$post_title = get_the_title(); //for use as term below
$post_date = get_the_date(); // may need to tweak the format, see codex ref.
    if( have_rows('toc_section') ): while ( have_rows('toc_section') ) : the_row(); //checks that ACF repeater exists
            $posts = get_sub_field('section_contents');
                if( $posts ):
                    foreach( $posts as $post): //checks for post relations
                        $postid = $post->ID;
                        wp_set_object_terms( $postid, $post_title, 'eco_issues', true ); //ad a tax term
                        wp_set_object_terms( $postid, 'Print', 'format', true ); //add a second tax term
            //how to set the date here?
// like so:
$date_update = wp_update_post(array(
    'ID' => $postid,
    'post_date' => $post_date,
),true);

                    endforeach;
                    wp_reset_postdata();
                endif;
        endwhile;
        wp_reset_postdata();
      endif;