42 CFR § 417.450 - Effective date of coverage.
(a) Basic rules. Except as specified in paragraph (b) of this section, and notwithstanding the provisions of § 417.440(d).
(i) Entitled to Medicare benefits; and
(2) The effective month of coverage may not be earlier than the first month after, nor later than the third month after the month in which CMS receives the information necessary to include the beneficiary as a Medicare enrollee of the HMO or CMP in CMS records.
(2) If an individual becomes an HMO or CMP enrollee before becoming entitled to Medicare Part B benefits, the effective month of coverage is the first month for which he or she becomes entitled to Medicare Part B benefits.
(c) Notice of effective date of coverage. For each beneficiary added to CMS's records as an enrollee of an HMO or CMP, CMS gives the HMO or CMP prompt written notice of the month with which CMS's liability begins.