Solar Terms or Lunar Calendar: What Does Bazi Use?
Understand why Bazi uses solar-term boundaries for year and month pillars while lunar dates remain useful for date conversion and display.
Overview
For the OpenFate Bazi calculation contract, the Year Pillar changes at the exact Li Chun instant and the Month Pillar changes at the twelve Jie boundaries. The Chinese lunar calendar remains useful for entering, converting and displaying dates, but lunar months do not directly determine Bazi months. Other traditions may declare another convention; the result must name it rather than blending systems.
At a glance
- Year rule
- Exact Li Chun in the OpenFate contract
- Month rule
- Exact twelve Jie boundaries
- Lunar-calendar role
- Input conversion, display and traditional date reference
- Not equivalent
- Lunar month number and Bazi month branch
- Required disclosure
- Calendar, time zone and boundary convention
- Boundary
- Do not merge incompatible conventions
Bazi month structure follows solar terms in this system
The twelve Bazi months begin at the twelve Jie, starting with Yin month at Li Chun. The sequence therefore follows solar longitude rather than the first day of each lunar month.
The same principle makes Li Chun the Year-Pillar boundary in the OpenFate engine. Exact instants matter for births close to a transition.
- Use Jie, not every one of the twenty-four terms, for month changes.
- Compute the event for the birth year and time zone.
The lunar calendar still has a valid, narrower role
Users may know a birth date only in the Chinese calendar. That date can be converted into a civil date once leap-month status and other inputs are clear.
The calendar is also useful for festivals and traditional date display. Those roles do not turn its month numbering into the Bazi Month Pillar.
- Validate leap-month input.
- Retain both original and converted date labels.
Different conventions should be compared, not silently mixed
Some practitioners use Lunar New Year for a Gan-Zhi year label or apply a different day boundary. Such conventions can be documented as alternate systems.
A calculator should apply one declared contract consistently. If it offers alternatives, each output needs its own label and provenance.
- One result, one coherent convention.
- Show sensitivity when an alternate rule changes a pillar.
Neither convention proves an interpretation
A calendar rule determines symbols; it does not prove a life event, trait or outcome. Interpretation begins only after the calculation is stable.
When the recorded time is uncertain, preserve that uncertainty. Do not manufacture precision or choose a boundary rule to make a chart look more favorable.
- Separate calculation from interpretation.
- No preferred chart selection after the fact.
Sources and editorial basis
- Hong Kong Observatory: The Chinese Agricultural Calendar香港天文台曆表同時列出公曆、農曆、節氣與干支，顯示日期表示和節氣邊界是不同資料層An official overview of how the Chinese agricultural calendar combines solar and lunar cycles, including months, leap months, and solar terms.
- Hong Kong Observatory: The 24 Solar Terms香港天文台二十四節氣說明與交節資料：太陽位置提供年、月邊界所需的時間點An official astronomical explanation of the 24 solar terms, their 15-degree divisions along the ecliptic, and the alternating major and minor terms.
- OpenFate Editorial Methodology產品以立春與節令處理八字年柱、月柱邊界，農曆日期只作日期對照；邊界附近必須使用交節時刻OpenFate separates deterministic chart calculation, traditional interpretation, and modern editorial explanation.