time stamp order string doesn't work on category pages

Hi,
This code uses a unix timestamp to sort events by date. Is there some reason for this not to work on category pages?

<?php $my_query = new WP_Query('category_name=music&showposts=20&meta_key=unix_time_stamp&orderby=meta_value&order=ASC'); ?>

<?php while (have_posts()) : the_post(); ?>

<div  class="upcomingevent">
 <ul class="time">
<li><?php echo get_post_meta($post->ID, 'Booking_info_no_link', true); ?></li>
 <li class="buy"><a target="blank" href="<?php echo get_post_meta($post->ID, 'buy_link', true); ?>"/><?php echo get_post_meta($post->ID, 'buy_text', true); ?></a></li>
 <li><?php echo get_post_meta($post->ID, 'start_date', true); ?></li>
 <li><?php echo get_post_meta($post->ID, 'end_date', true); ?></li>
 <li><?php echo get_post_meta($post->ID, 'time', true); ?></li>
     <li><?php echo get_post_meta($post->ID, 'price', true); ?></li>
     </ul>