Formula for identifying start date and finish date and associated number of days

I have a number of activities within a project with each activity having a start date and a finish date.

What I would like is a function that looks at the 1st activity start date and the last activity finish date and convert this into the total amount of workdays taken to complete the project.

Attached is a spreadsheet with generic data. Column A references the project with the cells I am looking to use highlighted in yellow columns B and C.

Also I only need the dates, not the time so would like to know how to remove the time as well.
Thanks in advance
FBS