Include end date
Checking Include end date in calculation counts the final day in your result.
If you want to count how many days something lasted, for example, an event that lasted from the 1st to and including the 4th day of the month, this should be checked (result: 4 days.) Otherwise, the last day would not be counted (returning 3 days).
If you want to calculate the age of something, leave this unchecked (the last/end date is not counted as one additional day).