Something I just learned and thought I’d share to save others the trouble: if you’re attempting to filter the results of get_posts(), for example with the posts_where filter, you need to disable suppress_filters.

For example, here’s how to fetch some posts from within the past 30 days (based on an example from the Codex):

<?php

function last_thirty_days( $where = '' ) {
	global $wpdb;

	$where .= $wpdb->prepare( " AND post_date > %s", date( 'Y-m-d', strtotime('-30 days') ) );

	return $where;
}

add_filter( 'posts_where', 'last_thirty_days' );

$some_posts = get_posts( array( 'suppress_filters' => false ) );

// Important to avoid modifying other queries
remove_filter( 'posts_where', 'last_thirty_days' );

?>