获取下一篇文章的时间WordPress相同的标签

Trying to get next post url, custom post type is portfolio. already have tag in posts in this post type. "star".

I was trying to get url for the next post by time. and should have the same tag.

anybody know that how possible to get that.

I have found that code which is able to get

$args=array(
  'portfolio-tag' => 'loop1',
  'showposts'=>5,
  'caller_get_posts'=>1
);
$my_query = new WP_Query($args);
if( $my_query->have_posts() ) {
  echo '5 recent Posts with tag1';
  while ($my_query->have_posts()) : $my_query->the_post(); ?>
    <p><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></p>
   <?php
  endwhile;
}

But i need url just for next post by time, if i am on fourth detail post, how can i get 5th post url. so it will be next in this collection.