//FUNCTION TO LIMIT SEARCH TO LAST 30 DAYS
<?php function filter_where( $where = '' ) {
	$where .= " AND post_date > '" . date('Y-m-d', strtotime('-30 days')) . "'";
	return $where;
}
add_filter( 'posts_where', 'filter_where' ); ?>

//THE QUERY AND LOOP
<?php
$query = new WP_Query( array( 'category__not_in' => array( '3', '8', '15' ) ) ); 
if ( have_posts() ) : ?>
	<?php while ( $query->have_posts()) : $query->the_post(); ?>
		<h3 class="entry-title"><?php the_title(); ?></h3>
		<?php the_content(); ?>
		<hr>		
	<?php endwhile; ?>	
<?php endif; ?>
<?php wp_reset_postdata(); ?>


//REMOVE FILTER THAT LIMITS SEARCH TO LAST 30 DAYS
<?php remove_filter( 'posts_where', 'filter_where' ); ?>