Date Add/Subtract Calculator

Date Add/Subtract Calculator moves a date forward or backward by a number of days, weeks, months or years. It shows the resulting date, its weekday, its ISO 8601 form and the number of calendar days between the two dates, applying real calendar rules for month ends and leap years.

Deadlines, notice periods, renewals and follow-ups are often expressed as “30 days from” or “6 months after” a date. Counting on a calendar is slow and month ends are easy to get wrong; a date-only calculation gives the same answer in every time zone.

1. Choose the starting date.
2. Choose Add or Subtract.
3. Enter a whole-number amount and pick days, weeks, months or years.
4. Press Calculate date.
5. Read the result date, weekday, ISO date and days between, then copy or download the summary.

• Add or subtract days, weeks, months or years
• Result date with weekday and ISO 8601 date
• Calendar days between the start and result
• Month-end clamping explained (31 January + 1 month = end of February)
• Leap-year aware; time-zone and daylight-saving safe
• Clear errors for invalid dates, decimals and out-of-range results

• Use days rather than months when a rule says “30 days”, as months vary in length.
• Check whether your deadline counts the starting day; this tool does not.
• Read the note when a month step lands on a day that does not exist.
• For working-day deadlines, use the Business Days Calculator instead.