I'd probably use something like a Worksheet_Change event for this.  These are
executed when something is changed on a worksheet.  You can selectively run
the code based on which cells change, etc.  

See example below:

Private Sub Worksheet_Change(ByVal Target As Range)
Dim myrange As Range
Dim r As Range
Dim EmptyRange As Boolean
Set myrange = Range("G7:I7")

if not intersect(target,myrange) is nothing then
EmptyRange = False
For Each r In myrange
   If IsEmpty(r) Then
       EmptyRange = True
       Exit For
   End If
Next r

If EmptyRange Then
   Cells("J7").Value = WorksheetFunction.Text(Now, "mm-dd-yyyy")
Else
   'Whatever you'd do if you already have something in all cells.
End If
End if
End Sub