Day Counter Calculator: Days Between Dates, Business Days & Add/Subtract Days
Count days between two dates or add/subtract business days. Include end day, exclude weekends and holidays (US, UK, Canada, custom). Doomsday Algorithm proof. Project timelines, deadlines, notice periods. Free, runs locally, no sign-up.
How Many Days Between Two Dates? The Math Behind the Count
Days between two dates come from the millisecond difference divided by 86,400,000: (or one less for exclusive end). Inclusive vs exclusive, leap years, and varying month lengths are handled automatically. Weekday and weekend counts use a constant-time formula from the start date's day-of-weekโno per-day loopโso even multi-year ranges return instantly.
Business Days vs Calendar Days: When to Use Each
Calendar days include every dayโuseful for lease terms, visa validity, or "X days from today" in legal or travel. Business days exclude Saturdays, Sundays, and optionally holidaysโthe norm for project timelines, SLAs, and notice periods. Duration Between mode gives the full breakdown; Add/Subtract with "Count Business Days Only" steps through weekdays only, skipping holidays you configure (US Federal, UK, Canada, or custom).
The Doomsday Rule: Finding the Day of the Week
The Doomsday Algorithm finds the weekday of any date. Each year has a "Doomsday"โa weekday that 4/4, 6/6, 8/8, and 12/12 fall on. The formula combines the century anchor d with year terms a = โyy/12โ, b = yy mod 12, c = โb/4โ. From the year's Doomsday, count forward or back to any date. The Information Hub's Doomsday Proof tab runs through it step-by-step for your start date.
How to Use the Day Counter Calculator
- Duration Between:Pick start and end dates. Toggle "Include End Day in Count" for inclusive vs exclusive. Read the Day Count Analysis for total days, weekdays, weekends, and holidays.
- Add/Subtract Days:Enter start date and number of days. Choose Add (+) or Subtract (โ). Enable "Count Business Days Only" to skip weekends and holidays.
- Holidays to Skip:Select US Federal, UK, Canada Federal, or enter custom YYYY-MM-DD dates. Excluded dates appear in the Holidays row and are skipped in business-day stepping.
Day Counter Calculator FAQ
? How do I count days between two dates?
Use "Duration Between" mode: pick start and end dates. Toggle "Include End Day in Count" for inclusive (both count) or exclusive. The breakdown shows total days, weekdays, weekends, and holidays.
? How do I add or subtract business days?
Use "Add/Subtract Days" mode, enable "Count Business Days Only," and enter start date and number of days. The calculator steps through weekdays only, skipping weekends and holidays (US Federal, UK, Canada, or custom YYYY-MM-DD). Good for deadlines, notice periods, and "10 working days from today."
? What is the difference between weekdays and business days?
Weekdays are MondayโFriday. Business days are weekdays minus holidays. Total days use where ฮt is the millisecond difference. Weekday count uses a constant-time formulaโno per-day loopโso results are instant for any range.
? How do I exclude custom holidays?
In Settings, choose "Custom dates..." and enter dates in YYYY-MM-DD format (comma or newline separated). Those dates are excluded and listed under Holidays. Also supports US Federal, UK, and Canada Federal.
? What is the Doomsday Algorithm?
The Doomsday Rule finds the weekday of any date. Each year has a "Doomsday"โa weekday that 4/4, 6/6, 8/8, 12/12 fall on. The formula uses the century anchor d and year terms a, b, c. The Information Hub's Doomsday Proof tab shows the step-by-step for your start date.