Take a look here:

   http://www.mcgimpsey.com/excel/timestamp.html

You'll have to modify the macro somewhat. Perhaps one way:

   Private Sub Worksheet_Change(ByVal Target As Excel.Range)
       Dim bValid As Boolean
       With Target
           If .Count > 1 Then Exit Sub
           If Not Intersect(Range("E2:E100"), .Cells) Is Nothing Then
               If IsNumeric(.Value) Then _
                       bValid = (.Value > 200) And (.Value < 226)
               On Error GoTo ErrorExit
               Application.EnableEvents = False
               With .Offset(0, -1)
                   If bValid Then
                       .NumberFormat = "dd mmm yyyy hh:mm:ss"
                       .Value = Now
                   Else
                       .ClearContents
                   End If
               End With
   ErrorExit:
               Application.EnableEvents = True
           End If
       End With
   End Sub