Sexagenary Day Counting: Continuous Cycle of Day Pillars

In bazi practice, the Day Pillar is the one pillar among the four that is not determined by solar terms. The Year Pillar changes at Beginning of Spring, and the Month Pillar changes at the twelve principal solar terms. The Day Pillar, by contrast, runs as a continuous sexagenary day count. 干支纪日 is the method by which that count is applied to a calendar date.

干支纪日是一条不间断的序列

The sexagenary cycle used for the day count is built from the ten Heavenly Stems and the twelve Earthly Branches. Because the Stems and Branches are paired by matching parity, a yang Stem only pairs with a yang Branch and a yin Stem only pairs with a yin Branch. The result is sixty valid pairs rather than one hundred and twenty. In day counting, this gives a cycle of approximately sixty days.

This sequence is treated as unbroken. It does not restart at a month boundary, a year boundary, or a solar term. A given day is simply the next sexagenary pair after the previous day, and the one before the next. For that reason, the Day Pillar is the only pillar in the Four Pillars that has no dependence on the position of the sun in the tropical year. The Year Pillar and Month Pillar are anchored to solar points, but the Day Pillar is anchored to a running ordinal sequence.

The sexagenary pairs themselves are the familiar combinations beginning with 甲子，乙丑，丙寅, and continuing through 癸亥. In 干支纪日, each successive civil day receives the next pair in that order. Because the cycle length is sixty, the same pair reappears after sixty days if no interruption is introduced. The practical question is therefore not whether the cycle is continuous, but where a given calendar date is placed within it.

日柱为什么不随节气改变

The Day Pillar does not change at solar terms because its underlying logic is a day count, not a seasonal division. The Year Pillar is defined by Beginning of Spring, and the Month Pillar is defined by the twelve sectional solar terms. Those boundaries mark changes in the annual and monthly energies. The Day Pillar has no corresponding boundary. Its unit is simply the civil day, and its sequence advances one step per day.

This also means that the Day Pillar is not tied to the month in which a date falls. A 甲子 day may occur in any month of the solar year, and the next day will be 乙丑 regardless of whether a solar term falls between them. In this respect, 干支纪日 is closer to a running serial number for days than to a calendrical layer that responds to seasonal movement.

The stability of the Day Pillar across solar terms is one reason it can serve as the anchor of a chart. The year and month change according to the solar calendar, but the day advances in a predictable and uninterrupted manner. Once the correct sexagenary number for a date is established, later dates follow without needing to consult a seasonal boundary.

本站把日期换成日柱的方式

This site converts a Gregorian date into a Day Pillar through Julian Day Number arithmetic. The procedure is mechanical. The engine first obtains the Julian Day Number for the given Gregorian date. It then subtracts the Julian Day Number of a fixed reference day whose sexagenary position is known. The difference is taken modulo sixty, and the remainder gives the position of that date within the sixty-day cycle.

The reference day is fixed, not recalculated for each user. Once the Julian Day Number of that reference point is known, any later date can be placed by simple subtraction. Because the Julian Day Number increases by one for each calendar day, the difference between two dates gives the number of days elapsed. Reducing that elapsed count by sixty gives the offset within the sexagenary sequence. This method does not rely on tables of past dates and does not need to know the month or solar term of the input date.

The result is expressed as one of the sixty pairs from 甲子 through 癸亥. That pair is the Day Pillar. The Stems and Branches of that pair are the components from which the Day Master and the Day Branch are read.

二十三时之后的进位

This site follows the convention of late 子 hour for cross-day births. Under this convention, a person born between 23:00 and 23:59 is assigned the Day Pillar of the next day, while the Hour Pillar is still calculated from the actual clock hour of birth.

The effect is that the Day Pillar advances at 23:00 rather than at midnight. A birth at 23:30 on a given calendar date is therefore treated as belonging to the next day for the Day Pillar. The Hour Pillar is not advanced in the same way; it is determined by the clock time, with 子 hour beginning at 23:00.

This is a convention, not a mathematical consequence of the sexagenary sequence. Different tools may use different conventions. Some place the day change at midnight, which would keep the Day Pillar on the current civil date until 00:00. This site uses the late 子 hour rule, so the crossover occurs one hour before midnight.

自己核对一个日柱的两种办法

There are two practical ways to check a Day Pillar without relying on a single converter.

The first is to compare against a printed perpetual calendar or an established reference table that lists the sexagenary day for each calendar date. If the table has been verified against a known epoch, the Day Pillar for a given date can be read directly. This is the traditional fallback in manual chart preparation. The Year Pillar, Month Pillar, and Hour Pillar can be derived by formula or mnemonic, but the Day Pillar has no such mnemonic. It must be taken from a table or computed.

The second way is to use an independent program or algorithm that also works from Julian Day Number, and compare the result with this site. Discrepancies between two tools that both use the same underlying day count are usually not caused by the day sequence itself. The usual points of divergence are the treatment of 子 hour and the handling of time zone. If one tool assigns the day change at 23:00 and another at midnight, their Day Pillars will differ for births in the 23:00–23:59 window. If one tool converts the birth time to a different time zone before assigning the date, the date itself may shift, and with it the Day Pillar.

When the same date and time produce different Day Pillars on different tools, the first thing to check is whether the tool uses the late 子 hour rule or a midnight rule. The second is whether the input time has been interpreted in the same time zone. The sexagenary day sequence itself is not usually the source of the discrepancy. Once the day boundary convention and time zone are aligned, the Day Pillar should agree.

In the Four Pillars, the Heavenly Stem of the Day Pillar is called the Day Master or 日主, also referred to as 日元. It is the central reference point for the analysis of the whole chart. The Earthly Branch of the Day Pillar is the Day Branch, and in chart reading it is treated as the spouse palace. These assignments do not change with the season; they depend only on the sexagenary day pair assigned to the birth date.