Section 121 - Accounting periods and methods(a) Accounting periods.-A taxpayer's taxable year under this title shall be the same as the taxpayer's taxable year for federal income tax purposes.(b) Accounting methods.-A taxpayer's method of accounting under this title shall be the same as the taxpayer's method of accounting for federal income tax purposes. In the absence of any method of accounting for federal income tax purposes, unincorporated business taxable income shall be computed under such method as in the opinion of the director of finance clearly reflects income.(c) Change of accounting period or method.- (1)If a taxpayer's taxable year or method of accounting is changed for federal income tax purposes, the taxable year or method of accounting for purposes of this title shall be similarly changed.(2) If a taxpayer's method of accounting is changed, other than from an accrual to an installment method, any additional tax which results from adjustments determined to be necessary solely by reason of the change shall not be greater than if such adjustments were ratably allocated and included for the taxable year of the change and the preceding taxable years, not in excess of two, beginning after January first, nineteen hundred sixty-six, during which the taxpayer used the method of accounting from which the change is made.(3) If a taxpayer's method of accounting is changed from an accrual to an installment method, any additional tax for the year of such change of method and for any subsequent year which is attributable to the receipt of installment payments properly accrued in a prior year, shall be reduced by the portion of tax for any prior taxable year attributable to the accrual of such installment payments, in accordance with regulations of the director of finance.N.Y. General City Model Law § 121