gistfile1.vb

Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Column = 1 Or Target.Column = 5 Then
        Dim value As String
        value = "" & Target.value
        If Len(value) = 6 Then
            Target.value = "平成" & Int(Mid(value, 1, 2)) & "年" & _
                Int(Mid(value, 3, 2)) & "月" & _
                Int(Mid(value, 5, 2)) & "日"
        End If
    End If
End Sub