; . (b) Applicability The amendments made by this section shall apply to determinations made with respect to cost-of-living computation quarters ending on or after September 30 of the second calendar year following the calendar year in which this Act is enacted. 4. Use of Consumer Price Index for the Elderly for cost-of-living adjustments of retired pay and retainer pay of members of the Armed Forces (a) In general Section 1401a(g)(1) of title 10, United States Code, is amended by striking Consumer Price Index (all items, United States city average) and inserting Consumer Price Index for the Elderly . (b) Application of amendment The amendment made by subsection (a) shall apply only to adjustments of retired and retainer pay of members of the Armed Forces effective after the date of the enactment of this Act. 5. Limitation on certain increases in Medicare part B premiums Section 1839(f) of the Social Security Act ( 42 U.S.C. 1395r(f) ) is amended in the first sentence— (1) by striking such increase would reduce and inserting the following: such increase— (1) for calendar years beginning before the date of the enactment of section 5 of the Seniors’ Security Act of 2017 , would reduce ; and (2) by striking the period at the end and inserting the following: ; and (2) for calendar years beginning on or after the date of the enactment of section 5 of the Seniors’ Security Act of 2017 , would exceed 30 percent of the difference between the amount of benefits payable to that individual for that December and the amount of benefits payable to that individual for that November (both amounts determined before the deduction of a premium under this section). .