Your Ba Zi month pillar depends on the 12 primary solar terms, not on the lunar month shown in a Chinese calendar. In Auspio's method, the month changes only when the Sun reaches the next primary 节 (jie, primary solar term), so a birth before and after that instant can belong to different Ba Zi months.
That distinction is easy to miss because Chinese calendars often place three things close together: Gregorian dates, lunar months, and 节气 (solar terms). Ba Zi does not use all of them in the same way.
For date selection and chart work, the lunar month matters in some parts of the calendar. But the Ba Zi month pillar is a solar-term boundary problem. It asks: which primary solar term has already arrived?
Auspio's calculation method is documented on the permanent How Auspio Calculates Your Dates page. This article zooms in on one part of it: why the month pillar follows 节气 rather than the lunar calendar.
The month pillar is a solar month, not a lunar month
A lunar month begins at a new moon. A Ba Zi month begins at a primary solar term, called 节 (jie, primary solar term).
Those are different clocks. The lunar calendar follows the Moon's phase cycle. The solar terms divide the Sun's apparent path through the year into 24 points, spaced 15 degrees apart along the ecliptic. Auspio's source code stores each solar term as the UTC instant when the Sun reaches that target apparent longitude.
Only 12 of those 24 solar terms begin a Ba Zi month. The other 12 are 中气 (zhongqi, major terms), which matter elsewhere in Chinese calendar logic but do not start the Ba Zi month pillar.
The 12 primary solar terms used for Ba Zi months
In Auspio's implementation, these 12 节 begin the Ba Zi months:
| Primary solar term | Chinese | Ba Zi month branch begun |
|---|---|---|
| Xiao Han | 小寒 | 丑 month |
| Li Chun | 立春 | 寅 month |
| Jing Zhe | 惊蛰 | 卯 month |
| Qing Ming | 清明 | 辰 month |
| Li Xia | 立夏 | 巳 month |
| Mang Zhong | 芒种 | 午 month |
| Xiao Shu | 小暑 | 未 month |
| Li Qiu | 立秋 | 申 month |
| Bai Lu | 白露 | 酉 month |
| Han Lu | 寒露 | 戌 month |
| Li Dong | 立冬 | 亥 month |
| Da Xue | 大雪 | 子 month |
The pattern is not arbitrary. 立春 (Li Chun, Start of Spring) begins the 寅 month, and each following primary solar term advances the month branch by one step.
Once the month branch is known, the month stem is derived from the year's Heavenly Stem using the 五虎遁 (Five Tigers) rule. That gives the full month pillar: one Heavenly Stem plus one Earthly Branch.
The Li Chun boundary example
Imagine two siblings born around 立春 (Li Chun, Start of Spring). One is born at 11 pm before the Li Chun instant. The other is born at 1 am after it.
If Li Chun falls between those two births, they do not share the same Ba Zi month boundary. The earlier birth still belongs to the previous solar month. The later birth belongs to the 寅 month that begins at Li Chun.
This is also why "same calendar day" is not always precise enough near a boundary. The important question is not only which date appears on the calendar, but whether the solar-term instant has already occurred.
How Auspio handles the boundary
Auspio computes the 24 solar terms astronomically for the calendar year and caches them. The solar-term service then asks for the most recent primary term at or before the birth instant. That answer determines the Ba Zi month.
The app does not decide the month pillar from the lunar month number. It also does not treat all 24 solar terms as month changes. The source rule is narrower: find the latest primary 节, then use the month branch that primary term begins.
For example, the source model maps Li Chun at 315 degrees of apparent solar longitude to the 寅 month, Jing Zhe at 345 degrees to the 卯 month, Qing Ming at 15 degrees to the 辰 month, and so on through the 12 branches.
Why this matters for date selection
The month pillar is part of the chart context used in Ba Zi. If the month pillar changes, the reading can change with it.
That does not mean every person born near a solar term should panic over their chart. It means the boundary is a real calculation point. If your birth time is close to a primary solar term, the exact instant matters more than a broad statement like "born in the lunar first month."
For auspicious-date selection, the same solar-month logic also appears in the Twelve Day Officers (建除十二神). Auspio's Day Officers service uses the jie-based month branch when it assigns the officer for a day, so the officer sequence shifts at the primary solar terms too.
What the lunar month still does
None of this makes the lunar calendar irrelevant. Lunar months still matter in Chinese calendrical practice, and Auspio uses lunar-month cautions in its date-scoring layer.
The narrower point is this: the Ba Zi month pillar is not the lunar month. It is tied to the Sun's seasonal markers, specifically the 12 primary solar terms.
Does the Ba Zi year also start at a solar term?
Yes. In Auspio's implementation, the Ba Zi year begins at 立春 (Li Chun, Start of Spring), not on January 1 and not automatically on Chinese New Year. The year pillar belongs to the calendar year of the most recent Li Chun at or before the instant being calculated.
Are all 24 solar terms used as Ba Zi month boundaries?
No. Auspio distinguishes the 12 primary terms, 节 (jie), from the 12 major terms, 中气 (zhongqi). The primary terms begin Ba Zi months; the major terms do not.
Can two people born on the same date have different month pillars?
Yes, if a primary solar-term instant falls between their birth times. Around Li Chun, for example, one birth can still belong to the previous solar month while a later birth belongs to the 寅 month that starts after Li Chun.