//add custom field column to post list
function add_admin_course_column_title( $columns ) {
  $columns['online_start'] = __( 'Online Start' );
  return $columns;
}
add_filter( 'manage_courses_posts_columns', 'add_admin_course_column_title' );

function add_admin_course_column( $column, $post_id ) {
    if ( 'online_start' === $column ) {
        $onlinestart = get_post_meta( $post_id, 'online_start', true );

        if ( ! $onlinestart ) {
                _e( 'n/a' );
        }
        else {
            echo $onlinestart;
        }
    }
}
add_action( 'manage_courses_posts_custom_column', 'add_admin_course_column', 10, 2);

function add_sortable_date_column( $columns ) {
  $columns['online_start'] = 'online_start';
  return $columns;
}
add_filter( 'manage_edit-courses_sortable_columns', 'add_sortable_date_column');