public function add_days($days)
    {
        
        if ($days < 0) {
            return $this->sub_days($days * -1);
        }
        
        if ($this->flag_jours_ouvres == true) {
            for ($d = 0; $d < $days;) {
                $this->time += self::$TIME2SEC['d'];
                $this->set_timestamp($this->time);
                
                if ($this->est_jours_ouvres()) {
                    $d++;
                }
            }
        }
        else {
            $this->time += $days * self::$TIME2SEC['d'];
            $this->set_timestamp($this->time);
        }
        
        return $this;
    }