Wordpress check new arrivals category

function add_new_arrival_meta( $post_id ) {

    // If this is just a revision, don't check.
    if ( wp_is_post_revision( $post_id ) )
        return;

    // Get all categories and look if the one with the slug new-arrivals is there
    $categories = wp_get_post_categories($post_id);
    $is_new_arrival = array_filter(
      $categories,
      function ($e) {
        return $e->slug == 'new-arrivals';
      }
    );

    // If it is there, then add a post meta
    if($is_new_arrival)
        add_post_meta($post_id,'arrived_at',date('Y-m-d'));
}
add_action( 'save_post', 'add_new_arrival_meta' );