Need help to understand how can I calculate time taken between 2 tasks in a project.very simply , like the time taken between date of Order confirmation and Date of Kick off meeting.I want to add this as a field in my custom form. Thanks for your assistance.

when I set up a Iteration for 2 weeks ( meaning 10 working days ) then why i see in project plan it is 9,38 days or more or less and not 10 days. If you can please help with the formula behind.  Thanks for assisting .