Determination of tax: stay periods and appeal-related exclusions extend statutory time limits and affect recovery. Where service of notice or issuance of order is stayed, the period of stay is excluded from limitation computations for reassessment; if an appellate body finds a fraud-based notice unsustainable, the proper officer must determine tax treating it as a notice under ordinary reassessment provisions. Orders directed by appellate fora must be issued within two years. Procedural protections require written hearing requests, limited adjournments, reasoned orders, demands confined to notice grounds, interest on unpaid tax payable irrespective of express specification, and corresponding adjustment of interest and penalty where appellate modification occurs.