WordPress Query Order by meta_key between meta_value

I would like to get posts ordered by meta_key between meta values. This is to get a list of events between two dates ordered by dates. The following code keeps me geting posts ordered by creation dates.

Can someone please help with this?

<?php
    $now = time();
    $in15days = time() + 24*3600*15;
    $args = array(
       'post_type' => 'events',
       'posts_per_page' => '10',
       'order' => 'ASC',
       'meta_key' => 'starttimestamp',
       'order_by' => 'meta_value',
       'meta_query'  => array(
                            array(
                                'value'   => array($now, $in15days),
                                'compare' => 'BETWEEN',
                                'type'    => 'NUMERIC'
                            )
                          )
      );
$the_query = new WP_Query( $args ); ?>