§ 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.