Stop hand-coding the Japanese Rokuyo calendar: LLM-generated lunar logic silently breaks

Originally published in Japanese on Qiita. This is the English edition. TL;DR Business requirements that touch the Japanese Rokuyo calendar are surging in the LLM era — wedding-date pickers, moving-day scorers, fortune-telling chatbots. But if you compute Rokuyo yourself, you get a class of bug that silently miscalculates on specific dates. The root cause is not the Rokuyo formula. It's the step before it: converting the Gregorian date to the Japanese lunar calendar requires astron...