Japanese Era Dates, Public Holidays, and Closing-Date Processing in Business Apps — Era-Resilient Design, JapaneseCalendar, and Business-Day Calculations in Practice (archived 2026-07-27)

A report that must show '令和8年' in the Japanese era, business-day calculations that exclude public holidays, payment due on the last business day of the month following a 20th-of-month closing — date handling unique to Japanese business applications carries 'specifications that change later': era transitions, legal changes to public holidays, and month-end rounding. This article covers era-resilient design using JapaneseCalendar for era display, why you should never hardcode public holidays plus how to run a holiday master table fed by the Cabinet Office's CSV, and closing-date/fiscal-year calculations that account for AddMonths' rounding behavior — all backed by code samples and decision tables. Archived version of https://comcomponent.com/en/blog/japanese-calendar-holidays-closing-date-guide/, as published on 2026-07-27. The live article is maintained and may change after this date. First published 2026-07-11.