How to get the numbers of days a case has been open in Excel? [Closed]

Report
-
I want show the number of days a case has been open in an Excel field. Example: a case was open in 01/01/2017 and has not been closed as of today. How many days the case has been open if no date is shown in the case closed field.
Colum 1: Case Open (date)
Colum 2: Case Closed (date)
Colum 3: How Many Days Case Has Been Open (Number of Days = ie. 100)
I want to show the numbers of days using today's date.