From Monday to Friday is four days apart. A Monday-through-Friday course takes place on five dates. Both answers can be right: they answer different questions.
Elapsed date difference
A simple date difference excludes the starting date. January 1 to January 2 is one day. January 1 to January 1 is zero days. Use this convention when you want to know how far apart two calendar dates are.
Inclusive date counting
If both the first and last dates belong in your total, add one to the ordinary difference. A course running from Monday through Friday includes five calendar dates. The date calculator provides an explicit checkbox for this choice, and labels the result so you can see which rule you used.
Ask a clearer question: “How many dates are included from July 1 through July 7?” includes both endpoints. “How many days after July 1 is July 7?” measures the six-day difference.
Calendar days are not always 24 hours
Moving from one local date to the next is a calendar operation. Measuring time between two instants is an elapsed-time operation. When local clocks change, midnight-to-midnight can be shorter or longer than 24 hours. Our date calculator works with calendar dates, so those clock changes do not alter its answer.
Weekdays need a definition
Here, weekdays mean Monday through Friday. A weekday total is not a country-specific business-day total: it does not remove holidays, regional closures, or organization-specific non-working days. In ordinary difference mode, the weekday count excludes the start and includes the end; inclusive mode counts both.
When adding weekdays, the start date is the anchor and is not counted. Adding one weekday to a Friday gives Monday. Adding zero leaves the date unchanged, even if it falls on a weekend. These are calculation conventions; confirm the rules that apply before using the result for a formal deadline.
Technical reference for separating calendar dates from elapsed time: Temporal.PlainDate documentation.