Month Pillar Calculation in Bazi
Follow the reproducible Month-Pillar sequence: normalize the birth instant, locate the Jie boundary, assign the month branch, then apply Five Tigers.
Overview
The Bazi Month Pillar is calculated in two linked steps. First, locate the normalized birth instant between exact Jie boundaries to obtain the Month Branch from Yin through Chou. Second, use the Bazi Year Stem and the Five Tigers rule to assign the Month Stem, advancing one stem per month. A lunar month number alone cannot produce the Month Pillar.
At a glance
- Step one
- Normalize the birth instant and time zone
- Step two
- Find the surrounding exact Jie boundary
- Month Branch
- Yin at Li Chun through Chou at Xiao Han
- Month Stem input
- Bazi Year Stem
- Month Stem rule
- Five Tigers starting at Yin month
- Invalid shortcut
- Lunar month number equals Month Branch
Start with a definite birth instant and Bazi Year Stem
The birth time is resolved through its historical time zone before comparison with the solar-term table. The selected true-solar and boundary settings must remain visible.
The Year Stem must already be calculated under the declared Li Chun convention because it controls the first Month Stem.
- Do not use a zodiac-year label as an unverified Year Stem.
- Preserve calculation settings with the result.
Locate the Month Branch from the twelve Jie intervals
A birth from Li Chun to just before Jing Zhe is in Yin month; the branch advances at each following Jie until Xiao Han begins Chou month.
The comparison uses exact instants. Two births on the same civil date can receive different Month Branches if a Jie occurs between them.
- Compare instants, not date strings.
- Use Jie rather than lunar-month starts.
Apply Five Tigers to the Year Stem and advance in order
The Five Tigers mapping supplies the stem of the Yin month for each Year-Stem pair. The stem then advances one place for every following Month Branch.
The result must pair the derived stem with the already determined branch. A lookup should expose both inputs so it can be audited.
- Start at the correct Yin-month stem.
- Advance exactly once per Month Branch.
The Month Pillar is calculation data, not a complete reading
The pillar contributes season, stem, branch and hidden-stem evidence, but it does not alone determine career, parents, personality or fate.
When uncertain input crosses a Jie, calculate both candidates and explain the downstream differences instead of concealing the ambiguity.
- No life conclusion from the Month Pillar alone.
- No false precision near a boundary.
Sources and editorial basis
- Hong Kong Observatory: The 24 Solar Terms香港天文台交節資料提供月支切換所需的節令時刻，不能以農曆初一或固定公曆日代替An official astronomical explanation of the 24 solar terms, their 15-degree divisions along the ecliptic, and the alternating major and minor terms.
- 《三命通會》 (Sanming Tonghui)《三命通會》論月令與年上起月：先依節令定月支，再由年干按五虎遁推月干A historical compilation covering stems and branches, Ten Gods, Nayin, Shen Sha, and multiple classical pattern systems.
- OpenFate Editorial Methodologytests/bazi/test-pillar-invariants.ts 驗證節令月界、寅月起點與五虎遁月干序列，輸入時區在界線判定前完成OpenFate separates deterministic chart calculation, traditional interpretation, and modern editorial explanation.