Source: http://uscode.house.gov/view.xhtml?req=granuleid:USC-prelim-title26-section45P&num=0&edition=prelim
Timestamp: 2019-04-23 11:54:30+00:00

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For purposes of section 38, the differential wage payment credit for any taxable year is an amount equal to 20 percent of the sum of the eligible differential wage payments for each of the qualified employees of the taxpayer during such taxable year.
The term "eligible differential wage payments" means, with respect to each qualified employee, so much of the differential wage payments (as defined in section 3401(h)(2)) paid to such employee for the taxable year as does not exceed $20,000.
The term "qualified employee" means a person who has been an employee of the taxpayer for the 91-day period immediately preceding the period for which any differential wage payment is made.
All persons treated as a single employer under subsection (b), (c), (m), or (o) of section 414 shall be treated as a single employer.
The amount of credit otherwise allowable under this chapter with respect to compensation paid to any employee shall be reduced by the credit determined under this section with respect to such employee.
(2) the 2 succeeding taxable years.
For purposes of this section, rules similar to the rules of subsections (c), (d), and (e) of section 52 shall apply.
The date of the enactment of this section, referred to in subsec. (d)(1), is the date of the enactment of Pub. L. 110–245, which was approved June 17, 2008.
2015-Subsec. (a). Pub. L. 114–113, §122(b)(1), struck out ", in the case of an eligible small business employer" after "section 38".
Subsec. (b)(3). Pub. L. 114–113, §122(b)(2), amended par. (3) generally. Prior to amendment, par. (3) defined "eligible small business employer".
Subsec. (f). Pub. L. 114–113, §122(a), struck out subsec. (f). Text read as follows: "This section shall not apply to any payments made after December 31, 2014."
"(1) Extension.-The amendment made by subsection (a) [amending this section] shall apply to payments made after December 31, 2014.
"(2) Modification.-The amendments made by subsection (b) [amending this section] shall apply to taxable years beginning after December 31, 2015."
Pub. L. 113–295, div. A, title I, §118(b), Dec. 19, 2014, 128 Stat. 4015 , provided that: "The amendment made by this section [amending this section] shall apply to payments made after December 31, 2013."
Pub. L. 112–240, title III, §308(b), Jan. 2, 2013, 126 Stat. 2329 , provided that: "The amendment made by this section [amending this section] shall apply to payments made after December 31, 2011."
Pub. L. 111–312, title VII, §736(b), Dec. 17, 2010, 124 Stat. 3318 , provided that: "The amendment made by this section [amending this section] shall apply to payments made after December 31, 2009."
Section applicable to amounts paid after June 17, 2008, see section 111(e) of Pub. L. 110–245, set out as an Effective Date of 2008 Amendment note under section 38 of this title.

References: §122
 §122
 §122
 §118
 §308
 §736