Four pillars: the year, the solar month, and the day boundary
The four pillars give the year, month, day, and hour one stem and one branch each. The year changes at the Beginning of Spring, the month follows twelve solar terms, the day defaults to 23:00, and each hour-branch lasts two clock hours. True solar time, when on, can move the hour pillar first.
Editorial review: 2026-09-22 · Reviewed by fatequery
The year pillar changes at the Beginning of Spring
1 January is not the year boundary. Before the Beginning of Spring the previous year’s pair still stands. In 1990 that term falls in February, so a June chart already uses Geng-Wu, not Ji-Si.
After the year is set, the month, day, and hour are cut on their own lines. The year stem is not the day master. The day master is always the day stem.
The month follows twelve solar terms
The month pillar does not follow the civil month. It follows the stretch between two solar terms. Grain in Ear to Slight Heat is the Wu month. 20:00 on 30 June 1990 sits after the Summer Solstice and before Slight Heat, so the month is Ren-Wu.
On the day a term changes, compare the clock with the term’s instant before fixing the month. Two charts that disagree around a term should be compared on this cut first.
The day boundary defaults to 23:00
Two day cuts are both legitimate, and the chart has to name which one it used. The default changes the day at 23:00, so 23:00–23:59 takes the next day’s pillar. Switched to 00:00, that hour still belongs to the same day.
The illustration is 20:00, and both cuts give Bing-Yin. To see the boundary move the day pillar, the clock has to fall after 23:00.
Each hour-branch lasts two clock hours
19:00–20:59 is the Xu branch. At 20:00 the hour pillar is Wu-Xu. If the birth clock is unknown, keep the year, month, and day, and do not invent an hour.
Ten gods that hang on the hour wait until that pillar exists. An empty hour leaves those gods empty.
True solar time can move the hour pillar
True solar time shifts the clock by the birthplace longitude. It is on by default and can be turned off. If the corrected instant crosses an hour boundary, the hour pillar changes. If it crosses the day boundary, the day pillar changes too.
This illustration hands 20:00 to the engine with no longitude correction, so the hour stays Wu-Xu. After true solar time is turned on, check whether the hour pillar moved before reading the ten gods.
Illustration: one birth, four pillars
Male, 20:00 on 30 June 1990, day boundary 23:00. The pillars are Geng-Wu, Ren-Wu, Bing-Yin, and Wu-Xu. The day master is the day stem Bing, fire. The year stem Geng, the month stem Ren, and the hour stem Wu are not the day master.
What BaZi looks at is on what BaZi is. Once the date, time, and place are in, continue at the chart page.
Common mistakes
- Changing the year pillar on 1 January.
- Replacing the solar-term month with the civil month.
- Filling in an hour pillar when the hour was unknown.
Use it in fatequery
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Branch relations, day-master combinations and element balance
Compare two BaZi charts →Frequently asked questions
Is the Beginning of Spring the lunar new year?
No. The year pillar follows the Beginning of Spring. The lunar new year does not change it.
Do 23:00 and 00:00 give this illustration different day pillars?
This clock is 20:00, so both cuts give Bing-Yin. The day pillar moves only when the clock falls between 23:00 and 23:59.
Does true solar time always change the hour pillar?
Only when the corrected instant crosses an hour boundary.
Sources and editorial basis
- 1. Yuanhai Ziping, transmitted text
Traditional names for the four pillars, the ten gods, and the month command. One text does not turn an illustration into something that has already happened.
- 2. Sanming Tonghui, transmitted text
Traditional names for the direct and indirect gods, the twelve stages, and nayin. The tables do not collapse into one luck sentence.
- 3. fatequery rule snapshot
The year changes at the Beginning of Spring, the month follows the twelve solar terms, hidden stems are weighted 1 / 0.5 / 0.3, and the day boundary defaults to 23:00, switchable to 00:00. It supports only the fields the chart prints.