Date Calculator

What Is This Date Calculator?

This date calculator adds or subtracts a chosen number of days, weeks, months, or years from a starting date and tells you exactly what date you land on — plus the day of the week. It's a quick way to answer questions like "what date is 90 days from today," "what was the date 6 weeks ago," or "what date falls 2 years after this contract signing date."

How to Use It

1. Pick a start date.
2. Choose "Add" or "Subtract."
3. Enter the amount you want to add or subtract.
4. Select a unit — days, weeks, months, or years.
5. Click "Calculate Date" to see the resulting date, its day of the week, and an ISO-formatted version.

How the Math Works

For days and weeks, the calculator simply moves the calendar forward or backward by that many 24-hour calendar days (a week being 7 days), correctly rolling across month and year boundaries — including leap years — because it operates on real calendar dates rather than a fixed day count.

For months and years, the calculator uses JavaScript's native date methods to move the month or year index forward or backward while keeping the same day-of-month where possible. This matters most at month boundaries: if the day-of-month you started with doesn't exist in the target month (for example, day 31 in a month with only 30 days, or day 29-31 in February), the date arithmetic overflows into the following month rather than clamping down to the last valid day.

Worked Example

Start with December 4, 2024, choose "Add," enter 90, and select Days. Ninety days later lands on Tuesday, March 4, 2025 — the calculator shows this as the resulting date along with the day of the week.

For the month-end overflow behavior described above, start with January 31, 2025, choose "Add," enter 1, and select Months. Since February 2025 (a non-leap year) only has 28 days, there's no "February 31" to land on. The underlying date arithmetic instead counts 31 days forward from the last valid day it can represent in February, which resolves to March 3, 2025. If you specifically need end-of-month results to stay at the end of the target month, adjust the resulting date manually or use the last day of the target month as your reference point instead.

Practical Notes

• All calculations use whole calendar days at local midnight, so there's no time-of-day component to worry about.
• Negative amounts aren't needed — use the Add/Subtract toggle instead of a negative number.
• Common uses include figuring out return-by dates, notice periods, contract renewal dates, trial expiration dates, and "N days from today" deadlines.
• To find the difference between two existing dates instead of projecting from one date, use the Date Difference Calculator.
• No data is sent to a server; everything is calculated locally in your browser.