N.Y. Comp. Codes R. & Regs. Tit. 20 §§ 1-1.13
The term "taxable year" means, in most cases, the taxpayer's taxable year for Federal income tax purposes, or the part thereof during which the taxpayer is subject to the tax imposed by article 9-A. In the case of a report made for a fractional part of the year, taxable year means the period for which the report is made. A taxable year must be a calendar year or a fiscal year ending during a calendar year. A taxable year shall not include more than 12 calendar months except in the case of a 52-53 week period. If a taxpayer does not have a taxable year for Federal income tax purposes, the taxable year must be a calendar year, unless the commissioner authorizes the use of a fiscal year. Any reference in article 9-A or 27 or this Subchapter to the term tax year or taxable period is a reference to taxable year as defined by this section.
N.Y. Comp. Codes R. & Regs. Tit. 20 §§ 1-1.13