Allows WP_Query to use a 'since' argument to query for relative date queries using `strtotime()`

wp-query-since.php

<?php
/**
 * Allows WP_Query to use a 'since' argument to query for
 * relative date queries.
 *
 * Usage: Query posts from last 30 days
 * $query = new WP_Query('since' => '-30 days'); 
 *
 * @uses strtotime()
 * @author Eddie Moya
 **/
function filter_where_add_since( $where = '', $query) {
    if( isset($query->query_vars['since']) ){
        $where .= " AND post_date > '" . date('Y-m-d', strtotime($query->query_vars['since'])) . "'";
    }
    return $where;
}
add_filter( 'posts_where', 'filter_where_add_since', 10, 2 );