private function ultimoDiaMes()
    {
        $ts = $this->ts;
        $mesAtual = strftime('%m', $ts);
        $mesTmp = $mesAtual;
        $i = 0;
        while ($mesAtual == $mesTmp) {
            $ts = $this->somaDia(++ $i);
            $mesTmp = strftime('%m', $ts);
        }

        return $this->somaDia(-- $i);
    }