A leap year contains 366 days because February has 29 days instead of 28. When a date interval crosses February 29, its total elapsed-day count may be one day greater than a comparable interval in a common year. Calendar-year and anniversary calculations may also require a consistent rule for February 29 dates.
Enter two dates to calculate exact years, months, weeks, days, weekdays, weekends, and inclusive date counts.
An interval crossing leap day must include it in the total elapsed-day count.
What Is a Leap Year?
A leap year contains 366 calendar days instead of 365.
The additional date is February 29.
The Date Difference Calculator counts every actual calendar day, including leap days.
Leap-Year Rule
Most years divisible by four are leap years.
Century years must also be divisible by 400.
This means 2000 was a leap year, while 1900 was not.
Worked Example: February 28 to March 1
In a common year, February 28 to March 1 is one elapsed day.
In a leap year, February 29 lies between the dates.
The leap-year interval is therefore two elapsed days.
When Leap Day Affects an Interval
February 29 affects the total only when it lies within the selected interval.
An interval entirely before or after leap day is unaffected.
A long interval may cross more than one leap day.
Leap Years and Full-Year Differences
A calendar year is counted from one anniversary date to the corresponding date in the next year.
A full calendar year may contain 365 or 366 elapsed days.
This is why calendar-year counts and decimal-year calculations can differ.
February 29 Anniversaries
A date such as February 29 has no identical date in a common year.
A calculation therefore needs a consistent rule for the anniversary.
Depending on the context, February 28 or March 1 may be used.
Leap Day and Inclusive Counting
Inclusive counting includes every boundary and intermediate date.
An interval including February 29 counts it like any other calendar date.
Read Inclusive vs Exclusive Date Counting for boundary rules.
Leap Years and Week Calculations
A leap day can change the remainder when total days are divided by seven.
In longer intervals, it may contribute to an additional complete week.
Always find the real total-day difference before converting it into weeks.
Leap Years and Month Calculations
Leap day changes February's elapsed-day length.
It does not make every February-to-March calendar interval one day longer in calendar-month terms.
Exact month calculations follow corresponding calendar boundaries.
Counting Multiple Leap Days
For long intervals, examine each year crossed by the dates.
Count February 29 only when it lies inside the interval.
Century rules must be applied correctly.
Common Mistakes
Do not assume that every fourth year is always a leap year without applying the century rule.
Do not add a leap day when February 29 is outside the interval.
Do not assume that every calendar year contains exactly 365 days.
Do not ignore the chosen anniversary rule for February 29 dates.
Conclusion
Leap years add February 29 and can increase an elapsed-day result by one.
Calendar calculations should use actual dates rather than fixed 365-day assumptions.
Use the Date Difference Calculator to see how many leap days an interval crosses.
FAQs
How many days are in a leap year?
A leap year contains 366 days.
When does leap day affect a date difference?
It affects the total when February 29 lies within the interval.
Is every year divisible by four a leap year?
Century years must also be divisible by 400.
Was 2000 a leap year?
Yes, because it is divisible by 400.
How is a February 29 anniversary handled?
A consistent rule may use February 28 or March 1 in a common year.