/**
     *    日期-獲取當月最后一天
     *  @return int
     */
    public function get_lastday() {
        if($this->month==2) {
            $lastday = $this->is_leapyear($this->year) ? 29 : 28;
        } elseif($this->month==4 || $this->month==6 || $this->month==9 || $this->month==11) {
            $lastday = 30;
        } else {
            $lastday = 31;
        }
        return $lastday;
    }
    /**
     *    日期-是否是閏年
     *  @return int
     */
    public function is_leapyear($year) {
        return date('L', $year);
    }