Function to run when Workbook opens

Hi all!

I need this function to work (to display the date/time sheet was last saved) everytime I open the Workbook.
The function works but only after I go to the cell where it is located and hit enter.

     Function LastSavedTimeStamp()
          LastSavedTimeStamp = ActiveWorkbook.BuiltinDocumentProperties("Last Saved Time")
     End Function

Thanks for any assistance.