Check whether a Japanese calendar date is a business day. Returns weekday, weekend/public-holiday flags and a business-day flag. Public holidays are bundled for 2020-2031.

Move a date forward or backward by N days, either in business days (skipping weekends and Japanese public holidays) or calendar days. Negative values move backward.

Count calendar days and business days between two dates (Japanese holidays excluded from business days). Order of the two dates does not matter.

Convert a Gregorian date to the Japanese imperial era format (令和/平成/昭和/大正/明治). Era boundaries are handled to the day.

Convert a Japanese era date to the Gregorian calendar. Warns when the given month/day falls outside that era.

Calculate the annual income-tax and residence-tax reduction from iDeCo contributions in Japan (contributions are fully deductible). Deduction effect only.

Calculate the furusato-nozei (hometown tax donation) limit where the out-of-pocket cost stays at 2,000 yen, using the residence-tax income levy from the annual tax notice.