Counting forward and back on the calendar
Deadlines are written as "within 30 days", "90 days after delivery" or "6 months from the start date", and counting them on a wall calendar is easy to get wrong across month ends and leap years. This calculator counts the start date as day 0, applies years and months on the calendar first, then adds weeks and days — the convention used by most U.S. contract, billing and court-deadline rules.
How the date is worked out
1. Start date + years × 12 + months (clamped to month end) 2. + weeks × 7 days 3. + days (calendar days, or Mon–Fri only if business days is on) Day of year: days since Jan 1 + 1 ISO week: weeks start Monday; week 1 contains Jan 4
Worked example
Start Friday, October 30, 2026, add 90 days: 1 day finishes October, 30 more finish November (31), 31 more finish December (62), and 28 into January makes 90 → Thursday, January 28, 2027.
Add 10 business days instead: two full Monday–Friday weeks → Friday, November 13, 2026 (14 calendar days later).
Notes
- Dates are calculated in UTC on whole days, so daylight-saving changes never shift the answer.
- Holidays are not excluded from business days; check federal holidays at OPM if your deadline depends on them.
Sources: U.S. Office of Personnel Management, Federal Holidays; ISO 8601-1:2019 (calendar dates and week numbering).
Frequently asked questions
What date is 90 days from today?
Enter today's date, choose Add and type 90 in Days. Counting from October 30, 2026, 90 days later is January 28, 2027. The start date itself is day 0, so "1 day from Monday" is Tuesday — the same way deadlines in contracts and court rules are usually counted.
How are months added when the day does not exist?
Months are added on the calendar and clamped to the last day of a shorter month: January 31 + 1 month = February 28 (or 29 in a leap year), and February 29, 2024 + 1 year = February 28, 2025. Years and months are applied first, then weeks and days.
How do business days work?
With "Count business days only" on, the Days value skips Saturdays and Sundays: Friday + 1 business day = the following Monday. Weeks still count as full calendar weeks. Public holidays are not skipped because they differ by state, country and employer — add one day for each holiday that falls inside the range.
Can I count backward?
Yes. Choose Subtract to find the date a number of days, weeks, months or years before the start date — for example a filing deadline 30 days before a hearing, or a birth date from an age.