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.
Leap Year Rules
A year is a leap year if it's divisible by 4, except centuries (divisible by 100) are not, unless the century is also divisible by 400. So 2024 is a leap year, 1900 was not, but 2000 was. This rule keeps the calendar aligned with the Earth's 365.2422-day orbit.
Business Days Per Month
A typical month has 20-23 business days (weekdays excluding holidays). The average across a year is about 21.7 business days per month, or roughly 260 business days per year. This is useful for estimating project timelines and delivery windows.
Quick Date References
- 30 days: Standard notice period, most return policies
- 90 days: Common probation period, visa windows
- 180 days: Many warranty periods, 6 months
- 365 days: One year (366 in leap year)
- 10,000 days: About 27.4 years