How to Sort Custom Field Admin Column by Date

The function add_sortable_date_column only registers columns as sortable.
Add one more function to tell wordpress how to sort your columns.

function courses_columns_orderby( $query ) {

    if( ! is_admin() )
        return;

    $orderby = $query->get( 'orderby');

    switch( $orderby ){
        case 'online_start': 
            $query->set('meta_key','online_start');
            $query->set('orderby','meta_value');
            break;
        default: break;
    }

}

add_action( 'pre_get_posts', 'courses_columns_orderby' );