The problem: one Chinese hour, two calendar days
Zi hour (子时) runs from 11:00 PM to 1:00 AM and is the only Chinese hour that straddles midnight. Whether 11:00 PM belongs to today's chart or tomorrow's has been debated in Chinese metaphysics for centuries. For the underlying framework, see How to Read a Bazi Chart: A Step-by-Step Guide for Beginners.
In the twelve-branch system, each Chinese hour spans two clock hours. Zi (子) is the first branch and corresponds to midnight — but not at midnight. It starts an hour before midnight (23:00) and ends an hour after (01:00). This means that a birth at 11:30 PM on March 5 and a birth at 12:30 AM on March 6 fall in the same Chinese hour, but on different calendar days.
The question is: which day pillar does the 11:00 PM birth get?
Two classical schools
The disagreement is old enough that both positions have names.
Early Zi (早子时) — the day changes at midnight
In this school, the period from 23:00 to 00:00 is called "early Zi" and still belongs to the current calendar day. The day pillar changes at midnight (00:00), not at the start of Zi hour. A birth at 23:30 on March 5 uses March 5's day pillar, and a birth at 00:30 on March 6 uses March 6's day pillar.
Both births share the Zi hour branch, but their day pillars — and therefore their Day Masters — may be different.
Late Zi (夜子时) — the day changes when Zi hour begins
In this school, the entire Zi hour belongs to the next day. A birth at 23:00 on March 5 already uses March 6's day pillar. The logic is that since Zi is the first branch of the new day cycle, the day must already have turned when Zi begins.
Under this rule, every birth from 23:00 onward gets the next day's stem-branch pair for the day pillar.
What this calculator does
This calculator follows the Early Zi convention: the day pillar changes at midnight, not at 23:00. A birth at 11 PM keeps today's day pillar. The method is easier to compare with Solar Terms and Month Pillars: why the month follows the sun, not the calendar.
The 23:00–00:00 window gets the Zi hour branch (子) but stays on the current day's stem-branch pair for the day pillar. The 00:00–01:00 window also gets the Zi hour branch, but uses the next day's day pillar since midnight has passed.
This matches the convention used in most modern Bazi software and the majority of practitioners trained in the Zi Ping (子平) tradition.
The hour stem correction at 23:00
The hour stem at 23:00 requires a special correction. The calculator re-derives it from the current day's stem using the Five-Rat Run formula. This point connects directly with Bazi Without a Birth Hour: what three pillars can still tell you.
Here is the technical issue: the astronomical library that computes stem-branch cycles uses a convention where the hour stem at 23:00 is derived from the next day's day stem. But under the Early Zi convention, the day pillar displayed is still today's day stem. If the calculator used the next day's hour stem alongside today's day stem, the hour pillar would be inconsistent with the day pillar shown on screen.
To fix this, the calculator applies the Five-Rat Run (五鼠遁, wǔ shǔ dùn) — the same formula that maps any day stem to its Zi-hour stem:
| Day stem | Zi hour stem | Zi hour pillar |
|---|---|---|
| Jia 甲 or Ji 己 | Jia 甲 | 甲子 |
| Yi 乙 or Geng 庚 | Bing 丙 | 丙子 |
| Bing 丙 or Xin 辛 | Wu 戊 | 戊子 |
| Ding 丁 or Ren 壬 | Geng 庚 | 庚子 |
| Wu 戊 or Gui 癸 | Ren 壬 | 壬子 |
When the true solar time falls in the 23:00 hour, the calculator takes the day stem of the displayed day pillar, looks up the corresponding Zi-hour stem from this table, and uses that as the hour stem. This ensures that the hour pillar is always derived from the same day stem that appears in the day pillar — no mismatch between the two.
What this means in practice
If you were born between 11:00 PM and midnight:
- Your hour branch is Zi (子) — the first branch in the cycle.
- Your day pillar is today's, not tomorrow's.
- Your hour stem is derived from today's day stem via the Five-Rat Run, not from tomorrow's day stem.
- All ten-god labels on the hour pillar are computed relative to your Day Master (today's day stem).
If you were born between midnight and 1:00 AM, your hour branch is also Zi, but your day pillar has already moved to the next day. No special correction is needed because the day and hour are naturally in sync.
Sifu Xion · 8th-generation Form School practitioner · M.Sc. Data Science, Shanghai.
Cast your own chart with the free Bazi calculator to see which configuration you carry. If you would rather have him read the whole chart himself, the full written reading covers all ten gods, your luck pillars and what to do with them.