26 U.S.C. § 6501 — Limitations on assessment and collection
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Where, within the 60-day period ending on the day on which the time prescribed in this section for the assessment of any tax imposed by subtitle A for any taxable year would otherwise expire, the Secretary receives a written document signed by the taxpayer showing that the taxpayer owes an additional amount of such tax for such taxable year, the period for the assessment of such additional amount shall not expire before the day 60 days after the day on which the Secretary receives such document.
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