Date Calculator

How to use the date calculator

1. Choose the start date. It is set to today by default.
2. Select Add to count forward or Subtract to count back.
3. Enter whole numbers in any of the four fields: years, months, weeks and days. Leave the others at 0. The days field starts at 90, a common deadline.
4. Read the result date, its weekday, the total days between the start and the result, and the ISO date (YYYY-MM-DD), which you can paste into a spreadsheet.

How the date is calculated

Years and months have different lengths, so the order of the steps matters. The calculator works like a paper calendar:

1. Years and months first. It moves the month and year by the amounts you entered and keeps the same day of the month.
2. End-of-month clamping. If that day doesn't exist in the target month, it uses the last day of that month. January 31 plus one month becomes February 28, or February 29 in a leap year. February 29 plus one year becomes February 28.
3. Then weeks and days. Weeks are converted to 7 days each and added to, or subtracted from, the date found in step 1, one calendar day at a time.

Dates are calendar days in the Gregorian calendar, with no time of day or time zone. Leap years follow the Gregorian rule described by the U.S. Naval Observatory: every year divisible by 4, except century years not divisible by 400. Results must fall between the years 1 and 9999.

Why the order matters

Starting on January 30, 2027, adding 1 month and 2 days gives Tuesday, March 2, 2027: one month later would be February 30, which doesn't exist, so it becomes February 28, and 2 days after that is March 2. If you added the days first you would reach February 1 and then March 1. The calculator always applies years and months before weeks and days.

Date calculator examples

90 days from October 12, 2026

The result is Sunday, January 10, 2027 (2027-01-10). A 180-day count from the same start lands on Saturday, April 10, 2027.

Six months after August 31, 2026

February 2027 has no 31st, so the result is clamped to Sunday, February 28, 2027, 181 days later. In the same way, January 31, 2027, plus 1 month is February 28, 2027, only 28 days later.

Subtract 2 years, 3 months, 1 week and 2 days from October 12, 2026

Years and months first take you back to July 12, 2024. Then 1 week and 2 days, or 9 days, take you to Wednesday, July 3, 2024. The two dates are 831 days apart.

Tips and limits

• Calendar days, not business days. Weekends and federal holidays are counted like any other day. If a deadline is stated in business days, use a business-day calculator.
• Months are not 30 days. Adding 1 month and adding 30 days often give different dates. Use the unit the contract, lease or rule actually states.
• Inclusive counts. The total days figure is the difference between the two dates. If a rule counts both the first and the last day, add 1.
• Legal deadlines may have their own counting rules, such as skipping the day of the event or moving a deadline that falls on a weekend. Check the rule that applies to you.