How to remove/exclude specific posts in a category from RSS feeds after particular date [closed]

Question

I want to remove/exclude some posts of a category from RSS feeds after particular date.

First of all i want to specify that same requirement i already accomplish for front page posts. I have two categories x and y and i am showing the posts of these categories on front page. I have created custom field name 'expiration' in which i am giving the date like 2011/08/12 as value which will remove the particular post from front page on given date.

Below is the code to remove particular post where custom field expiration is set.

<?php if (have_posts()) :query_posts($query_string .'&cat=3'); while (have_posts()) : the_post(); 
        $currentdate = date("Ymd");
        $expirationdate = get_post_custom_values('expiration');
        if (is_null($expirationdate)) {
        $expirestring = '30005050'; //MAKE UN-EXPIRING POSTS ALWAYS SHOW UP;
        } else {

        if (is_array($expirationdate)) {
        $expirestringarray = implode($expirationdate);
        }
        $expirestring = str_replace("/","",$expirestringarray);
        } //else
        if ( $expirestring > $currentdate ) 
        {
         // Enter your post display code after that.?>
            <a href="<?php the_permalink(); ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>" class="recent_news"><?php the_title(); ?></a><br/>
            <p class="postmetadata"><?php the_time('l, jS F Y'); ?> at <?php the_time('g:i a'); ?></p>

  <?php } endwhile; endif; ?>

In the above code you can see that '&cat=3' is mention which is my category X and similar for Category Y.

Now come to the point how can i set the date once in custom field expiration for front page posts as well as RSS feeds.

i used ( exclude ) tag whose id is ('7') on the posts and filter out but problem is when i put the exclude tag it immediately remove/exclude the post from RSS Feeds. I want to exclude on the basis of expiration date.

Below is the function placed in function.php file which filter out the tag on basis of ID.

<?php function filter_feed_posts($query) {
if($query->is_feed) {

$exclude_tags = array(7);
$query->set('tag__not_in',$exclude_tags);
}
}
add_filter('pre_get_posts','filter_feed_posts');
?>

Please help me to get this done. Your help is appreciated.