private function updateJalaliFromGeorgian($force = false) {
        
        if ($this->converted && ! $force) {
            return;
        }
        
        list($year, $month, $day) = self::julian2jalali(self::georgian2julian($this->carbon->year, $this->carbon->month, $this->carbon->day));
        
        $this->year  = $year;
        $this->month = $month;
        $this->day   = $day;
        
        // it tells that the jalali core is updated
        $this->converted = true;
        
    }