run yin stems forward like yang ones.
• Void branches: the two branches that the day pillar’s ten-day run (旬) skips. The run from 甲子 to 癸酉, for example, skips 戌 and 亥.
• Counting: the five-element count takes each visible stem and each branch’s main hidden stem once. The family weights count those as 1 each and every other hidden stem as 0.3.

Where the numbers come from: Ipchun 2027

Solar-term moments come from lunar-typescript’s astronomical tables, converted to an exact moment in UTC. For 2027 our engine puts Ipchun at 01:46 UTC on February 4, which is 10:46 in Seoul and Tokyo (UTC+9). Japan’s National Astronomical Observatory lists the same minute (NAOJ 2027 almanac).

The same moment falls on the evening of February 3 in the Americas. A baby born in New York at 8:00 PM that night has a 丙午 year pillar; one born at 9:00 PM has 丁未.

The free chart in your browser uses a table of solar-term moments generated from the same library, running from Ipchun 1919 to Minor Cold (小寒) in January 2028. That is also why we don’t publish month dates past early 2028 yet.

How the free chart picks a 2027 month

The free chart names one month of 2027 for you. That pick is a house rule of ours, not a classical one, and it describes where the chart’s emphasis falls, not what will happen. The rule:

• For each of the twelve 2027 months, we take the ten-god family (companion, output, wealth, officer or resource) that the month’s stem, and its branch’s main hidden stem, bring to your Day Master.
• A month ranks higher when those families are scarce in your chart (below an even one-fifth share) and lower when they already fill more than about a third of it. The stem counts twice as much as the branch.
• A month whose branch clashes with your day branch ranks lower; one that combines with it ranks a little higher. Clashes and combinations with your other branches count less.
• Ties go to the month that brings your scarcer family, then to the earlier month.

What we check automatically

Automated tests in our code check, among other things, that:

• the engine’s Ipchun 2027 falls on the minute published in the 2027 almanacs;
• a birth one second before Ipchun and one second after get different year pillars, and the same moment gives the same pillars in Seoul and New York;
• clock times skipped or repeated by daylight saving time are rejected or need a choice, including in half-hour and quarter-hour time zones;
• all 100 ten-god pairs and the hidden-stem table match the calendar library’s own tables;
• the browser’s date-only chart matches the server engine for every birthday from 1920 through 2008.

What we don’t claim

Saju is a calendar-based tradition, not a science and not statistics. There’s no evidence that it predicts events, and we don’t use it that way: no predictions about health, money, relationships or dates, and no promise that a reading will match your life. Passing the tests above means the code follows its rules; it says nothing about whether saju describes anyone.

Readings describe tendencies the tradition associates with a chart, for entertainment and self-reflection. They are not advice about health, money or legal matters.

What happens to your birth details

The free chart is calculated in your browser. The birth date you type there isn’t sent to our server, to an AI or to analytics. If you continue to the payment screen for a reading, your birth details and answers are sent to our server and saved with your order, even if you don’t pay; unfinished checkouts are deleted within 7 days, and reading data is kept until you delete it or for 365 days after purchase, whichever comes first. The details are in the privacy policy.

Who runs this site

Horan’s Saju Room is run by AboutUsAgency, Inc., a company in the United States. You can write to support@horansaju.com. Horan and Kachi are fictional; the real Korean folk painting behind them, the magpie and tiger, is on the legend page.

Changes to these rules

When a rule changes, we date it here.

• 2026-09-29: page published, describing chart engine horan-chart-1.0.0/lunar-typescript-1.8.6.

About this page

Written with AI (Claude) from the sources listed and our calculation engine. Horan and Kachi are fictional characters, and nothing here predicts events. The rules behind every chart: how we calculate.