If macro's are not disabled you can use this event to add the date/time in a cell
every time you save the file (I use A1 in :Sheet1")
Copy this in the Thisworkbook module of the workbook
Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
Sheets("Sheet1").Range("A1").Value = Format(Now, "dd-mmm-yy h-mm-ss")
End Sub