26 U.S.C. § 31: Tax withheld on wages

The amount withheld as tax under chapter 24 shall be allowed to the recipient of the income as a credit against the tax imposed by this subtitle. The amount so withheld during any calendar year shall be allowed as a credit for the taxable year beginning in such calendar year. If more than one taxable year begins in a calendar year, such amount shall be allowed as a credit for the last taxable year so beginning. The Secretary may prescribe regulations providing for the crediting against the tax imposed by this subtitle of the amount determined by the taxpayer or the Secretary to be allowable under section 6413(c) as a special refund of tax imposed on wages. The amount allowed as a credit under such regulations shall, for purposes of this subtitle, be considered an amount withheld at source as tax under section 3402. Any amount to which paragraph (1) applies shall be allowed as a credit for the taxable year beginning in the calendar year during which the wages were received. If more than one taxable year begins in the calendar year, such amount shall be allowed as a credit for the last taxable year so beginning. Any credit allowed by subsection (a) for any amount withheld under section 3406 shall be allowed for the taxable year of the recipient of the income in which the income is received. Except as otherwise provided in this section, the amendments made by this title [enacting sections 3406 and 6705 of this title, amending this section and sections 274, 275, 643, 661, 3402, 3403, 3502, 3507, 6011, 6013, 6015, 6042, 6044, 6049, 6051, 6365, 6401, 6413, 6652, 6653, 6654, 6676, 6678, 6682, 7205, 7215, 7431, 7654, and 7701 of this title, repealing sections 3451 to 3456 of this title, enacting provisions set out as notes under sections 1, 3451, and 6011 of this title, and repealing provisions set out as a note under section 3451 of this title ] shall apply with respect to payments made after December 31, 1983 . The amendments made by section 102 [amending this section and sections 274, 275, 643, 661, 3403, 3502, 3507, 6013, 6015, 6042, 6044, 6049, 6051, 6365, 6401, 6413, 6654, 6682, 7205, 7215, 7654, and 7701 of this title, repealing sections 3451 to 3456 of this title, enacting provisions set out as a note under section 3451 of this title , and repealing provisions set out as a note under section 3451 of this title ] shall take effect as of the close of June 30, 1983 . The amendments made by sections 104(b) and 107 [amending sections 6682, 7205, and 7431 of this title] shall take effect on the date of the enactment of this Act [ Aug. 5, 1983 ].”