Add to Date Calculator
Want to know what date it will be in 30 days, 6 weeks or 3 months? Enter your start date, the amount and the unit, and the calculator adds it up exactly according to the calendar, including leap years and months of different lengths.
結果
フィールドに入力して結果を確認してください。
公式
Result = start date + (amount × unit)
For days and weeks, the calculator simply adds the number of days (1 week = 7 days). For months, the calendar month shifts forward; if the start date doesn't exist in the target month (for example 31 January plus 1 month), the date is clamped to the last day of that month (28 or 29 February, depending on a leap year) rather than overflowing into March.
ステップ解説
- 1Enter the start date.
- 2Enter the amount and choose days, weeks or months.
- 3Read the resulting date.
計算例
15 January 2026 plus 10 days
→ 25 January 2026.
31 January 2026 plus 1 month
→ 28 February 2026 (not 3 March, since 2026 is not a leap year).
31 January 2028 plus 1 month
→ 29 February 2028, since 2028 is a leap year.
Reference table
| Unit | In days | Special case |
|---|---|---|
| 1 day | 1 | Always exact. |
| 1 week | 7 | Always exact. |
| 1 month | 28-31 | Date is clamped on overflow. |
| February (leap year) | 29 | Every 4 years, with century exceptions. |
Months don't have a fixed number of days; the calculator accounts for this automatically.
よくある間違い
- Expecting that adding months at the end of a month always lands on the same day next month.
- Confusing weeks and months: 1 month is not a fixed 4 weeks, but 4.35 weeks on average.
- Forgetting that a negative amount is needed to calculate backwards to an earlier date.
活用シーン
- Calculating the expiry date of a contract or warranty.
- Estimating the delivery date of an order after a processing time in business days or weeks.
- Planning reminders, such as a check-up appointment 3 months after a treatment.
よくある質問
Can I also calculate backwards?
Yes, enter a negative amount to subtract days, weeks or months from the date.
What happens if the date doesn't exist in the target month?
The date is clamped to the last day of that month. For example, 31 March plus 1 month becomes 30 April, not 1 May.
Does the calculator correctly account for leap years?
Yes. The calculation uses the real calendar, so 29 February automatically appears in leap years and is skipped in other years.
Can I add days to a date across a year change?
Yes, the calculator automatically accounts for the year boundary; for example, 20 December plus 20 days becomes 9 January of the following year.
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