Five Rats Method (五鼠遁): Calculate the Hour Stem
Use the Five Rats Method to derive the Bazi Hour Stem from the effective Day Stem and Hour Branch, with the complete five-pair starting table.
Overview
The Five Rats Method (五鼠遁) derives the Hour Stem after the effective Day Stem and Hour Branch are known. At Zi hour, Jia or Ji days begin with Jia; Yi or Geng with Bing; Bing or Xin with Wu; Ding or Ren with Geng; and Wu or Gui with Ren. The Hour Stem then advances one step for every branch hour. A day-boundary choice can change the Day Stem near 23:00 or midnight, so that choice must be resolved before this table is applied.
At a glance
- Jia or Ji day
- Jia-Zi begins the hour sequence
- Yi or Geng day
- Bing-Zi begins the hour sequence
- Bing or Xin day
- Wu-Zi begins the hour sequence
- Ding or Ren day
- Geng-Zi begins the hour sequence
- Wu or Gui day
- Ren-Zi begins the hour sequence
- Required input
- Effective Day Stem under the selected day boundary
The method needs two already-validated inputs
First assign the Hour Branch from the normalized birth time. Then determine the effective Day Stem under the declared 23:00 or midnight day-boundary rule.
Five Rats does not calculate the birth time, time zone or Day Pillar. Applying it before those inputs are stable can produce a precise-looking but wrong Hour Pillar.
- Resolve civil time and any solar-time setting first.
- Store the day-boundary mode with the result.
Five Day-Stem pairs set the Zi-hour starting stem
Jia–Ji start at Jia-Zi, Yi–Geng at Bing-Zi, Bing–Xin at Wu-Zi, Ding–Ren at Geng-Zi, and Wu–Gui at Ren-Zi.
The pairing covers all ten Day Stems exactly once. Similar Five-Phase names are not a substitute for the fixed table.
- Match the exact Day Stem pair.
- Use Zi as the fixed starting branch.
Advance the stem once for each branch hour
After the Zi-hour stem is selected, Chou takes the next Heavenly Stem, Yin the next, and so on through Hai. The stem cycle wraps from Gui back to Jia.
The final Hour Pillar joins that derived stem to the already assigned Hour Branch. Showing both derivation inputs makes the result auditable.
- Advance stems and branches in lockstep.
- Verify the ten-stem wraparound.
The formula is not an interpretation or a birth-time guess
Five Rats deterministically maps validated symbols; it does not reveal personality, children, career or future events by itself.
If the birth hour is unknown, the system should omit or compare candidate Hour Pillars. It must not invent a time merely to complete the formula.
- No life claim from the Hour Stem alone.
- No fabricated Hour Pillar for missing input.
Sources and editorial basis
- 《三命通會》 (Sanming Tonghui)《三命通會》日上起時法：甲己還加甲、乙庚丙作初等五組起時規則，從子時順推時干A historical compilation covering stems and branches, Ten Gods, Nayin, Shen Sha, and multiple classical pattern systems.
- Hong Kong Observatory: Heavenly Stems and Earthly Branches香港天文台天干地支資料提供十干、十二支及六十干支的固定次序，作為時干順推的現代對照An official modern reference for the canonical stem and branch orders, the sexagenary cycle, and calendrical use.
- OpenFate Editorial Methodologytests/bazi/test-pillar-invariants.ts 驗證五組日干起子時、十二時辰順推與日界後重新取日干；未知時辰不補造時柱OpenFate separates deterministic chart calculation, traditional interpretation, and modern editorial explanation.