以下のガイドはまだ日本語に翻訳されていないため、英語で表示しています。計算機自体はすべて日本語に対応しています。
この計算機の使い方
- Choose a start date (today by default).
- Choose add or subtract.
- Enter any combination of years, months, weeks and days.
計算式
Years and months are applied first, keeping the day of the month where possible. If that day doesn't exist (for example the 31st in a 30-day month), the result uses the last day of the month. Weeks and days are then added as exact numbers of days, so leap years are handled automatically.
Jan 31 + 1 month = Feb 28 (or Feb 29 in a leap year)
Mar 1 − 1 day = Feb 28 or 29
Example
- 8 November 2026 + 90 days = 6 February 2027.
- 31 August 2026 + 6 months = 28 February 2027.
よくある質問
Does the start date count as day 0 or day 1?
The start date is day 0, so adding 1 day gives tomorrow. To count both start and end dates, use the day counter.
How do I count only business days?
Use the business days calculator, which skips weekends and optional holidays.
Why is adding 30 days different from adding 1 month?
Months have 28 to 31 days. Adding a month moves to the same day number in the next month; adding 30 days always moves exactly 30 days.