Display number of posts in category since last visit

Hi

I want to display the number of new posts in three separate categories since the users last visit. When the user reads the posts of one category, I create a cookie with the date of their visit. Next time he visits I create a filter which filters out the posts which were published after the users visit. However, when I try to combine it with a query category, it displays the number of all of the posts instead of just the posts from the category current.

I’m guessing that the posts_where hook overrides the query and it displays the number of all of the new articles.

Here is the code, please help me, how can I solve this issue.

function filter_a() {
    $where .= " AND post_date >='" . date('Y-m-d H:i:s', $_COOKIE['a'] / 1000) . "'";
    return $where;
  }  

function notifications_since($notifcateg) {
    switch($notifcateg){
      case 'a':
        add_filter('posts_where','filter_a');
        break;
    }

 $query = array (
    'category_name' => $notifcateg
  );

  $queryObject = new WP_Query($query);
  $postcount = $queryObject->post_count;
  remove_all_filters('posts_where');
  wp_reset_postdata();
  return $post_count;

}