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function hp_checkout_add_delivery_time( $order_id ) {    

	if ( WC()->session->get('delivery_time') ) {

		update_post_meta( $order_id, 'Delivery Time', esc_attr( WC()->session->get('delivery_time' ) ) );
	}
}
add_action('woocommerce_checkout_update_order_meta', 'hp_checkout_add_delivery_time');