(a) In general
For purposes of section 38, in the case of an eligible employer, the retirement auto-enrollment credit determined under this section for any taxable year is an amount equal to—

(1) $500 for any taxable year occurring during the credit period, and

(2) zero for any other taxable year.

(b) Credit period
For purposes of subsection (a)—

(1) In general
The credit period with respect to any eligible employer is the 3-taxable-year period beginning with the first taxable year for which the employer includes an eligible automatic contribution arrangement (as defined in section 414(w)(3)) in a qualified employer plan (as defined in section 4972(d)) sponsored by the employer.

(2) Maintenance of arrangement
No taxable year with respect to an employer shall be treated as occurring within the credit period unless the arrangement described in paragraph (1) is included in the plan for such year.

(c) Eligible employer
For purposes of this section, the term "eligible employer" has the meaning given such term in section 408(p)(2)(C)(i).