Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Cells(1, 1).Column <> 12 Then Exit Sub
Dim c As Range
For Each c In Target
If c.Column = 12 Then
If c.Value = "" Then
c.Offset(0, 1).Value = ""
Else
c.Offset(0, 1).Value = Format(Now, "yyyy-mm-dd hh:mm:ss")
End If
End If
Next
End Sub