Applies the 'close comments after x days' to all WordPress post types

check_close_comments.php

<?php
/*
Plugin Name: Link PDF Attachment
Plugin URI: http://premium.wpmudev.org
Description: Adds a link to the top of a WordPress post to the first PDF attachment
Author: Chris Knowles
Version: 1.0
Author URI: http://twitter.com/ChrisKnowles
*/

function check_comments_open() {

	global $post;
	
	// if the post comment status is not open then no need to worry
	if ( $post->comment_status != 'open' ) return false;

  // if not closing comments on old posts then also don't worry
	if ( !get_option('close_comments_for_old_posts') ) return $post->comment_status;
	
	// calculate the age (in days) of the post 
	$post_date = date_create( $post->post_date );
	$today = date_create();
	$age = date_interval_format( date_diff( $post_date , $today ) , '%a' );
	
	// if the post is older than the 'close after x days' then close comments
	if ( $age > get_option('close_comments_days_old') ) {
		return false;
	} else {
		return true;
	}
}

add_filter( 'comments_open' , 'check_comments_open' );