Solar Terms and Calendar Conversion: Why the Month Pillar Does Not Follow the Lunar Month
How solar terms decide the year and month pillars, how the solar-term instant and leap months are handled, and how true solar time relates to standard time.
Direct answer
The year and month pillars in BaZi are divided by solar terms (节气), not by lunar months. The year pillar changes at the Start of Spring (立春) and the month pillar at each of the twelve sectional terms, and which side a birth falls on is decided by the exact solar-term instant (交节), not by the date. A leap month in the lunar calendar does not affect the stem-branch year or the stem-branch month; it only changes the numbering of lunar months.
01The month pillar switches at the twelve sectional terms, not at the first of the lunar month
Stem-branch months are divided by the twelve sectional terms: Start of Spring, Awakening of Insects, Clear and Bright, Start of Summer, Grain in Ear, Minor Heat, Start of Autumn, White Dew, Cold Dew, Start of Winter, Major Snow and Minor Cold. The span between two sectional terms is one stem-branch month, and it does not coincide with the beginning and end of a lunar month.
A lunar month is set by the synodic month and begins on its first day; a solar term is set by the sun's position on the ecliptic. The two cycles have different lengths, so the boundary of a lunar month and that of a stem-branch month can differ by as much as about half a month. That is exactly why a BaZi month is not a lunar month.
02The solar-term instant decides which month pillar applies
A solar term is an astronomical moment accurate to the minute and second, not a whole day. For someone born on the day a term falls, you have to see whether the specific time comes before or after the instant; otherwise the month pillar can be off by a full position, and the Ten Gods (十神) mapping shifts with it.
That is also why saying "born on the solar term day" is not enough: what is needed is the moment, and that moment has to be compared under the same time-zone convention used to cast the chart.
03The year pillar changes at the Start of Spring, not at Chinese New Year
The stem-branch year switches at the Start of Spring, while the lunar new year (the first day of the first lunar month) can fall several days to half a month away from it, and further apart in a year with a leap month. So for anyone born around the new year, the year pillar in particular needs checking.
Folk custom also differs on which day the zodiac animal changes, some going by Chinese New Year and some by the Start of Spring. That is a difference of convention rather than a calculation error, but within one analysis it has to be consistent.
04Leap months and the stem-branch calendar
A leap month is a whole month inserted into the lunar calendar to reconcile the difference in length between the tropical year and the synodic month, following rules such as seven leap months in nineteen years. What it adjusts is the numbering of lunar months; it does not change the division of stem-branch years and stem-branch months.
So "which month is a leap-month birth" is not a problem in BaZi: the month pillar is decided by solar terms alone. What a leap month does affect is lunar birthdays, traditional festivals and other civil uses, and the two should not be mixed up.
05How true solar time relates to standard time
Standard time is the time-zone time fixed by administration; true solar time (真太阳时) is measured from the sun's actual position. The conversion takes two steps: first correct for the difference between the longitude of the birthplace and the standard meridian, at roughly four minutes per degree, then add the equation of time, which varies by more than ten minutes over the year.
Whether to use true solar time is a choice of school and of product convention, and both practices have users. It mainly affects people born near an hour boundary, and it can also affect which side of a solar-term instant a birth falls on, so whether it is enabled should be stated plainly.
06A re-checkable order of conversion
The re-checkable order is: confirm the birthplace; then the time zone in force at the time and whether daylight saving applied; then decide whether to convert to true solar time; then settle the rule for the day change at the zi hour; then use the solar-term instants to fix the year and month pillars — and only after the four pillars are cast discuss the Major Cycles (大运) and interpretation.
This whole chain is deterministic calculation that can be verified programmatically, and it should be done by an algorithm that records its intermediate results. Asking a language model to derive four pillars straight from a birth date invites errors that cannot be reproduced.
07Checklist
- The year pillar was divided at the Start of Spring and the month pillar at the twelve sectional terms; lunar months were not used
- For a birth on a solar-term instant or on a solar term day, the exact moment was checked rather than the date alone
- The birthplace, the time zone in force at the time, and whether daylight saving applied were all confirmed
- It is clear whether the convention in use enables true solar time, and how the day changes at the zi hour
- It is understood that a leap month only affects the numbering of lunar months, not the stem-branch year or month
Frequently Asked Questions
Is a BaZi month the same as a lunar month?
No. Stem-branch months switch at the twelve sectional terms, with no relation to the first day of a lunar month; the two boundaries can differ by about half a month.
How is the month pillar decided for a birth on a solar term day?
By the exact solar-term instant: before it belongs to the previous month pillar, after it to the next.
Does being born in a leap month affect a BaZi chart?
Not directly, for the stem-branch year and month. A leap month only adjusts the numbering of lunar months; the month pillar is still decided by solar terms.
Does the Start of Spring or Chinese New Year decide the year pillar?
The year pillar changes at the Start of Spring. Chinese New Year is the lunar new year, and the two dates can fall several days to half a month apart.
Must true solar time always be applied?
No. It is one correction convention; whether to use it depends on the school and product rules you accept. What matters is stating it clearly and staying consistent.
How much difference does longitude make?
About four minutes per degree of longitude, plus an equation of time that can add more than ten minutes, so people born near an hour boundary need to pay most attention.
Related entries
- Birth Time, Time Zones and True Solar Time: Get the Time Right FirstBirthplace, time zones, daylight saving, hour boundaries and the true solar time dispute — and why casting a chart should be separate from AI interpretation.
- Reading the Year, Month, Day and Hour PillarsWhat information each of the four pillars carries, and why no pillar can be read on its own, apart from the whole chart.
- What Is BaZi? The Basic Structure of Four Pillars ReadingStarting from the Four Pillars, Heavenly Stems and Earthly Branches, Five Elements and the Day Master, what BaZi actually analyses — and how it is best used.
Practical guides
- How 大運 and 起運 are calculated: from solar term to ten-year cyclesFix the direction from the year stem, measure the distance to the nearest 節, convert at 3 days = 1 year, and read the eight ten-year pillars.
- How to read a 流年: put the year inside its 大運Fix the annual pillar at 立春, find which ten-year 大運 it falls inside, then read how it relates to the 大運 and the natal chart.
Updated 2026-09-20