Display popular post of last 15 days

Using a Theme Junkie theme Freshlife at my blog https://www.digmlm.com and in the sidebar there is a default option to show all time popular post, but I wanted to show popular post of last 30 days.

I have extracted these code as below and you might help what should be change to show the last 30 days popular post.

// Tabber: Get Most Popular Posts
function tj_tabs_popular( $posts = 5, $size = 35 ) {
	$popular = new WP_Query('orderby=comment_count&posts_per_page='.$posts);
	while ($popular->have_posts()) : $popular->the_post();
?>

<li>
 	<?php tj_show_thumb($size, $size); ?>
 	<div class="info">
 	<a>" href="<?php the_permalink() ?>"><?php the_title(); ?></a>
 	<span class="meta"><?php comments_popup_link('0 Comment', '1 Comment', '% Comments', 'comments-link', ''); ?></span>
	</div> <!--end .info-->
	<div class="clear"></div>
</li>
<?php endwhile;
}

Thanks