Posted Date plus 30 days

See if this plugin can work for you https://www.ads-software.com/plugins/wp-date-and-time-shortcode/

If you’d rather custom code your template, add something like this:
<?php echo date('Y-m-d', get_post_timestamp() + 2592000 ); // add 30 days as seconds ?>
assumes we’re in the loop, otherwise a post ID is needed to get the right timestamp.