How this calculator works
Adding days or weeks is plain calendar arithmetic — the calculator moves forward or back the exact number of days, so leap years and month lengths are handled automatically. Months and years keep the same day of the month; if that day doesn’t exist in the target month (adding one month to January 31), the date moves to the last day of the month, which is how most contracts and banks count.
Business days skip Saturdays and Sundays and, if you choose, the 11 US federal holidays on the days they are observed. The start date itself is not counted — “10 business days from Monday” lands on the Monday two weeks later.
Assumptions and tips
- The start date is day 0; counting begins the next day.
- US federal holidays follow 5 U.S.C. 6103 (observed on Friday/Monday when they fall on a weekend).
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Start date | 2026-11-05 |
| Add or subtract | Add (+) |
| Amount | 90 |
| Unit | Days |
| Result | Value |
|---|---|
| New date | Wednesday, February 3, 2027 |
| Calendar days | 90 days |
| Start date | Thursday, November 5, 2026 |
| In weeks | 12 weeks, 6 days |
| ISO date | 2027-02-03 |
Step by step:
- Start: 2026-11-05 (day 0)
- Add 90 days
- Move that many calendar days (month lengths and leap years are handled automatically)
Common deadlines from November 5, 2026
Calculated with this tool from Thursday, November 5, 2026.
| Add | New date |
|---|---|
| 7 days | Thursday, November 12, 2026 |
| 10 business days | Friday, November 20, 2026 |
| 14 days | Thursday, November 19, 2026 |
| 21 days | Thursday, November 26, 2026 |
| 30 days | Saturday, December 5, 2026 |
| 45 days | Sunday, December 20, 2026 |
| 60 days | Monday, January 4, 2027 |
| 90 days | Wednesday, February 3, 2027 |
| 120 days | Friday, March 5, 2027 |
| 180 days | Tuesday, May 4, 2027 |
| 6 months | Wednesday, May 5, 2027 |
| 1 year | Friday, November 5, 2027 |
Common mistakes to avoid
- Counting the start date as day 1 — “30 days from today” starts counting tomorrow.
- Adding 30 days when a contract says “one month” (or the other way round) — they are different.
- Forgetting holidays when a deadline is in business days.
Frequently asked questions
What is 90 days from November 5, 2026?
Wednesday, February 3, 2027.
What date is 30 days from today?
Enter today (or leave the start date blank), choose Add, 30, Days. The answer updates for whatever day you open the page.
How do I count 10 business days?
Choose “Business days”. Weekends — and, if selected, federal holidays — are skipped, and the start day is not counted.
What is one month after January 31?
February 28 (or February 29 in a leap year) — the calculator uses the last day of the shorter month.
Sources
Built and reviewed by Anirban Kundu, Creator & Editor · M.Sc Physics, 20 years teaching science