How to Calculate Days Between Dates
Finding the exact number of days between two dates requires accounting for months of varying lengths (28-31 days) and leap years. This calculator handles all of that automatically.
Calendar Days vs Business Days
Calendar days count every day — weekdays, weekends, and holidays. Business days count only Monday through Friday. The difference matters for deadlines, shipping estimates, and project planning.
For a 30-calendar-day span, you'll typically have about 22 business days (though this varies based on which day of the week you start).
Adding and Subtracting Days
Need to know what date falls 90 days from now? Or what date was 60 days ago? The add/subtract mode handles this. Enter a positive number to go forward, negative to go backward.
Common Date Calculations
- 90-day notice period: Enter today + 90 days
- Project deadlines: Days between start and due date
- Shipping estimates: Business days from order to delivery
- Pregnancy due date: 280 days from last menstrual period
- Visa expiration: Days remaining on a visa or document
Date Arithmetic Formula
For total days: days = floor((endDate - startDate) / 86400000)
For business days, the calculator iterates through each day and counts only weekdays (day of week 1-5).
The year/month/day breakdown uses calendar-aware subtraction, borrowing days from the previous month when the day value would go negative.