Assessment of unregistered taxpayers permits best-judgment tax assessment with right to be heard within specified time limit. Where a person liable to register fails to obtain registration or has registration cancelled but remains tax liable, the proper officer may assess that person's tax liability to the best of his judgment for the relevant tax periods and issue an assessment order within the time frame measured from the date prescribed for furnishing the annual return for the financial year to which the unpaid tax relates, provided the person is given an opportunity of being heard.