Section 2(j) defines 'turnover ' as meaning the aggregate of the amounts of sale prices and parts of sale prices received or receivable by a dealer in respect of the sale or supply of goods or in respect of the sale or supply of goods in the carrying out of any contract effected or made during the prescribed period; and the expression 'taxable turnover ' means that part 'of a dealer 's turnover during such period which remains after deducting therefrom his turnover during that period in respect of the sale of goods declared tax free under section 6 The definition of the term 'year ' as provided in section 2(1) to the extent necessary for our present purpose reads: "year ' means the 12 months ending on 31st day of March, or if the accounts of the assessee are made up to any other day in respect of a year ending on any date other than the 31st day of March, than at the option of the assessee the year ending on the day to which his accounts have been so made up. . . ."