Why is 31 January plus one month not 3 March?

February has no 31st, so the result clamps to the last day of February, matching spreadsheets and most legal practice. If you actually want 31 days later, switch the unit to days.

Yes — enter a negative amount. -90 days gives the date 90 days ago, and the same works for weeks, months, and years.

DATE AND TIME · CALCULATED IN YOUR BROWSER

Date Add and Subtract Calculator

Move a date forward or back by days, weeks, months, or years.

Enter a valid date.

Input

Starting date

Amount

Unit

About this tool

Pick a starting date, enter an amount and a unit, and get the resulting date and its weekday. Adding months clamps to the last day of the target month, so 31 January plus one month lands on 28 February (29 in a leap year) instead of sliding to 3 March. Common shortcuts such as 100 days, six months, and one year are one click away.

How it works

Enter the starting date.
Enter an amount and pick days, weeks, months, or years; negatives go backwards.
Read the resulting date and weekday.

Assumptions

Month arithmetic clamps at the end of the month, so 31 January plus one month is 28 or 29 February rather than spilling into March.

Privacy

Every calculation runs inside this browser. Dates and birthdays are never uploaded, stored, or written to the URL or any log.

Frequently asked questions

Why is 31 January plus one month not 3 March?: February has no 31st, so the result clamps to the last day of February, matching spreadsheets and most legal practice. If you actually want 31 days later, switch the unit to days.
Can I go backwards?: Yes — enter a negative amount. -90 days gives the date 90 days ago, and the same works for weeks, months, and years.

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