日付加減算・○日後○日前計算機 - 無料オンライン計算ツール

Need the date that falls 90 days from today? Or the date 6 months ago?

Pick a start date, hit Add or Subtract, and enter years, months, weeks and days. You instantly get four things: the full date, the weekday, the ISO format (like 2026-10-30) and how many calendar days you moved. If the result lands on a Saturday or Sunday, the tool flags it and shows the next business day. It also handles the classic month-end trap (like January 31 + 1 month) for you.

Keep in mind: it counts calendar days. Public holidays and working-day rules are not included.

First: holidays are not counted. The business-day alert only looks at Saturday and Sunday, and it always points forward, even when you subtract. In some countries (Saudi Arabia, for example) the weekend is Friday-Saturday, so the alert does not fit.

Second: legal and contract deadlines have their own counting rules, and they differ by country.

India: under the General Clauses Act, 1897, s.9, the word "from" excludes the first day and "to" includes the last day. Section 10 lets an act be done on the next open day if the court or office is closed on the last day - but s.10 does not apply where the Limitation Act, 1963 applies, which has its own rules.

USA (FRCP 6(a), federal courts): the trigger day is excluded, every calendar day counts, and a last day that falls on a weekend or legal holiday rolls to the next day that is not one. Check the current rule text before relying on it.

Germany and France: civil procedure codes (BGB §§187-193, CPC art. 641-642) similarly exclude the trigger day and extend weekend/holiday expirations to the next open working day. See the localized sections for details.

Third: "month" is not "30 days". The tool adds real calendar months, so the two can land on different dates (section 4, example g).

Fourth: order matters. Years and months first, then weeks and days. That is why January 30 + 1 month + 2 days gives March 2, not March 1. Treat the result as a solid first estimate, not legal advice.