Example of how to change the default order_by used in for datetimes within the EE REST API.

tw_ee_set_ee_rest_datetime_order_by.php

<?php //Do not include the opening PHP tag if you already have one

add_filter('FHEE__Read__create_model_query_params', 'tw_ee_set_ee_rest_datetime_order_by', 10, 3);
function tw_ee_set_ee_rest_datetime_order_by( $model_query_params, $query_params, $model ) {
    if($model instanceof EEM_Datetime) {
        // Match default order_by array to compare.
        $default_order_by['DTT_ID'] = 'ASC';
        // Check if the order_by currently set is the default order_by.
        if( $default_order_by === $model_query_params['order_by']) {
            // Order by DTT_ID => ASC so override that with DTT_EVT_Start => DESC
            $model_query_params['order_by'] = array('DTT_EVT_start' => 'DESC');
        }        
    }
    return $model_query_params;
}