Resetting Results of query_post

OK, I think I understand the nature of the problem, but not the solution.

I’m trying to create a contextual archive list: the next five posts and the previous five posts after the current post a visitor is viewing. Since there is no native WP function that will do this, I tried using query_posts to accomplish this task in the following way:

function iabc_context_archive($current_post) {
	global $wpdb;
	if(is_single()) :
		// get newer posts:
		add_filter('posts_where', 'iabc_filterWhere_newer');
		$newer = query_posts('showposts=5&sortby=data&sort=DESC');
		remove_filter('posts_where', 'iabc_filterWhere_newer');
		// get older posts:
		add_filter('posts_where', 'iabc_filterWhere_older');
		$older = query_posts('showposts=5&sortby=data&sort=ASC');
		remove_filter('posts_where', 'iabc_filterWhere_older');
		// merge returns into single array. Add the current post and mark it as such:
		if($current_post) :
			$current_post->thepost = 1;
			$newer[] = $current_post;
		endif;
		$list = array_merge((array)$newer, (array)$older);
	else:
		// just get the latest posts
		$list = get_posts('numberposts=5');
	endif;
	// Output:
	print('<ul id="archive">');
	foreach((array)$list as $next) :
		$title = str_split($next->post_content, 40);
		$next->thepost ? $class = ' class="current" ' : $class = '';
		print('<li'.$class.'><a href="'.get_permalink($next->ID).'" title="'.$title[0].'">'.$title[0].'</a></li>');
	endforeach;
	print('</ul>');
}

function iabc_filterWhere_newer($where = '') {
	global $post;
	$where .= " AND post_date > '".$post->post_date."'";
	return $where;
}

function iabc_filterWhere_older($where = '') {
	global $post;
	$where .= " AND post_date < '".$post->post_date."'";
	return $where;
}

The problem I am finding is that the links don’t work with this active. They go to the correct URL, but at that URL, WordPress seems to be pulling the wrong content: instead of grabbing post ID 21, it seems to be pulling post ID 12, which is the last published article on the site.

I take this to mean that the posts_where is not getting reset from my last operation, even though I am removing the filter. I have no idea how to correct this problem, and would be very thankful for anyone who can provide a solution!

An example of the problem can be seen here:
https://iam.bit-character.com/archives/21
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