Days between two dates, adding or subtracting from a date, or counting down to something. It shows both counting conventions, because from 1 January to 2 January is either one day or two and most calculators never tell you which one they used.
Ask how long it is from 1 January to 2 January and there are two defensible answers. The difference is one day. The number of dates involved is two. Neither is wrong.
Which you want depends entirely on what you are counting. A hotel charges you for one night. A contract giving you two days' notice counts both dates. Annual leave usually counts every day you are away, so both ends. Interest on a loan almost always counts the difference.
The trouble is that calculators pick one and say nothing, so two people can look up the same two dates and get answers that differ by a day. On a short span that is a big proportional gap. This one shows both and labels what each is for.
Working days here means the total with Saturdays and Sundays taken out. It does not remove public holidays, and that is a deliberate choice rather than an omission.
Public holidays are not national in the way people assume. Australia sets them state by state, so Labour Day lands in different months depending on where you are, and some states have holidays that exist nowhere else. The United States has federal holidays plus state ones. The United Kingdom splits three ways. On top of that, dates move each year, and a holiday landing on a weekend gets observed on a different day.
Quietly assuming one country's calendar would give most of the world a wrong answer with no warning. Counting weekends and saying so lets you subtract the holidays that actually apply to you.