Bu kod satırı:

echo "<strong> {$this->author->last} {$this->date->shortYear()}</strong> ...";

bana bu hata veriyor:

Fatal error: Call to undefined method Date::shortYear() in /f5/debate/public/libs/Card.php  on line 22

Hatta (Card.php dahil) Date.php olsa:

class Date {
    public $day;
    public $month;
    public $year;

    public function shortYear() {
        return substr($this->year, -2);
    }
}