Time of recognition of turnover determines tax liability; accounting method, Commissioner approvals, and government timing rules govern. The provision fixes the time of recognition of turnover, turnover of purchases and related tax adjustments on the basis of a dealer's regularly and systematically maintained accounts that give a true and fair view and are used for commercial or income tax purposes. The Commissioner may permit or require alternative recording methods (cash or accrual) for specified classes of dealers, and a dealer may change methods only with the Commissioner's consent and conditions. The Government may prescribe the timing for recognition of turnover, purchases and adjustments for classes of transactions.