Date Calculator

Find the date that is a given number of days, weeks, months or years before or after a starting date.

How to use the Date Calculator

  1. Pick the start date.
  2. Choose the operation.
  3. Enter the years.
  4. Enter the months.
  5. Enter the weeks.
  6. Enter the days.
  7. Press Calculate. The result updates automatically as you type.

About the Date Calculator

Use the date calculator to find deadlines and due dates: a 90-day return window, a 30-day notice period, the end of a 6-month lease, a warranty that expires in 2 years, or the date 40 weeks after a known start. Switch to Subtract to look backward, for example to find when a 180-day period began.

Note that "30 days" and "1 month" are not the same thing. Contracts often specify one or the other, so enter the amount exactly as it is written.

Formula

Years and months are applied first on the calendar, then weeks and days are counted:

result = start date ± (12 × years + months) calendar months ± (7 × weeks + days) days

Adding a month keeps the same day number (March 15 → April 15). If that day does not exist, such as January 31 + 1 month, the result is the last day of the month (February 28 or 29).

Example calculation

Using the default values:

  • Start date = 2026-10-12
  • Operation = Add (+)
  • Years = 0
  • Months = 0
  • Weeks = 0
  • Days = 90

the calculator returns:

  • Resulting date: Sunday, January 10, 2027
  • Date (YYYY-MM-DD): 2027-01-10
  • Calendar days between: 90 days

Frequently asked questions

What date is 90 days from October 12, 2026?

Adding 90 days to Monday, October 12, 2026 gives Sunday, January 10, 2027.

Does adding days count the start date?

No. Day 1 is the day after the start date, which is how deadlines such as "within 30 days" are normally counted.

What happens when I add one month to January 31?

February has no 31st, so the result is the last day of February (the 28th, or the 29th in a leap year).

Can I add business days instead?

Use the Business Days Calculator, which skips weekends and optionally US federal holidays.

Last updated: · Category: Time & Date calculators

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