Count the gap for elapsed days
From June 1 to June 2, one calendar-day boundary passes. That is one elapsed day. From June 1 to June 1, none passes, so the difference is zero. This is the default in our date-difference tool.
Count both ends for an inclusive span
A list containing June 1 and June 2 has two dates. Inclusive counting describes that list, so it adds one to the elapsed difference. This can be useful when listing the dates covered by an event or activity.
Write the convention beside the answer
The number by itself can be ambiguous. Write “elapsed days” or “including both dates” when you share a result. Keep the start and end dates visible too; that makes it much easier for someone else to reproduce your calculation.