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Date Calculator

Pick a start date and add or subtract days, weeks, months, or years, for example 15 days from September 20. The date calculator returns the new date with its weekday.

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Operation

Resulting date
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How to use the Date Calculator

  1. Choose the Start date, or press Use today.
  2. Choose Add or Subtract, then fill in any mix of years, months, weeks, and days.
  3. Press Calculate. The new date appears with its weekday, ISO date, day of the year, and ISO week number.

How it works

The period is applied in a fixed order:

  1. Years are added or subtracted.
  2. Months are added or subtracted. If the day doesn't exist in the target month, it is moved to the last day of that month.
  3. Weeks × 7 + days are added or subtracted as calendar days.

The order matters. January 31 plus 1 month and 1 day gives March 1: the month is applied first and clamped to the last day of February, and only then is the day added. All dates follow the Gregorian calendar.

Examples

  • 2026-03-15 + 90 days = Saturday, June 13, 2026.
  • 2026-01-31 + 1 month = Saturday, February 28, 2026.
  • 2024-02-29 + 1 year = Friday, February 28, 2025.

March 15, 2026 plus 90 days

Date Calculator: start date March 15, 2026, add 90 days, result Saturday, June 13, 2026, ISO date 2026-06-13, day of the year 164, ISO week 24
Adding 90 days to March 15, 2026.

The first example in the calculator: 90 days after March 15, 2026 is Saturday, June 13, 2026, day 164 of the year, in ISO week 24. This example is one of the automated tests for the tool, so a code change that gives a different answer fails the test run.

Counting in days or in months

Adding days and adding months give different answers. 3 months from January 31 is April 30, but 90 days from January 31 is May 1 in a non-leap year. If a contract or notice says "90 days", count days. If it says "3 months", count months.

Common uses

  • Deadlines: 30, 60, or 90 days from an invoice or contract date
  • Notice periods: 4 weeks or 3 months from today
  • Follow-ups: 6 weeks after an appointment
  • Looking back: the date 18 months before a given day

Limitations

  • Adding a month to the 29th, 30th, or 31st can land on the last day of a shorter month.
  • February 29 plus one year becomes February 28 when the next year is not a leap year.
  • Results must fall between the years 1000 and 9999.
  • Only calendar days are counted. To skip weekends and holidays, use the business days calculator.

Sources

Frequently asked questions

How do I add days to a date?

Enter the start date, choose Add, and type the number of days. The calculator handles month lengths and leap years for you.

What happens when I add one month to January 31?

February has no 31st, so the result moves to the last day of February: the 28th, or the 29th in a leap year.

How are leap years handled?

The calculator follows the Gregorian calendar. Years divisible by 4 are leap years, except century years not divisible by 400. So 2000 was a leap year and 1900 was not.

Can I subtract weeks or months?

Yes. Choose Subtract and fill in any combination of years, months, weeks, and days.

Often used together with Date Calculator.