How do you calculate the number of days between two dates?

Count the remaining days in the start month, add the days in each full month between, then add the days in the end month. For larger gaps, use a date duration calculator for accuracy.

How many business days are in a year?

Approximately 260-262 business days per year (52 weeks × 5 working days), minus public holidays. The exact number varies by year and country.

Do leap years affect date calculations?

Yes. Leap years add February 29, adding an extra day to any calculation that spans that date. Most calculators handle leap years automatically.

Should I include the end date when calculating duration?

It depends on the context. Hotel nights and flight durations use exclusive counting. Project deadlines and legal terms may use inclusive counting. Always verify which convention applies.

Date Duration Calculator: How to Calculate Days Between Dates

Need to know how many days are between your vacation start date and the day you return? How many business days until a project deadline? Or how long ago a historical event occurred? This guide covers every way to calculate the duration between two dates — with step-by-step methods and real-world examples.

Calculating Days Between Two Dates

The simplest calculation counts the total number of days from one date to another. This can be done by hand for nearby dates or with a calculator for larger gaps.

Manual Method for Nearby Dates

Count the remaining days in the starting month
Add full months between the dates (using the correct number of days per month)
Add the days in the ending month

Example: Days from March 15 to April 28

Days remaining in March: 31 - 15 = 16
Days in April: 28
Total: 16 + 28 = 44 days

Example: Days from January 1 to December 31 (same year)

Jan: 30 (remaining after Jan 1), Feb: 28/29, Mar: 31, Apr: 30,
May: 31, Jun: 30, Jul: 31, Aug: 31, Sep: 30, Oct: 31,
Nov: 30, Dec: 31
Total (non-leap year): 364 days (365 - 1)

Expressing Duration in Years, Months, and Days

For longer spans, it is more useful to express the duration as a combination of years, months, and days:

Count full years from the start date
Count full months after the last full year
Count remaining days after the last full month

Example: Duration from June 15, 2020 to September 3, 2025

Unit	Calculation
Years	June 15, 2020 → June 15, 2025 = 5 years
Months	June 15 → Sept 15 = 3 months (but Sept 3 < Sept 15, so 2 months)
Days	Aug 15 → Sept 3 = 19 days
Result	5 years, 2 months, 19 days

Counting Business Days

In business contexts, you often need to exclude weekends (Saturday and Sunday) from the count. Some calculations also exclude holidays.

Manual Method

Calculate the total calendar days between dates
Count the number of full weeks (each has 5 business days)
Count the remaining days and determine how many are weekdays
Sum the business days from full weeks + remaining weekdays

Example: Business days from Monday, March 3 to Friday, March 21, 2025

Total calendar days: 18 (inclusive: March 3-21)
Full weeks: 2 (10 business days)
Remaining days: Mon-Fri = 5 weekdays, but only 3 days remain (Wed-Fri)
Weekends: 2 Saturdays + 2 Sundays = 4 weekend days
Business days: 18 - 4 = 14 business days

Leap Year Considerations

Leap years add an extra day (February 29) to the calendar. This affects any date calculation that spans February in a leap year.

Leap year rules: A year is a leap year if it is divisible by 4, except for years divisible by 100 (unless also divisible by 400).

2024: leap year (divisible by 4)
2000: leap year (divisible by 400)
1900: NOT a leap year (divisible by 100 but not 400)
2025: not a leap year

Should You Include the End Date?

Different contexts treat the end date differently:

Exclusive: Count days between dates (start +1, end -1). March 15 to March 16 = 1 day.
Inclusive: Count both start and end dates. March 15 to March 16 = 2 days.

Tip: Hotel nights, flight durations, and rental periods use the exclusive count. Project deadlines and legal deadlines often use inclusive counting. Always verify which convention applies to your situation.

Common Use Cases

Project planning: How many business days until the deadline?
Travel planning: How many days between your departure and return?
Age calculation: How many days/months/years since birth?
Legal deadlines: How many days until a contract expires?
Event countdown: How many days until a wedding, graduation, or holiday?
Interest calculations: How many days between loan start and end?

Key Takeaways

Simple duration: count days remaining in start month + full months + days in end month
Complex duration: break into years → months → days for readability
Business days: subtract weekends (and optionally holidays) from total calendar days
Leap years add an extra day in February — watch for spans crossing February 29
Verify whether to count the end date inclusively or exclusively
Use our Date Duration Calculator for instant results including business days, weekends, and inclusive counting