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Chinese astrology uses the Sun's actual position in the ecliptic, not the lunar calendar. The year is divided into 24 solar terms, each separated by 15° of solar longitude. Bazi month boundaries are set by the 12 Jié (節) terms — starting with Lìchūn (立春, 315°) which also marks the Bazi New Year.
All date arithmetic uses the Julian Day Number, a continuous count of days since 4713 BCE. This eliminates calendar ambiguities and makes day-difference calculations trivial. The proleptic Gregorian calendar formula (Meeus Ch. 7) is used throughout.
The Sun's ecliptic longitude λ is computed using the low-precision formula from Jean Meeus, Astronomical Algorithms (Ch. 25), accurate to ±0.01° for 1900–2100. Exact solar term crossings are found by Newton-Raphson iteration until |λ − target| < 0.0001° (≈ 8 seconds of time).
Clock time is corrected in two steps:
1. Longitude correction: adj = (longitude − UTC_offset × 15) × 4 minutes — every degree of longitude = 4 minutes.
2. Equation of Time (Spencer 1971): accounts for Earth's elliptical orbit and axial tilt, varying roughly ±16 minutes across the year.
TST determines the Hour Pillar and the traditional 23:00 day boundary.
One solar month (the interval between two Jié terms) spans ~30 days and represents a full Luck Decade of 10 years. So 3 days ≈ 1 year. The starting age is found by counting days from birth to the nearest Jié crossing, then dividing by 3. Each remainder day counts as 4 months.