Year Pillar (Lichun boundary) 年柱 · 立春 Glossary
In BaZi, the year pillar flips at the exact astronomical instant of Lichun — the sun reaching solar longitude 315° — not at the lunar new year.
Niánzhù · Lìchūn · 年柱·立春 (simplified) · 年柱·立春 (traditional)
Source credibility (S/A/B/C/X): S A Rule computability (A/B/C): A
What it is
The four pillars of a birth moment each take a stem-branch pair （干支）. The year pillar is the first of them, and the whole question is: when does the old year's pillar stop and the new one's begin? The mainstream Zi Ping answer is Lichun （立春）, the "beginning of spring" solar term, defined astronomically as the moment the sun's apparent longitude crosses 315°. A birth one minute before that instant belongs to the previous year's pillar; one minute after, to the new one. This is why the boundary is a clock time, not a calendar day — on Lichun day itself, the flip happens at a specific hour and minute.
This makes the BaZi year a solar year. It is not the same object as the civil lunar year that begins at Chinese New Year — Zhengyue Chuyi — even though popular zodiac articles treat the two as interchangeable. They are two different year concepts that happen to share a vocabulary.
How far apart are the two boundaries?
Across the 200 years from 1900 to 2100, Lichun and the lunar new year fall on different days in 195 years — an average gap of 7.4 days, and as much as 16 days apart in extreme years. Only in the rare years when the two dates coincide does the choice of convention make no difference. For everyone born in the window between Lunar New Year and Lichun (or after Lichun in a year where the order reverses), the two conventions genuinely produce different year pillars — and through the month-stem derivation, different month pillars too. This is a measured property of the calendars, not an opinion; the project's School Dispute Register records the boundary question as entry D-01, and the related question of when on the boundary day the pillar flips (date granularity vs the exact instant) as entry D-07.
How this engine computes it
- Default: the year pillar flips at the precise Lichun instant, computed from the engine's solar-longitude pipeline (with aberration and nutation corrections), not from a published date table.
- The Lunar-New-Year convention is also implemented — both are shipped, and the choice is an explicit, recorded parameter rather than a hidden default.
- The zodiac / lunar year and the BaZi year pillar are two independent fields in the output; the interface shows how many days apart the two boundaries are for the year in question.
- Boundary behaviour is covered by dedicated differential tests: samples on Lichun day at minute granularity assert that the pillar flips at the instant, and the divergence from mainstream tools is confined to the boundary-window hours.
Related entries
Chinese New Year (Zhengyue Chuyi) — the other year boundary, and why it must be a separate entry · Early / Late Zi Hour · True Solar Time · Yun Period
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