#WordPress: self expiring posts

functions.php

<?php
    if (have_posts()) :
        while (have_posts()) : the_post(); ?>
            $expirationtime = get_post_custom_values('expiration');
            if (is_array($expirationtime)) {
                $expirestring = implode($expirationtime);
            }
        
            $secondsbetween = strtotime($expirestring)-time();
            
            if ( $secondsbetween > 0 ) {
                the_title();
                the_excerpt();
            }
        endwhile;
    endif;
?>