order by custom meta field date

order-by-meta-value.php

$args = array(
    'post_type' => 'event',
        'order' => 'DESC',
        'orderby' => 'meta_value',
        'meta_key' => 'start_date',
    'meta_query' => array(
        array(
            'key' => 'start_date',
            'value' => date('Ymd'),
            'compare' => '>='
        )
    )
);
$query = new WP_Query( $args );