20 CFR §418.2110

1. (a)Generally, an income-related monthly adjustment amount determination will be effective for all months that you are enrolled in a prescription drug coverage plan during the year for which we determine you must pay an income-related monthly adjustment amount.
2. (b)When we have used modified adjusted gross income information from the IRS for the tax year 3 years prior to the effective year to determine your income-related monthly adjustment amount, and modified adjusted gross income information for the tax year 2 years prior later becomes available from the IRS, we will review the new information to determine if we should revise our initial determination concerning the income-related monthly adjustment amount. If we revise our initial determination, the effective date of the new initial determination will be January 1 of the effective year, or the first month your enrollment or re-enrollment in a Medicare prescription drug coverage plan became effective if later than January.
3. (c)When we use your amended tax return, as described in § 418.2150, the effective date will be January 1 of the year(s) that is affected, or the first month in that year that your enrollment or re-enrollment in a Medicare prescription drug coverage plan became effective if later than January.
4. (d)When we use evidence that you provide to proves the IRS modified adjusted gross income information we used was incorrect, as described in § 418.2335, the effective date will be January of the year(s) that is affected or the first month in that year that your enrollment or re-enrollment in a Medicare prescription drug coverage plan became effective if later than January.
5. (e)When we use information from a more recent tax year that you provide due to a major life-changing event, as described in § 418.2201, the effective date is described in § 418.2230.