How a BaZi Chart Is Built (Solar Terms, Hour, Unknown Birth Time)

A BaZi chart is built by turning a birth instant into four stem-and-branch pillars. The year waits for Start of Spring. The month waits for the solar term. The day follows the sexagenary day count. The hour follows the double-hour on the casting clock, with a special rule at midnight. The result is guidance: a professional reading of that calendar, not a promise about what the life will earn or become.

Key facts

The month pillar changes when the sun reaches the next solar term. It does not change on the first of a Western month or at the lunar new moon.
The year pillar used here changes at Start of Spring, not on 1 January and not at lunar new year.
Typed time is civil time in the place’s timezone, including historical daylight saving. With a longitude, the pillars use apparent solar time.
With no city, the chart uses a fixed UTC+8 reading. It does not apply a fake Beijing longitude correction.
Unknown hour: the hour pillar is blank, the day pillar is taken from local noon, and luck cycles are not counted.
From 23:00 to 23:59, late zi advances the day pillar. From 00:00 to 00:59, early zi keeps the new day. Both hours are the zi branch.

The order of the cast

The form takes a date, an optional time, an optional place, and, for the full chart’s luck layer, sex at birth. Those fields are converted to an instant, then to a solar clock, then to four pillars through the sexagenary rules. The same fields return the same pillars. If you change the city or the hour, you are changing the instant, so the pillars are allowed to change.

Start of Spring is the first gate. Before that term, the year pillar still belongs to the previous solar year even if the Western calendar already says January or early February. After the term, the year pillar is the new one. The term falls on different clock times each year. A birth on the morning of the term and a birth on the evening of the term can stand on different year pillars when the term itself falls between them.

The month gate is the next solar term, not the next page of a phone calendar. The traditional solar months begin at the terms often called the jie: Start of Spring, Awakening of Insects, Clear and Bright, Start of Summer, Grain in Ear, Slight Heat, Start of Autumn, White Dew, Cold Dew, Start of Winter, Great Snow, and Lesser Cold. Two birthdays both labeled early February can sit on different month pillars if a solar term falls between them. The lunar new moon can fall in the middle of a solar month and the month pillar will not move.

Why the Western month is the wrong boundary

A person born on 4 February and a person born on 5 February are both “February” on a civil form. In a year when Start of Spring falls on the 4th in the afternoon, one of those births can still be in the old year and the old month, and the other can be in the new year and the new month. The chart prints the pillars that the term supports. It does not average them to protect the civil label.

Why the lunar new moon is also the wrong boundary

Lunar new year is a real festival and a real calendar. It is not the switch this chart uses for the year branch or the month pillar. A birth on lunar new year’s day still takes its year from Start of Spring and its month from the solar term in force. Mixing the two calendars is a common source of “the website disagrees with my aunt’s handwritten chart.” Often the handwritten chart and the website are both consistent with different switches. This house states its switch: solar terms, and Start of Spring for the year.

Civil time, then solar time

You type the time as people spoke it in that place: the civil clock. The site reads it in the IANA timezone of the city, so a region that observed daylight saving in the year of the birth is handled by that zone’s history. China between 1986 and 1991 is included through Asia/Shanghai. The hour branch is the branch of the corrected instant, not the branch of the uncorrected summer digits.

When the city has a longitude, all four pillars are cast on apparent solar time. The civil instant is shifted by longitude, about four minutes per degree from the zone’s meridian, and by the equation of time, which is about a minute and varies through the year. After that shift, the year and the month are read from the solar clock. They are not left behind on a Beijing wall-clock reading.

If you name no city, the instant is taken as Asia/Shanghai and then read on the fixed UTC+8 clock, so a China daylight-saving hour is not left sitting on the pillar. Longitude is not applied. If you type a place that is not in the city table, the chart does the same: a fixed UTC+8 reading, not a guessed longitude, and not a claim that the digits were Beijing wall time. The note under the pillars says which of these paths was used.

The hour, and the blank hour

Each earthly branch covers a double-hour. Zi, chou, yin, mao, chen, si, wu, wei, shen, you, xu, and hai tile the day. Most of those blocks sit inside a single civil date. Zi does not. It straddles midnight, so the casting clock splits it.

Late zi is 23:00–23:59. The hour branch is zi, and the day pillar advances to the next sexagenary day even though the civil date has not flipped yet. Early zi is 00:00–00:59. The hour branch is zi, and the day pillar keeps the new day that the civil date has already entered. A birth at 23:30 and a birth at 00:30 the same night can share both the zi hour and the day pillar. A chart that used the civil date alone at 23:30 would print the previous day and would disagree with this rule.

If the hour and minute are left empty, the form casts local noon and then blanks the hour pillar. Noon gives the day, month, and year a stable instant in the middle of the civil date, which avoids the midnight boundary. The hour cell stays empty so nobody confuses noon with a real birth hour. Luck cycles are not counted. The full chart’s other layers can still describe the pillars that exist. They do not describe a zi or a wu hour that was never supplied.

Sex at birth is asked only when luck steps are in view. It sets the classical direction of the ten-year count. It does not change the four natal pillars. A chart without that field still has its year, month, day, and, if known, hour.

What you see after the cast

On the light chart the pillars are followed by ten gods, element balance, a useful-element sketch, and a short reading. On the full chart the same pillars are followed by hidden stems, nayin, void, structure, symbolic stars, combinations and clashes, and, when hour and sex at birth are both known, eight 10-year steps with their starting ages and the annual pillars of the first step. The build of the pillars does not change between the two prices. The print depth changes.

The reading is a measured judgement of those characters. It can say that a day master is relatively strong in its season, or that a useful element is water, or that a branch clash is present. It cannot say that a promotion will arrive in March. The clock can be precise. A life is not a clock.

Checking a chart you already have

If you compare this cast with a chart from another tool, compare the gates first. Did the other chart change the year at lunar new year, at 1 January, or at Start of Spring? Did it change the month at the Western month, or at the solar term? Did it treat 23:30 as the old day pillar? Did it apply a Beijing longitude to a birth in another country? Did it invent an hour?

Those five questions explain most disagreements. The remaining disagreements are usually the solar-time shift: two clocks a few minutes apart, near a term, can sit on different pillars for a real astronomical reason. The chart note records the clock that was used so you can see the instant, not only the prose.

What this is not

Not a lunar-calendar conversion that flips pillars at the new moon.
Not a Beijing solar correction applied silently to every city, and not a guessed longitude when the city is missing.
Not a filled-in hour when you left the time blank.
Not a luck-cycle calculation without both the hour and sex at birth.
Not medical, legal, or financial advice, and not a promise that the cast predicts an event.
Not the entertainment past-life quiz. The quiz does not use this clock.