Solar Terms (节气): Why the Chinese Month Does Not Start on the 1st

The Chinese calendar most people know is lunar: months follow the moon, and the new year falls somewhere between late January and late February. BaZi does not use it. BaZi runs on the solar terms (节气), twenty-four fixed points in the solar year that mark where the sun is rather than where the moon is. Twelve of them are major terms, and those twelve are the boundaries of the BaZi months.

This has an immediate practical consequence: the BaZi year begins at 立春, the start of spring, around 4 February — not at the lunar new year. In 2026 the lunar new year falls on 17 February, but the Fire Horse year in BaZi terms began on 4 February. Someone born on 10 February 2026 is a Horse in BaZi and, by the popular lunar-calendar reckoning, still a Snake. Both statements have a defensible basis. They are answers to different questions, and a calculator has to say which one it is using.

The month pillar changes on the same principle. 惊蛰 (Insects Waken, around 5-6 March), 清明 (Clear and Bright, around 4-5 April), 立夏 (Start of Summer, around 5-6 May) — twelve boundaries falling between roughly the 4th and the 8th of each Gregorian month. This is why serious monthly forecasts in this tradition update in the first week of the month rather than on the 1st, and why a forecast published on 1 January is still describing the previous BaZi month for several days.

The terms themselves are agricultural and unusually concrete: Grain Rain, Lesser Heat, White Dew, Frost Descends. They were developed as a farming calendar for the Yellow River basin and are old enough to appear in Han dynasty texts. UNESCO inscribed the system as intangible cultural heritage in 2016. Reading the list in order is one of the quickest ways to understand why the five elements are seasonal before they are anything else.

For chart calculation the practical points are these. A birth in the first days of February needs checking against 立春 before you assume the year. A birth within a day or two of any major term needs the actual time of the term, which varies by year and by longitude, not just the date. And a birth near a term boundary is one of the few cases where a small error in the recorded birth time genuinely changes the chart.

Our calculation resolves all pillars from solar terms rather than from calendar dates, which is why the pillars it produces should match any other correctly implemented BaZi calculator. Where a convention is genuinely contested — the late 子 hour, for instance, where schools disagree about whether an 11pm birth belongs to the following day — we declare which convention we use rather than picking one silently.