What does a birth date say in the Korean saju system?

The room this came out of

Before any of this was a web page it was a book. The manse-ryeok, a perpetual calendar of sixty-year cycles, is the object a saju reading sits on: pages of stems and branches, the solar terms listed with their times, the year turning at a point in early February rather than at midnight on January 1.

The colours on this site come from that older setting rather than from a star chart: the five directional colours, the mulberry-paper ground, the painted rafter patterns of a shrine gate, the figures standing at the edge of the page. They are here as atmosphere. What runs underneath them is arithmetic on a calendar.

What a birth date turns into: four pillars, eight characters

A date, a time and a place of birth become four columns: year, month, day, hour. Each column carries two characters, one heavenly stem above one earthly branch, so a full chart is eight characters. "Saju" is the word for those four columns.

A different rule decides each column, and this is where most of the surprises are:

The year column is decided by a solar term, not by New Year's Day. A birth date in January or early February can fall in the previous year's column.

The month column is decided by which solar term section the date falls in, not by the number on the calendar month.

The day column is decided by a continuous count through the sixty-year cycle. Where the day is taken to begin — at midnight, or at 23:00 — can move it by one.

The hour column is decided by the time of birth after the time zone in force on that date has been applied, and after any correction for true solar time.

The five elements, and why "horoscope" is a loose fit

Each of the eight characters carries one of five elements: wood, fire, earth, metal, water. Counting how many of each appear, and which of the five does not appear at all, is what "element" points at in this system.

English has no word for the whole thing, so "Korean horoscope" is what most people reach for, and it is a loose fit. There is no zodiac belt here, no planets, no houses, no rising sign. Nothing is observed in the sky at the moment of birth. The whole object is a date located inside a calendar, and the calendar is the part that has to be right.

What the tradition then does with eight characters

From there the tradition reads the characters as dispositions. The day stem is treated as the centre of the chart, and each stem carries an old image: the tradition reads gap (甲) as a large tree, something that keeps a direction once it has taken one, and eul (乙) as a vine, something that reaches the same place by bending around what is in the way.

Those are descriptions of a tradition. This page keeps them as descriptions of a tradition. What it can give you without leaving that ground is the other half: which rule put each of your four columns where it is, and where a different rule would have put it.