Inclusive vs exclusive days between dates
Elapsed days count the gap between dates. Counting both endpoint dates adds one to a forward range: Monday to Tuesday is one elapsed day but two dates.
Choose what the number represents
For the dates 2026-10-05 and 2026-10-06, the calendar difference is one day. Including both dates counts two dates. For the same start and end date, the gap is zero while the inclusive count is one.
Use the endpoint control in the date-difference tool to match that meaning. A count of calendar dates is not a count of working days or a guarantee of a legal deadline.
Adding working days has its own start rule
The business-days tool excludes Saturday and Sunday by default. In Count mode it includes the start date when that date is a working day. Add mode starts counting after the start date.
Enter the holidays that apply to your situation. The tool has no built-in country holiday calendar. Duplicate holiday dates count once.
Keep weeks and months distinct
A week is seven calendar days. Calendar months do not all contain the same number of days, so choose the unit you mean in the date calculator rather than treating a month as a fixed number of weeks.