For projects that don't start on the prescribed date or changes in project plan...
In the scheduling process, an Activity's type determines which assigned calendar will be used to calculate its start and finish dates. Calendars specify valid working periods and nonworking periods such as weekends, holidays, and exception time.
To assign Duration Type to an activity:
Step 1: Navigate to the Activities page
Step 2: From the list view, click the + symbol corresponding to the activity
Step 3: Click Add Activity to expand and select Duration