Tax Law, § 1147(b)
Except as otherwise provided in this section, no assessment of tax imposed by article 28 of the Tax Law or imposed pursuant to the authority of article 29 and administered by the Department of Taxation and Finance will be made after the expiration of three years from the date of the filing of a return. For purposes of this subdivision, a return filed before the last day prescribed by law or regulation for its filing or before the last day of any extension of time for its filing is deemed to be filed on such last day. If a return has not been filed as required by, or pursuant to, law, no time period limits assessment.
Cross-reference:
For time requirements on the filing of returns, see section 533.3 of this Title.
An assessment is timely if a notice of determination of the tax due is mailed by the Division of Taxation within three years of the filing of the taxpayer's return, or within the extended period consented to by the taxpayer pursuant to paragraph (b)(2) of this section. Timeliness of the assessment is determined by the date on which such notice is mailed by the Division of Taxation, as prescribed in section 535.1 of this Part, not by the date of receipt of such notice by the taxpayer.
N.Y. Comp. Codes R. & Regs. Tit. 20 § 535.3