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.
