I have a calculated column with specific formula, which returns single number. The number represnts number of days between task created and task closed. I want to display an average counter to this task. When I go to "modify this view" then "totals" my column is not displayed there . How do I add average counter to it?
You cannot use a calculated column for totals within a view. You could use a workflow to move the information over to a numeric field and then choose average within your views for that column.
Your formula could be:
This would calculate the number of days between the two dates.