public function convertToDatabaseValue($value)
    {
        if (!$value instanceof \DateTime) {
            return null;
        }
        
        $timezone  = $value->getTimezone()->getName();
        $timestamp = $value->getTimestamp();
        $date      = new \MongoDate($timestamp);
        
        return array(
            'date' => $date,
            'tz'   => $timezone,
        );
    }