How dates are added
Years and months are calendar steps: the month number moves forward or back and the day of the month stays the same. If that day does not exist in the new month, the date is moved to the month's last day. Weeks and days are then added as exact counts of 7 and 1 days.
Formula: result = clampToMonthEnd(start + years×12 + months) ± (weeks×7 + days)
Worked example: 31 January 2024 + 1 month + 10 days
- One month after 31 January would be 31 February, which does not exist.
- 2024 is a leap year, so the last day of February is the 29th.
- Add 10 days: 29 February + 10 days = 10 March 2024.
Practical uses
- Finding a due date such as “90 days from today” for invoices or notices.
- Setting a renewal or expiry date a number of months ahead.
- Working backward from a deadline to a start date by subtracting.
- Planning schedules in weeks, such as training or pregnancy-week calendars.
Limitations
- Results are limited to years 1–9999.
- Calendar math is not reversible at month ends: 31 Jan + 1 month − 1 month gives 29 Jan in a leap year.
To measure the gap between two dates, use the Date Difference Calculator.
Frequently asked questions
What is 31 January plus one month?
The last day of February: 28 February, or 29 February in a leap year. When the day does not exist in the target month, the calculator uses that month's last day.
In what order are the amounts applied?
Years and months are applied first on the calendar, then weeks and days are added as exact day counts.
Is adding 12 months the same as adding 1 year?
Yes. Both move the date to the same day in the following year (with the same end-of-month rule).
What date is 90 days after 1 January 2025?
1 April 2025. Days are counted exactly, so the result does not depend on month lengths. Check whether a contract counts the first day, since some legal deadlines do.