The month pillar follows solar terms, not the calendar
In Bazi, the month does not change on the first of the calendar month. It changes at a solar term — a precise astronomical moment when the sun crosses a specific ecliptic longitude. For the underlying framework, see How to Read a Bazi Chart: A Step-by-Step Guide for Beginners.
This surprises people who expect "born in March" to mean the same thing in Bazi as it does on the Gregorian calendar. A person born on March 3 and a person born on March 8 can end up in different Bazi months, because the solar term boundary (Jing Zhe, 惊蛰) typically falls around March 5 or 6.
The 24 solar terms and the 12 that set month boundaries
The Chinese astronomical calendar divides the year into 24 solar terms (节气, jié qì). These come in alternating pairs: a jié (节, "node") followed by a qì (气, "breath"). In Bazi, only the 12 jié terms — the odd-numbered ones — mark the start of a new month. The 12 qì terms fall in the middle of each month but do not trigger a pillar change.
| Bazi month | Branch | Starting jié (节) | Typical Gregorian date |
|---|---|---|---|
| Month 1 | Yin 寅 | Li Chun 立春 (Start of Spring) | Feb 3–5 |
| Month 2 | Mao 卯 | Jing Zhe 惊蛰 (Awakening of Insects) | Mar 5–7 |
| Month 3 | Chen 辰 | Qing Ming 清明 (Clear Brightness) | Apr 4–6 |
| Month 4 | Si 巳 | Li Xia 立夏 (Start of Summer) | May 5–7 |
| Month 5 | Wu 午 | Mang Zhong 芒种 (Grain in Ear) | Jun 5–7 |
| Month 6 | Wei 未 | Xiao Shu 小暑 (Minor Heat) | Jul 6–8 |
| Month 7 | Shen 申 | Li Qiu 立秋 (Start of Autumn) | Aug 7–9 |
| Month 8 | You 酉 | Bai Lu 白露 (White Dew) | Sep 7–9 |
| Month 9 | Xu 戌 | Han Lu 寒露 (Cold Dew) | Oct 8–9 |
| Month 10 | Hai 亥 | Li Dong 立冬 (Start of Winter) | Nov 7–8 |
| Month 11 | Zi 子 | Da Xue 大雪 (Major Snow) | Dec 6–8 |
| Month 12 | Chou 丑 | Xiao Han 小寒 (Minor Cold) | Jan 5–7 |
The "typical Gregorian date" column shows the range over which each jié usually falls. The exact moment varies by year because the solar terms are defined astronomically, not by calendar arithmetic.
The year pillar also changes at a solar term
The Bazi year does not start on January 1. It starts at Li Chun (立春), the same solar term that starts Month 1. The method is easier to compare with Historical Time Zones: why the timezone at your birth may differ from today's.
A person born on January 28 in a year when Li Chun falls on February 4 still belongs to the previous Bazi year. Their year pillar carries the previous year's stem-branch pair, and their Chinese zodiac animal is the previous year's animal. This is one of the most common sources of confusion: someone born in late January or early February may have a different zodiac animal in Bazi than the one the Lunar New Year calendar assigns, because the Lunar New Year and Li Chun are different events that fall on different dates.
Why solar terms, not lunar months
The traditional Chinese lunisolar calendar uses the moon for its month structure — each month starts at the new moon. Bazi does not follow that structure for the month pillar. Instead, it follows the solar terms, which track the sun's ecliptic longitude in 15-degree steps. This point connects directly with The Midnight Hour (Zi): how the calculator handles births between 11 PM and 1 AM.
The reason is that the month pillar is supposed to encode the season — the quality of qi (气) that the sun is producing at that moment. The moon cycles every 29.5 days and drifts relative to the seasons, which is why the lunisolar calendar needs occasional leap months (闰月) to stay aligned. The solar terms never need a leap month because they are tied directly to the sun's position. A month pillar that followed the lunar calendar would sometimes map to the wrong season.
What happens at the boundary
Solar terms are computed to the minute. On the day a jié falls, a birth before the exact jié moment belongs to the previous month; a birth after it belongs to the new month. The calculator uses astronomical tables (via the lunisolar library) to determine the exact moment of each jié for the birth year, then compares the birth's true solar time against it.
If your birth date is within a day of a jié boundary and you are not sure of your exact birth time, the month pillar could go either way. In that case, running the calculator with two slightly different times will show whether the month pillar changes.
The month stem follows the year stem
The month branch is fixed by the solar term (as shown in the table above). The month stem is then derived from the year stem using a fixed cycle called the Five-Tiger Run (五虎遁, wǔ hǔ dùn). This means two people born in the same solar-term month but in different years will share the same month branch but may have different month stems, because their year stems differ.
The calculator handles this derivation automatically. You do not need to look up the Five-Tiger Run table yourself.
Sifu Xion · 8th-generation Form School practitioner · M.Sc. Data Science, Shanghai.
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