Vol. I · No. 1

柱Eastern Pillars

Methodology

How we calculate it

Most calculators are black boxes. We name the algorithm library, the school, the edge cases, and the translation choices. If you can read this page and still disagree with our output, you at least know which fork we took.

Four Pillars (BaZi)

Algorithm

We use the open-source lunar-javascript library by 6tail as our base computation engine. It implements the GB/T 33661-2017 national standard for Chinese calendar calculation, including:

Year boundary: lichun (立春)

The year pillar changes at the solar term lichun, not at the Lunar New Year. This is the academic consensus reflected in San Ming Tong Hui, Di Tian Sui, and modern scholarship. A person born on, say, February 3rd belongs to the previous year's pillar; one born on February 5th belongs to the next.

Some popular calculators and almanacs (especially those marketed at a casual audience) use the Lunar New Year as the boundary. We do not. We test against canonical lichun edge cases on every release.

Zi-hour school (子时流派) — three schools, two options surfaced

The two-hour zi block (23:00–01:00) crosses a calendar day. Three schools have evolved to handle this and they disagree:

The honest position is that most "Late Zi" calculators in English are actually split-zi calculators that have labeled themselves wrong. We are labeling ours correctly. If your teacher uses pure late-zi and our default doesn't match, that's the reason — and we'd rather you know it than be silently wrong.

True solar time correction

Clock time and true solar time can differ by up to roughly 30 minutes, depending on (a) how far your birth longitude is from the central meridian of your time zone, and (b) the seasonal Equation of Time correction (up to ±16 minutes).

For births near a time zone boundary or near a solstice/equinox, this matters: it can shift the hour pillar. We offer optional correction in the advanced options. The formula is: 

Δ_minutes = (longitude − timezone_offset × 15) × 4 + EOT(date)

where EOT(date) ≈ 9.87·sin(2B) − 7.53·cos(B) − 1.5·sin(B)
      B = 2π·(N − 81)/365,  N = day of year

If you provide longitude and time zone, we apply this correction before computing the hour pillar. We disclose the correction (in minutes) on the result page.

Luck pillars (大运)

Luck pillars are ten-year cycles. Direction is determined by gender + year stem polarity: yang-male and yin-female travel forward; yin-male and yang-female travel backward. Starting age is computed at 3 days per year (1 day = 4 months) from the nearest enclosing solar term, per San Ming Tong Hui.

Zi Wei Dou Shu (Purple Star)

Algorithm

We use iztro by SylarLong as our base library. It implements the Zi Wei Dou Shu chart layout including 14 major stars and 100+ minor stars across 12 palaces.

School

iztro implements the San-He (三合) school. This is the most widely-taught lineage in modern Taiwan and Hong Kong, and the most rigorously documented in open-source form. We declare this transparently because:

If you have learned Zi Wei from a Flying Star teacher, our star placements will mostly agree—but our interpretive notes will not match what you've been taught for the same chart. We chose San-He because it's the most internally consistent open-source implementation available; we plan to add Flying Star support in a future release.

Year boundary: Lunar New Year

Unlike BaZi, Zi Wei uses the Lunar New Year as its year boundary—not lichun. This is because Zi Wei is a lunar-system astrology (based on the moon's position relative to the 28 lunar mansions), whereas BaZi is a solar-system astrology (based on the sun's ecliptic position). Mixing these conventions is a common beginner error.

Translation conventions

We use the academic English conventions for technical terms. Most importantly:

A fuller glossary is forthcoming.

Version + revision history

This methodology document carries a version number. When we change a default, switch a library, or update a translation convention, we update the version and note what changed.

Current: v0.1.0-mvp · 2026-06-13 · Initial public release.