How date arithmetic works
- Days and weeks are exact: one week is always 7 days.
- Months move to the same day number in the target month. If that day does not exist, the result is the last day of that month.
- Years are 12 months, so February 29 plus one year becomes February 28.
Example: 90 days after September 28, 2026 is Sunday, December 27, 2026. One month after January 31, 2024 is February 29, 2024 (a leap year); one month before March 31, 2026 is February 28, 2026.
Common uses
- Payment and contract deadlines (net 30, 60 or 90 days).
- Notice periods, probation periods and warranty expiry dates.
- Medication schedules and follow-up appointments.
- Project planning and countdowns.
Frequently asked questions
What is 90 days from today?
Enter today’s date (the default), choose “Add” and type 90 in the days field. The result shows the exact date and day of the week.
What happens when I add a month to January 31?
There is no February 31, so the result moves to the last day of February (28th, or 29th in a leap year). This “end-of-month” rule is the one used by most banks and spreadsheet software.
In what order are years, months, weeks and days applied?
Years and months are applied first, then weeks and days. Adding “1 month and 1 day” to January 31, 2026 gives February 28 + 1 day = March 1, 2026.
Can I skip weekends?
This calculator counts calendar days. To add only working days, use the business days calculator.