public static function get_last_modified_gmt( $post_types, $return_all = false ) {

    global $wpdb;

    static $post_type_dates = null;

    if ( ! is_array( $post_types ) ) {
      $post_types = [ $post_types ];
    }

    foreach ( $post_types as $post_type ) {
      if ( ! isset( $post_type_dates[ $post_type ] ) ) { // If we hadn't seen post type before. R.
        $post_type_dates = null;
        break;
      }
    }

    if ( is_null( $post_type_dates ) ) {

      $post_type_dates = [];
      $post_type_names = WPSEO_Post_Type::get_accessible_post_types();

      if ( ! empty( $post_type_names ) ) {
        $post_statuses = array_map( 'esc_sql', self::get_post_statuses() );

        $sql = "
					SELECT post_type, MAX(post_modified_gmt) AS date
					FROM $wpdb->posts
					WHERE post_status IN ('" . implode( "','", $post_statuses ) . "')
						AND post_type IN ('" . implode( "','", $post_type_names ) . "')
					GROUP BY post_type
					ORDER BY date DESC
				";

        foreach ( $wpdb->get_results( $sql ) as $obj ) {
          $post_type_dates[ $obj->post_type ] = $obj->date;
        }
      }
    }

    $dates = array_intersect_key( $post_type_dates, array_flip( $post_types ) );

    if ( count( $dates ) > 0 ) {
      if ( $return_all ) {
        return $dates;
      }

      return max( $dates );
    }

    return false;
  }