DaTE

DATE

<?php 
add_action( 'set_object_terms', 'bcw_set_date', 10, 6 );

function bcw_set_date( $object_id, $terms, $tt_ids, $taxonomy, $append, $old_tt_ids ) {

    $item = get_post( $object_id );

    if ('items' == $item->post_type && 'relations' == $taxonomy && 0 < count( $tt_ids)){

        // Form an array of data to be updated
        $latest_cpt = get_posts([
            'posts_per_page'=> 1,
            'post_type'=>'post',
            'tax_query'=>[[
               'taxonomy'=>'relations',
               'field'=>'term_taxonomy_id',
               'terms'=> $tt_ids[0],
            ],],
        ]);

        // Get the current time
        $time = current_time('mysql');

        // prevent issues with metabox save function.
        $_POST[‘themeprefix_metabox_nonce’] = array();



        // Form an array of data to be updated
        $post_data = array(
            'ID'           => $latest_cpt[0]->ID, 
            'post_modified'   => $time,
            'post_modified_gmt' =>  get_gmt_from_date( $time )
        );

        // Update the latest post with the same term
        wp_update_post( $post_data );
    }
}
?>