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Date Calculator

Calculate the number of days between two dates or add/subtract days from a date. Includes business days option and year/month/week breakdown.

Days Between Worksheet

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

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Frequently Asked Questions

How do I calculate days between two dates?

Enter the start and end dates. The calculator computes the exact number of calendar days, weeks, months, and years between them.

What are business days?

Business days are weekdays (Monday through Friday), excluding weekends. This calculator can count business days only by toggling the option.

Does the calculator account for holidays?

The business days calculation excludes weekends but not public holidays, as holidays vary by country and region.

Can I add or subtract days from a date?

Yes. Switch to "Add/Subtract Days" mode, enter a start date and number of days (negative to subtract), and see the resulting date.

How does it handle month boundaries?

When adding days across months of different lengths, the calculator uses actual calendar arithmetic. Adding 30 days to January 15 correctly lands on February 14.

Does adding one month always mean adding 30 days?

No. Adding one calendar month moves the date by whatever days are in that specific month. January 31 plus one month lands on March 3 (or March 2 in a leap year) because February has no 31st. The calculator uses calendar months, not fixed 30-day blocks.

How do I count days inclusive of both the start and end date?

Add 1 to the result. Date difference calculations count elapsed time, so the span from the 1st to the 5th is 4 days. If you need to count both endpoints as full days, add 1 to the displayed day count.

What is a practical example of adding days to a date?

A project starts March 7 with a 45-day deadline. Enter March 7 as the start, set the mode to add 45 days, and the calculator returns April 21. This avoids manual month-boundary errors that are easy to make by hand.