N.Y. Comp. Codes R. & Regs. tit. 20 § 266.4
Tax Law, § 12(d) of § 1340(c); Codes and Ordinances of the City of Yonkers, § 92-109(d)
Where a taxpayer changes resident status to or from the City of Yonkers during the taxable year, the exclusion allowable under section 2(b) of the Model Local Law contained in section 1340(c) of the Tax Law and section 92-99(b) of the Codes and Ordinances of the City of Yonkers must be prorated according to the number of months of the taxable year covered by the City of Yonkers earnings tax return for the nonresident period. That is, the allowable exclusion will equal the amount which bears the same ratio to the full amount of the exclusion as the number of months of the nonresident period bears to 12 months. For this purpose, a fractional part of a month will be disregarded unless it amounts to more than one-half month, in which case it will be considered to be a month.
N.Y. Comp. Codes R. & Regs. Tit. 20 § 266.4