public function publish_date() {

    if ( ! is_singular( 'post' ) ) {
      
/**
 * Filter: 'wpseo_opengraph_show_publish_date' - Allow showing publication date for other post types.
 *
 * @api bool $unsigned Whether or not to show publish date.
 *
 * @param string $post_type The current URL's post type.
 */
      if ( false === apply_filters( 'wpseo_opengraph_show_publish_date', false, get_post_type() ) ) {
        return false;
      }
    }

    $post = get_post();

    $pub = $this->date->format( $post->post_date_gmt );
    $this->og_tag( 'article:published_time', $pub );

    $mod = $this->date->format( $post->post_modified_gmt );
    if ( $mod !== $pub ) {
      $this->og_tag( 'article:modified_time', $mod );
      $this->og_tag( 'og:updated_time', $mod );
    }

    return true;
  }