The BaZi hour pillar and your birth time
The hour pillar is the most fragile of the four, because it rests on a two-hour window and a clock reading. Here is how the window is set, where the boundaries fall, and why the last hour of the day causes an argument that has never been settled.
Twelve branches, twelve windows
The day is divided into twelve two-hour branches, and the branch your birth moment falls into becomes the lower character of your hour pillar. The upper character follows from the day stem, so once the window is fixed the whole pillar is fixed.
| Branch | Animal | Window (true solar time) |
|---|---|---|
| Zi | Rat | 23:00 - 00:59 |
| Chou | Ox | 01:00 - 02:59 |
| Yin | Tiger | 03:00 - 04:59 |
| Mao | Rabbit | 05:00 - 06:59 |
| Chen | Dragon | 07:00 - 08:59 |
| Si | Snake | 09:00 - 10:59 |
| Wu | Horse | 11:00 - 12:59 |
| Wei | Goat | 13:00 - 14:59 |
| Shen | Monkey | 15:00 - 16:59 |
| You | Rooster | 17:00 - 18:59 |
| Xu | Dog | 19:00 - 20:59 |
| Hai | Pig | 21:00 - 22:59 |
These are solar-time windows. On a clock they shift, sometimes by well over an hour, which is the subject of the rest of this page. The BaZi basics page covers the branches and their elements.
The midnight problem: the late zi hour
The zi branch opens at 23:00, one hour before the calendar day ends. A birth at 23:30 therefore belongs to the new two-hour window while the date has not yet turned over. Two conventions exist:
- The day pillar stays on the birth date. That 23:00 to midnight hour is treated as the tail of the old day.
- The day pillar rolls to the next day. Because the zi branch has already begun, the new day is treated as having started.
The choice is not cosmetic. It changes the day pillar, the day master, and the hour pillar that is derived from the day stem:
| True solar time | Day pillar, stays | Day pillar, rolls |
|---|---|---|
| 1988-06-01 23:30 | 丁亥 | 戊子 |
Both settings are in the calculator. Pick the one your existing readings use, and if you have never had a reading, pick one and stay consistent.
What moves your hour pillar
Before the convention even comes into play, the window itself depends on getting the birth moment right. Three corrections do most of the damage.
| Birth (clock) | Place | True solar time | Hour pillar on the clock | Hour pillar corrected |
|---|---|---|---|---|
| 1990-08-01 09:00 | Shanghai | 07:59 | 丁巳 | 丙辰 |
| 1988-06-01 23:30 | Beijing | 22:17 | 壬子 | 辛亥 |
The first row is summer time: the clock was an hour ahead of standard time and the birthplace sat west of the zone meridian, so the solar time fell back across the boundary into the previous window. The second row is more dramatic still — a birth the clock places at 23:30, in the zi hour, is moved by the sun to 22:17, in the hai hour, before the midnight question can even arise.
How to pin down your own hour pillar
- Get your birth time from a record rather than from memory. A birth certificate, a hospital note or a family diary will beat recollection.
- Enter the clock reading exactly, including any daylight saving that was in force.
- Read the true solar time, then check how close it is to a branch boundary. If it is within a few minutes, treat both neighbouring hour pillars as candidates and say so.
- Decide the late zi convention before you read the chart, not after seeing which answer you prefer.
The true solar time page explains where each correction comes from and how large it can be.
Frequently asked questions
What decides the BaZi hour pillar?
The two-hour branch window your birth moment falls into, measured in true solar time, together with the day stem. The branch comes from the window; the stem is derived from the day stem by a fixed rule. Change the window and both characters of the hour pillar can change.
What is the late zi hour?
The zi branch begins at 23:00, so births between 23:00 and midnight sit at the very start of the new two-hour window while still belonging to the calendar day that is ending. Schools disagree about whether the day pillar has already turned over at that point, which is the late zi hour debate.
Which convention should I use?
The one your existing readings use. If you have had a chart read before and it matched your experience, keep that convention so the charts stay comparable. The calculator lets you switch, and it shows the day pillar either way.
Can daylight saving really change my hour pillar?
Yes. In Shanghai on 1 August 1990 the clock read 09:00 during summer time, but the sun said 07:59, which moves the birth from the si window into the chen window. Same clock, same place, different hour pillar.
My birth time is close to a boundary. What should I do?
Read both charts. If the true solar time is within a few minutes of a boundary, say so honestly and treat the neighbouring hour pillar as a live possibility. Picking one silently would be false precision.
How do I get my true solar time?
Enter your birth city, date and clock time in the calculator. It applies the timezone rule, the longitude correction and the equation of time, then shows the solar time and the hour pillar that follows from it.