Can my Medicare start date be backdated if I signed up late or got wrong information?

CMS says Part A coverage starts the month a person turns 65 if they apply within 6 months of that month. If they apply more than 6 months after turning 65, Part A coverage is retroactive for 6 months [1]. This is the CMS rule for premium-free Part A based on age [1].

Part B is different. Medicare.gov lists special enrollment periods, and each one says when coverage begins. For several of them, coverage begins the month after you sign up [2]. For people released from incarceration on or after 1/1/2023, you can select retroactive coverage back to your release date, but not before it. You can only request retroactive coverage up to 6 months in the past [2]. To choose a start date in the past, fill out form CMS-10797 and send it to your local Social Security office by fax or mail [2].

If you missed a chance to sign up because your health plan or employer gave you inaccurate or misleading information, Medicare.gov lists a special enrollment period. It starts the day you tell Social Security, ends 6 months later, and coverage begins the month after you sign up [2]. The sources here do not say that this period can backdate coverage. They also do not say how to correct a start date that Social Security recorded wrongly. "Don't know yet" for your case.

Open the Medicare.gov page in Source 2 and read the table of special enrollment periods for the one that matches your story. Your own Social Security record shows your start dates - "Don't know yet" here.