Determination of tax: non-fraud shortfalls subject to show-cause notice, interest and prescribed penalty within time limits. Determination of tax addresses cases where tax is unpaid, short paid, erroneously refunded or input tax credit wrongly availed for reasons other than fraud or wilful misstatement. The proper officer must serve a show-cause notice requiring payment of tax, interest and penalty; a later statement can extend the same grounds to other periods and is deemed a notice. Voluntary pre-notice payment prevents notice for that amount; paying tax with interest within thirty days of notice avoids penalty. The officer must issue an order fixing tax, interest and penalty within the statutory time limit.