public function getDateMonday($format = 'Y-m-d')
    {
        $weekDay = $this->format('N') - 1;
        $dateTime = clone $this;
        if ($weekDay) {
            $dateTime->add(sprintf('- %d days', $weekDay));
        }
        if ($format) {
            return $dateTime->format($format);
        }
        return $dateTime;
    }