Kairo
Korean Saju
Patterns before predictions
The Hour Pillar: How Saju Divides a Day Into 12
The Hour Pillar comes from a two-hour block and a rule that hangs off the Day Stem, and in Korea two conventions disagree about where every block starts.
The Hour Pillar (시주, 時柱, siju) is the fourth column of a Saju chart. Its branch comes from which of twelve two-hour blocks the birth falls in, starting with the Rat hour at 23:00. Its stem is derived from the Day Stem by a fixed rule. Two conventions complicate this in Korea: a 30-minute shift that some practitioners apply to every block, and a disagreement about whether the day changes at midnight or at 23:00.
The twelve double-hours
A traditional day is twelve double-hours (시진, 時辰, sijin), one per Earthly Branch (지지, 地支), in the same order as the branches of the year. The standard clock spans are:
| Branch | Animal | Korean | Standard span |
|---|---|---|---|
| Ja 子 | Rat | 자시 | 23:00 – 01:00 |
| Chuk 丑 | Ox | 축시 | 01:00 – 03:00 |
| In 寅 | Tiger | 인시 | 03:00 – 05:00 |
| Myo 卯 | Rabbit | 묘시 | 05:00 – 07:00 |
| Jin 辰 | Dragon | 진시 | 07:00 – 09:00 |
| Sa 巳 | Snake | 사시 | 09:00 – 11:00 |
| O 午 | Horse | 오시 | 11:00 – 13:00 |
| Mi 未 | Goat | 미시 | 13:00 – 15:00 |
| Sin 申 | Monkey | 신시 | 15:00 – 17:00 |
| Yu 酉 | Rooster | 유시 | 17:00 – 19:00 |
| Sul 戌 | Dog | 술시 | 19:00 – 21:00 |
| Hae 亥 | Pig | 해시 | 21:00 – 23:00 |
Noon sits in the middle of the Horse hour and midnight in the middle of the Rat hour. That is the design: the O 午 block is centred on the sun's highest point, and the Ja 子 block on its lowest. Everything that follows about the 30-minute question comes from the fact that a clock and the sun disagree about when those points happen.
How the hour stem is derived
The branch is read off the table. The stem is computed from the Day Stem (일간, 日干) by the "five Rats" rule (오자둔, 五子遁): the Day Stem decides which stem the Rat hour gets, and the remaining eleven hours follow in the normal stem order.
| Day Stem | Rat hour (23:00) starts at | Then |
|---|---|---|
| Gap 甲 or Gi 己 | Gap-Ja 甲子 | Eul-Chuk, Byeong-In, Jeong-Myo … |
| Eul 乙 or Gyeong 庚 | Byeong-Ja 丙子 | Jeong-Chuk, Mu-In, Gi-Myo … |
| Byeong 丙 or Sin 辛 | Mu-Ja 戊子 | Gi-Chuk, Gyeong-In, Sin-Myo … |
| Jeong 丁 or Im 壬 | Gyeong-Ja 庚子 | Sin-Chuk, Im-In, Gye-Myo … |
| Mu 戊 or Gye 癸 | Im-Ja 壬子 | Gye-Chuk, Gap-In, Eul-Myo … |
Twelve hours times ten stems gives sixty pairs per five days, so the same hour pillar returns every fifth day. A birth in the Sa 巳 hour on a Gyeong 庚 day gets Sin-Sa 辛巳; the same hour on the following day, a Sin 辛 day, gets Gye-Sa 癸巳.
Because the stem hangs off the Day Stem, any convention that moves the Day Pillar moves the hour stem with it. That matters at the end of this article.
The Korean 30-minute question
Korea keeps its clocks on UTC+9, the meridian at 135°E. Seoul is at about 127°E. Every degree of longitude is four minutes of solar time, so the mean sun over Seoul reaches its highest point around 12:32 on the clock rather than 12:00.
Many Korean practitioners take this seriously and shift every double-hour by 30 minutes. Under that table the Rat hour runs 23:30 to 01:30, the Horse hour 11:30 to 13:30, and so on. Korean manseryeok apps commonly offer this shifted table as a setting.
