_close_comments_for_old_post( bool $open, int $post_id )

function _close_comments_for_old_post( $open, $post_id ) {
	if ( ! $open )
		return $open;

	if ( !get_option('close_comments_for_old_posts') )
		return $open;

	$days_old = (int) get_option('close_comments_days_old');
	if ( !$days_old )
		return $open;

	$post = get_post($post_id);

	/** This filter is documented in wp-includes/comment.php */
	$post_types = apply_filters( 'close_comments_for_post_types', array( 'post' ) );
	if ( ! in_array( $post->post_type, $post_types ) )
		return $open;

	// Undated drafts should not show up as comments closed.
	if ( '0000-00-00 00:00:00' === $post->post_date_gmt ) {
		return $open;
	}

	if ( time() - strtotime( $post->post_date_gmt ) > ( $days_old * DAY_IN_SECONDS ) )
		return false;

	return $open;
}