Ascending Date Modification

I am trying to modify the function in order to enable the posts to display in date ascending. In widget-recent-posts-extended.php, I have tried this;

$args = array(
				'numberposts' => $limit,
				'cat' => $cat,
                                'orderby'=>'post_date',
                                'order'=>'ASC',
				'post_type' => $post_type
			);

This does not work. Can you tell me how I can make this possible?

Thanks

http://ww.wp.xz.cn/plugins/recent-posts-widget-extended/