; and (C) in paragraph (3)— (i) in subparagraph (A)(i), by striking units and inserting billing units ; and (ii) by striking subparagraph (B) and inserting the following: (B) Total number of billing units For purposes of subparagraph (A)(i), the total number of billing units with respect to a part B rebatable drug is determined as follows: (i) Determine the total number of units equal to— (I) the total number of units, as reported under subsection (c)(1)(B) for each National Drug Code of such drug during the calendar quarter that is two calendar quarters prior to the calendar quarter as described in subparagraph (A), minus (II) the total number of units with respect to each National Drug Code of such drug for which payment was made under a State plan under title XIX (or waiver of such plan), as reported by States under section 1927(b)(2)(A) for the rebate period that is the same calendar quarter as described in subclause (I). (ii) Convert the units determined under clause (i) to billing units for the billing and payment code of such drug, using a methodology similar to the methodology used under this section, by dividing the units determined under clause (i) for each National Drug Code of such drug by the billing unit for the billing and payment code of such drug. (iii) Compute the sum of the billing units for each National Drug Code of such drug in clause (ii). . (2) Change of base year for rebate calculation Section 1847A(i) of the Social Security Act (42 U.S.C. 1395w–3a(i)) is amended— (A) in paragraph (3)— (i) in subparagraph (D), by striking July 1, 2021 and inserting July 1, 2016 ; and (ii) in subparagraph (E), by striking January 2021 and inserting January 2016 ; and (B) in paragraph (4)— (i) in subparagraph (A)— (I) by striking December 1, 2020 and inserting December 31, 2015 ; and (II) by striking January 2021 and inserting January 2016 ; (ii) in subparagraph (B), by striking December 1, 2020 and inserting December 31, 2015