Others use the clock as recorded, on the argument that the tradition always used civil time and that a 32-minute correction for Seoul is a different number in Busan (129°E, about 24 minutes) or in Los Angeles. Practitioners disagree, and no text settles it.
Two things follow for any birth recorded in Korea. A birth at 11:20 pm sits inside the Rat hour by the standard table and inside the Pig hour by the shifted one. And a birth anywhere within 30 minutes after an odd-numbered hour (01:00, 03:00, 05:00 and so on) gets a different branch under the two conventions.
Early Rat, late Rat, and where the day ends
The Rat hour straddles midnight. That creates a second question, separate from the 30-minute one: the moment at which the Saju day changes.
Under the Rat-hour convention, the day changes at 23:00, when the Ja hour begins. A birth at 11:20 pm belongs to the next calendar day's pillar.
Under the midnight convention, the day changes at 00:00. The hour from 23:00 to midnight is the late Rat hour (야자시, 夜子時), attached to the day that is ending; the hour from midnight to 01:00 is the early Rat hour (조자시, 早子時), attached to the day beginning. A birth at 11:20 pm keeps the calendar day's pillar. Schools that use this convention also differ on which Day Stem drives the late-Rat hour stem.
Most English calculators pick one of these silently. Kairo's engine flags a birth that falls near the midnight and Rat-hour boundary as uncertain, rather than choosing one convention and moving on.
A worked example: 11:20 pm, July 20, 2026, Seoul
July 20, 2026 is an Eul-Mi 乙未 day, in the Mi 未 month of the Byeong-O 丙午 year. The Day Stem is Eul, so by the five-Rats rule the Rat hour on that day is Byeong-Ja 丙子 and the Pig hour is Jeong-Hae 丁亥.
| Convention | Day Pillar | Hour branch | Hour Pillar |
|---|---|---|---|
| Standard table, Rat-hour day change | Byeong-Sin 丙申 (July 21) | Ja 子 | Mu-Ja 戊子 |
| Standard table, midnight day change | Eul-Mi 乙未 (July 20) | Ja 子 (late Rat) | Byeong-Ja 丙子 or Mu-Ja 戊子, by school |
| 30-minute shift | Eul-Mi 乙未 (July 20) | Hae 亥 | Jeong-Hae 丁亥 |
Three defensible conventions, three different fourth columns, and two of them also change the Day Master from Eul Wood to Byeong Fire. The chart's reference point moves.
Nothing in the birth record resolves this. The recorded time is 23:20; the disagreement is about what to do with it. This is why Kairo attaches a flag to the chart for births in this window instead of presenting one pillar as the answer, and it is the strongest argument for finding an exact recorded time rather than a family estimate of "around eleven."
What the Hour Pillar is read for
Traditionally the Hour Pillar is read as the last of the four positions: what comes after the established self, in time and in output. The associations that recur across Korean texts are later life, children, the fruits of work, and private aims that the earlier pillars do not show. Its branch also contains hidden stems that can root a Day Master or supply an element absent from the other six characters. And the Hour Pillar is one of two inputs, with the solar-term distance, to the exact start age of the daewoon (대운, 大運) sequence.
Those are the traditional positions. None of them is a fact encoded in a clock, and the site's article on why birth time matters covers the boundaries of what a fourth column can carry.
Pattern questions
- Was your birth time recorded, or reconstructed later from memory? "Around eleven" spans two hour branches under either table.
- Does your recorded time fall within 30 minutes after an odd hour? If so, ask any service you use which table it applied.
- Were you born between 23:00 and midnight? Then your Day Master itself depends on a convention, and a service that has not flagged this has chosen for you.
- Have you compared two calculators and found different hour pillars? Check the 30-minute shift and the day boundary before assuming either is wrong.
What this can and cannot tell you
Under a stated convention the Hour Pillar has one right answer, and this article gives the two tables needed to compute it by hand. What it cannot do is settle which convention is correct, because the tradition never did. The hour associations in the section above are interpretive, and Saju is an interpretive tradition, not a validated predictive method; Kairo does not present it as one. A missing birth time removes this pillar and nothing else, and Kairo leaves it out rather than guessing.