The short answer
Traditional Chinese timekeeping divides the day into twelve double-hours (时辰, shichen), each two hours long and named after one of the twelve earthly branches. Your BaZi hour pillar's branch is simply the shichen you were born in; its stem is then derived from the day stem by a fixed rule. That is why a BaZi reading needs your birth *hour*, not just the date.
The full shichen table
| Shichen | Branch | Clock time |
|---|
|---------|--------|-----------|
| 子 Zi | Rat | 23:00 – 00:59 |
|---|---|---|
| 丑 Chou | Ox | 01:00 – 02:59 |
| 寅 Yin | Tiger | 03:00 – 04:59 |
| 卯 Mao | Rabbit | 05:00 – 06:59 |
| 辰 Chen | Dragon | 07:00 – 08:59 |
| 巳 Si | Snake | 09:00 – 10:59 |
| 午 Wu | Horse | 11:00 – 12:59 |
| 未 Wei | Goat | 13:00 – 14:59 |
| 申 Shen | Monkey | 15:00 – 16:59 |
| 酉 You | Rooster | 17:00 – 18:59 |
| 戌 Xu | Dog | 19:00 – 20:59 |
| 亥 Hai | Pig | 21:00 – 22:59 |
Note the boundaries: a birth at 08:59 is a Dragon-hour birth; 09:00 is already Snake hour.
The midnight problem: early vs late Zi hour
Zi hour straddles midnight (23:00–00:59), which raises a classical question: does a 23:30 birth belong to today's day pillar or tomorrow's?
- Late-Zi school (晚子时): 23:00–23:59 keeps today's day pillar but uses the next day's Zi-hour stem.
- Mainstream modern convention: the day pillar changes at 23:00 — a 23:30 birth on the 5th is charted as the 6th's Zi hour.
Practitioners differ; what matters is that your calculator states which rule it applies. Our free BaZi calculator follows the late-Zi convention: a birth at 23:00–23:59 keeps that day's day pillar (verified against the engine directly — a 23:30 birth on January 5, 2026 charts as the 己卯 day, not the next day's 庚辰). Boundary births are worth reading under both rules.
How the hour stem is derived (the "Five Rats" rule)
The hour pillar's heavenly stem is not independent — it follows from the day stem via the traditional 五鼠遁 ("Five Rats escaping") formula. Days whose stem is 甲 or 己 start their Zi hour with 甲; 乙/庚 days start with 丙; 丙/辛 days with 戊; 丁/壬 days with 庚; 戊/癸 days with 壬. The remaining eleven hours then follow the stem cycle in order.
So a 甲-day birth in Mao hour (05:00–06:59) has the hour pillar 丁卯 — counted four steps from 甲子.
Why the hour matters so much
The hour pillar governs the chart's "time palace" — classically read for later life, children, and the day-to-day expression of the day master. Two people born on the same day but in different shichen can have meaningfully different charts: a different hour branch changes the five-element balance, and a different hour stem changes which "ten god" sits beside the day master.
If you don't know your birth hour, a chart can still be read from three pillars — but any statement that depends on the hour pillar should be treated as undetermined rather than guessed.