Date Calculator

Find the date a set number of days, weeks, months or years before or after any date — for deadlines, due dates, notice periods and "90 days from today".

Result date
Saturday, January 9, 2027
Day of the week
Saturday
ISO date
2027-01-09
Calendar days after the start date
90
Same span in weeks
12 weeks, 6 days

How adding days, weeks, months and years works

Days and weeks are exact: a week is always 7 days, so adding 3 weeks is the same as adding 21 days, and the calculator simply counts forward on the calendar, crossing month and year boundaries and leap days as it goes.

Months and years are calendar units, not fixed lengths. Adding one month moves to the same day number in the next month, whether that month has 28, 29, 30 or 31 days. Adding a year moves to the same date next year. That is how leases, subscriptions and most contracts count months.

When you enter both, years and months are applied first and then weeks and days. The order matters near month-ends: in 2026, January 30 plus 1 month is February 28, and 1 more day gives March 1; but adding the day first gives January 31, and 1 month after that is February 28.

The month-end rule: January 31 plus one month

February has no 31st, so January 31 plus one month has no exact answer. This calculator uses the most common convention, also used by spreadsheet functions such as EDATE: when the day doesn't exist in the target month, the result is the last day of that month. January 31 plus 1 month is February 28 (February 29 in a leap year), and March 31 plus 1 month is April 30.

The rule only affects days 29, 30 and 31. Adding 1 month to January 15 is always February 15. If a contract or a lender defines month-ends differently, follow its wording.

Leap years follow the Gregorian rule: a year divisible by 4 is a leap year, except century years, which are leap years only if divisible by 400. So 2000 was a leap year, 2100 will not be, and February 29, 2028 is the next leap day. Adding 1 year to February 29, 2024 gives February 28, 2025.

Counting days for deadlines

The result here is the date that is exactly the given number of calendar days away, the way "30 days from March 1" is usually meant: day 1 is March 2 and day 30 is March 31. Weekends and holidays are included.

Legal and government deadlines often have their own counting rules — for example, excluding the day of the event, or rolling a deadline that lands on a weekend or holiday to the next business day. If the deadline counts business days only, use the business days calculator instead, which can also skip US federal holidays.

How the result is calculated

Step 1: date₁ = start date moved by (12 × years + months) calendar monthsIf that day doesn't exist in the target month, use the month's last dayStep 2: result = date₁ + (7 × weeks + days) calendar days
  • Subtracting works the same way with the months and days counted backward.
  • Calendar days include weekends and holidays.

Worked example

  1. Start on Saturday, January 31, 2026 and add 1 month and 10 days.
  2. Step 1: one month after January 31 would be February 31, which doesn't exist. 2026 is not a leap year, so the month-end rule gives Saturday, February 28, 2026.
  3. Step 2: add 10 days to February 28: March 1 is day 1, so day 10 is Tuesday, March 10, 2026.
  4. In total the result is 38 calendar days after the start date (5 weeks and 3 days).
  5. A second check: 90 days after Sunday, October 11, 2026 is Saturday, January 9, 2027.

Frequently asked questions

What date is 90 days from today?

Leave the start date as today, choose Add and enter 90 days. As an example, 90 days after October 11, 2026 is Saturday, January 9, 2027. The count includes weekends and holidays and doesn't count the start day itself.

Is adding 1 month the same as adding 30 days?

No. One month moves to the same day number in the next month, which can be 28 to 31 days later. From January 31, 2026, 30 days later is March 2, but 1 month later is February 28 under the month-end rule.

What is January 31 plus 1 month?

February 28, or February 29 in a leap year. There is no February 31, so the result is moved back to the last day of February. That is the same convention spreadsheet date functions use.

Does the calculator count the start date?

No. Adding 1 day to Monday gives Tuesday. If you need a count that includes both the first and last day, use the days between dates calculator with the include-end-date option.

How do I subtract days from a date?

Choose Subtract and enter the days. For example, subtracting 1 month from March 31, 2026 gives February 28, 2026, again using the month-end rule.

Sources

Last reviewed for 2026. How we calculate.