If you have ever tried to build a Korean fortune-telling or astrology app, you already know the dirty secret: the fun part is the interpretation, and the brutal part is the date math. Saju (사주), the Korean "Four Pillars of Destiny," is computed from a person's birth year, month, day, and hour. Sounds like four lookups. It is not. I built and shipped a small HTTP API to do exactly this part for me, because I got tired of every Western reimplementation quietly returning the wrong pillars. This is