WordPress Query Order by meta_key

Hi there,

I am trying to get posts ordered by meta values. What I need is to get events between today and today+15 days order by dates.
Dates are stored in meta as timestamps.

This is the code I am having at the moment but it keeps me getting posts order by id..

<?php
        setlocale(LC_ALL, "fr_FR.UTF-8");
        $now = time();
        $in15days = time() + 24*3600*15;
        $args = array(
            'post_type'  =>  'events',
            'posts_per_page' => 10,
            'meta_key'   =>  'eventtimestamp',
            'order_by'   =>  'meta_value_num',
            'order'      => 'ASC',
            'meta_query'  => array(
                                array(
                                    'key'     =>  'eventtimestamp',
                                    'value'   =>  array($now, $in15days),
                                    'compare' =>  'BETWEEN',
                                    'type'    =>  'NUMERIC',
                                )
                              )
          );
$the_query = new WP_Query( $args ); ?>