Public Function Func(ByVal Arg As String)
        DieEigentlicheFunktionArg , New DateTime(0))
    End Function
    Public Function Func(ByVal Arg As String, ByVal d As DateTime)
        DieEigentlicheFunktion(Arg , d)
    End Function
    Private Function DieEigentlicheFunktion(ByVal Arg As String, ByVal d As _
      DateTime)
        ' Dein Code
        MsgBox(d.ToString)
    End Function
' In Form
        Func("Argument")
        Func("Argument", New DateTime(2003, 10, 30, 10, 10, 10, 10))