12. Time at which turnover, turnover of purchases and adjustments arise.- (1) Subject to sub-sections (2), (3) and (4) of this section, the amount of the turnover and the turnover of purchases of a dealer which arises during any tax period shall be the amount recorded in the accounts of the dealer where those accounts are regularly and systematically prepared and maintained, give a true and fair view of his dealings, and are employed by the dealer in determining the turnover of the dealers business for commercial or income tax purposes.
(2) The Commissioner may by notification
(a) permit certain classes of dealer to record turnover based on amounts paid or received; and
(b) require certain classes of dealer to record turnover based on amounts payable or receivable.
(3) Where a dealer wishes to change the method of determining the turnover and turnover of purchases, he may only make the change with the consent of the Commissioner and on such terms and conditions as the Commissioner may impose.
(4) The Government may prescribe the time at which a dealer shall treat the
(b) turnover of purchases; and
(c) adjustment of tax or adjustment to a tax credit;
as arising for a class of transactions.