src/Birthday/Birthday.php
public function format($format = null): string
    {
        if (empty($format)) {
            return $this->birthday;
        }

        return date($format, strtotime($this->birthday));
    }
public function format($format = null): string
    {
        if (empty($format)) {
            return $this->birthday;
        }

        return date($format, strtotime($this->birthday));
    }
Return the birthday in given format

@param  string   $format
@return string