How to order by multiple meta keys?

This is something that WordPress added support for in 4.2: https://make.wordpress.org/core/2015/03/30/query-improvements-in-wp-4-2-orderby-and-meta_query/

In your case you'll probably want to do something like this:

$args = array(    'post_type'  => 'event',    'meta_query' => array(        'relation' => 'AND',        'event_start_date_clause' => array(            'key'     => '_event_start_date',            'compare' => 'EXISTS',        ),        'event_start_time_clause' => array(            'key'     => '_event_start_time',            'compare' => 'EXISTS',        ),     ),    'orderby' => array(        'event_start_date_clause' => 'ASC',        'event_start_time_clause' => 'ASC',    ),);$loop = new WP_Query( $args );

You have to use meta_query at add your meta keys as everyone said, but also you have to customize order by function.

$args = array(    'post_type' => 'event',    'meta_key' => '_event_start_date',    'meta_query' => array(        array('key' => '_event_start_date'),        array('key' => '_event_start_time')    ),);add_filter('posts_orderby','customorderby');$loop = new WP_Query( $args );remove_filter('posts_orderby','customorderby');

And put this on your functions.php

function customorderby($orderby) {    return 'mt1.meta_value ASC, mt2.meta_value ASC';}

You can read more at http://dotnordic.se/sorting-wordpress-posts-by-several-numeric-custom-fields/

From WP_Query documentation, use meta_query and fill up with an array of meta key/values:

$args = array(    'post_type' => 'product',    'meta_query' => array(        array(            'key' => 'color',            'value' => 'blue',            'compare' => 'NOT LIKE'        ),        array(            'key' => 'price',            'value' => array( 20, 100 ),            'type' => 'numeric',            'compare' => 'BETWEEN'        )    ));$query = new WP_Query( $args );