Why every BaZi calculator disagrees with the almanac

Every Four Pillars calculator — saju in Korea, BaZi in China — agrees on the easy 95% of the job. Feed it a birth date and it maps that instant onto a traditional calendar: four pillars, each a heavenly stem paired with an earthly branch. The remaining 5% is boundaries. And at the boundaries, nearly all of them quietly disagree with the printed almanac they claim to reproduce. I maintain a saju reading service, and getting these four cases right was most of the actual engineering. Here they are, with the failing inputs. 1. A solar term is an instant, not a date The year pillar does not turn on January 1, and not on lunar new year either. It turns at 입춘 (ipchun, "start of spring") — one of the 24 solar terms, defined by the sun's apparent longitude. In 2024 that moment was February 4, 16:27 KST . A calculator that applies solar terms at day granularity says "February 4 → new year pillar" and hands the wrong year to everyone born that morning. npx k-saju 2024-02-04 04:00 # year 癸卯 — still the old year pillar, because 04:00 < 16:27 The fix is unglamorous: store term boundaries as instants and compare instants. The subtlety is that this correction applies to the year and month pillars only — the day pillar runs on its own sexagenary count and must not be touched. 2. The 23:00 hour belongs to two days at once Traditional practice starts the day at 23:00, not midnight — the hour of the Rat (자시). So for a birth at 23:31, there are two defensible answers about which day's stem the hour pillar derives from, and schools split on it. The convention this engine declares: the day pillar keeps clock midnight , while the hour stem takes the next day's stem (the 야자시 rule). npx k-saju 2000-05-15 23:31 # day 癸酉, hour 甲子 I am not claiming this is the One True Rule. I am claiming it should be written down. Most tools pick a side in silence, which is how two calculators give one person two charts and neither can explain why. 3. The clock is not the sun Korea keeps time on the 135°E meri