source/php/EventArchive.php
public function addEventDateArgsToPermalinks($permalink, $post, $leavename)
    {
        if (!isset($post->start_date) || $post->post_type != $this->eventPostType) {
            return $permalink;
        }

        return esc_url(add_query_arg('date', preg_replace('/\D/', '', $post->start_date), $permalink));
    }
public function addEventDateArgsToPermalinks($permalink, $post, $leavename)
    {
        if (!isset($post->start_date) || $post->post_type != $this->eventPostType) {
            return $permalink;
        }

        return esc_url(add_query_arg('date', preg_replace('/\D/', '', $post->start_date), $permalink));
    }
Add date arguments to event permalink URL