US Medicare Provider and Supplier Enrollment (42 CFR Part 424 Subpart P) 424.521: Retrospective billing by the listed supplier types is limited to 30 days before the effective date (90 days in a Presidentially declared disaster) and only where all program requirements were met and services were furnished at the enrolled location

Physicians, non-physician practitioners and their organisations, ambulance suppliers, opioid treatment programs, Part B hospital departments, CLIA laboratories, intensive cardiac rehabilitation facilities, mammography centers, mass immunizers and pharmacies, radiation therapy centers, home infusion therapy suppliers and physical, occupational and speech therapists may bill retrospectively for services furnished up to 30 days before their effective date where circumstances precluded enrolling in advance, or up to 90 days where a Presidentially declared disaster under the Stafford Act precluded it, provided the provider or supplier met all program requirements including State licensure at the time and the services were furnished at the enrolled practice location; the same applies to reassignments.