How to add 90 days to a date
Inputs: Base date June 23, 2026, Add 90 Days.
Expected result: Monday, September 21, 2026.
Date Calculator
Count days between dates, add or subtract days, weeks, months, or years, and calculate business days.
Calculate a Date
Use the date picker or type a date. Ambiguous typed dates follow the selected format; ISO dates are always accepted.
Common Presets
Count business days
Add or subtract business days
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How Calculations Work
- Calendar days: exclusive days use
end date - start date. Inclusive days add one day when counting forward or backward. - Inclusive vs exclusive: inclusive counts both endpoints; exclusive skips the start date and counts completed days up to the end date.
- Business days: count each date in the selected range when it matches the business-day settings and is not an excluded holiday.
- Leap years: dates are valid Gregorian calendar dates, so February 29 is included in leap years.
- End-of-month clamping: adding months or years keeps the day number when possible; otherwise it uses the last valid day of the target month.
- Today and timezones: Today presets use your browser’s local date. Calculations normalize selected dates at midnight UTC to avoid daylight-saving changes.
Practical Examples
How to count days between two dates
Inputs: April 1, 2026 to April 30, 2026.
Expected result: 29 exclusive days or 30 inclusive days.
Shipping deadline
Inputs: June 23, 2026 + 7 calendar days.
Expected result: Tuesday, June 30, 2026.
Contract notice period
Inputs: April 1, 2026 to April 30, 2026, inclusive.
Expected result: 30 calendar days.
School term duration
Inputs: September 2, 2025 to December 19, 2025, inclusive.
Expected result: 109 calendar days.
Invoice net-30 or net-60
Inputs: Invoice date June 23, 2026, add 30 or 60 days.
Expected result: Net-30 is Thursday, July 23, 2026; net-60 is Saturday, August 22, 2026.
Medical waiting period
Inputs: March 3, 2026 + 14 calendar days.
Expected result: Tuesday, March 17, 2026.
Project deadline with business days
Inputs: June 23, 2026 + 10 business days, excluding common U.S. federal holidays.
Expected result: Wednesday, July 8, 2026.
Calculation Transparency
Runs locally in your browser
Uses Gregorian calendar rules
Leap years included
Last reviewed: June 23, 2026
To report a calculation issue, include the start date, end date, selected options, and copied summary when contacting Starlight Tools.
Date Calculator: FAQs
Does the calculator include the end date?
Use Inclusive when the end date should be counted. Exclusive counting skips the start date and counts completed days up to the end date.
How do I calculate 30, 60, or 90 days from today?
Use the preset buttons for 30, 60, or 90 days from today, or choose Add/Subtract, set the base date to today, choose Add, enter the number of days, and calculate.
Are weekends counted?
Calendar-day results include weekends. Business-day results exclude Sundays and exclude Saturdays unless you turn on Saturday as a business day.
Are holidays excluded?
Holidays are excluded only when you turn on common U.S. federal holidays or add dates to the custom holiday list.
Is Saturday a business day?
Saturday is not counted by default. Turn on Saturday as a business day if your schedule or deadline includes Saturdays.
What happens when adding months to the 31st?
Month and year calculations clamp to the last valid day of the target month, so January 31 plus one month becomes the last day of February.
What timezone is today based on?
The Today presets use the browser's current local date, while calculations normalize selected dates at midnight UTC to avoid daylight-saving time shifts.
Can I calculate dates in the past?
Yes. You can count backward between dates, subtract time from a base date, or use the days-ago preset buttons.
Is my data private?
Yes. All calculations run locally in your browser. No dates are uploaded or stored on a server.