Quick overview

Missing a Medicare deadline can cause higher premiums or gaps in coverage that last months or years. This checklist gives you the timelines and tasks to complete before, during, and after your eligibility period so you enroll correctly for Medicare Part A (hospital), Part B (medical), Part C (Medicare Advantage), Part D (prescription drugs), and Medigap (supplement) if applicable.

Note: This is educational information updated for 2025. For personalized advice, consult a licensed Medicare counselor or financial planner. See the Professional Disclaimer at the end.

The checklist (action items you can do now)

  1. Confirm your eligibility date. If you or your spouse have 40 Social Security work credits, Part A is usually premium-free at age 65. If you receive Social Security or Railroad Retirement benefits, enrollment may be automatic; otherwise you must sign up. (See Medicare.gov for details.)
  2. Mark the Initial Enrollment Period (IEP) on your calendar: the 7-month window that begins 3 months before the month you turn 65, includes your birth month, and ends 3 months after. Enroll during the IEP to avoid late penalties.
  3. If you’re covered by employer insurance, confirm whether it is current-employer coverage (20+ employees often counts differently) and ask HR how Medicare coordinates with your plan. Get that confirmation in writing.
  4. Decide about Part B: many defer Part B if working and covered by a qualifying employer plan; document employer coverage start/end dates and get a letter from HR showing creditable coverage if you later enroll in Part D (to avoid Part D penalty).
  5. Evaluate Medicare Advantage (Part C) and Part D options during Annual Enrollment Period (AEP, Oct 15–Dec 7) and other plan-change windows. Keep a note of AEP dates each year.
  6. Research Medigap (Medicare Supplement) rules in your state and whether guaranteed-issue rights apply at your Enrollment Period.
  7. Keep a file with copies of enrollment confirmations, Social Security notices, employer letters, and any correspondence from Medicare.
  8. If you miss an enrollment window, identify whether you qualify for a Special Enrollment Period (SEP) and gather supporting documents immediately.

Timeline: When to act

  • Before age 65: talk to your employer benefits team, review retiree plan documents, and begin researching Part B and Part D options. If you’ll retire before 65, see options for bridging to Medicare Bridging to Medicare: Health Coverage Strategies Pre-65.

  • IEP (7-month window around your 65th birthday): enroll in Part A and choose whether to enroll in Part B. Enroll in Part D if you need prescription coverage.

  • If you miss the IEP and still have employer coverage, you may be eligible for a SEP after employer coverage ends. If not, late-enrollment penalties may apply.

  • AEP (Oct 15–Dec 7): annually evaluate and change Medicare Advantage and Part D plans. Selections take effect Jan 1.

  • Additional windows: Open Enrollment for Medicare Advantage and switching back to Original Medicare (Jan 1–Mar 31) and SEPs for life events.

Authoritative resources: official enrollment timelines are on Medicare.gov and CMS. (See Resources at the end.)

Understanding penalties (what can happen if you delay)

  • Part B late-enrollment penalty: generally 10% of the standard premium for each full 12-month period you were eligible but didn’t sign up for Part B. This penalty is added to your monthly premium for as long as you have Part B.

  • Part D late-enrollment penalty: calculated based on the national base beneficiary premium and the number of months without credible prescription drug coverage. The penalty may be added to your monthly Part D premium when you enroll.

  • Coverage gaps: delaying enrollment can leave you without hospital or outpatient coverage for periods when you need care, and you may not be able to add supplemental coverage (Medigap) without medical underwriting outside guaranteed-issue windows.

Sources: Medicare.gov and CMS explain these penalties and formulas; check current pages for up-to-date calculations.

Employer coverage and special cases

  • If you have employer coverage at age 65: If your employer has 20 or more employees, employer coverage often pays before Medicare for active employees; you can delay Part B without penalty while you have that coverage. If your employer has fewer than 20 employees, Medicare usually pays first and you’ll likely need Part B.

  • Spousal coverage: If you’re covered through a spouse’s plan, confirm how long your spouse’s coverage remains active and whether switching to Medicare will affect premiums and benefits.

  • Disability and ESRD/ALS: People under 65 who qualify through disability, end-stage renal disease (ESRD), or ALS have different timelines and rules—verify those with Medicare.gov or a counselor.

  • Retiree plans and COBRA: Understand whether retiree coverage is creditable. Some retiree plans are not considered creditable for Part D, which can trigger a Part D penalty later if you delay.

Medicare.gov has specific pages explaining employer coverage and SEPs—save the URLs and any HR letters for proof if you need a SEP.

Choosing plans without gaps

  • Part C (Medicare Advantage): If you enroll in a Medicare Advantage plan, make sure it covers the providers you use and pharmacy coverage you need. Use the AEP (Oct 15–Dec 7) to change if needed. For details on choosing Advantage plans, see our guide on how to shop for Medicare Advantage plans How to Shop for Medicare Advantage Plans: Key Questions.

  • Part D (prescription): Check formularies, pharmacy networks, and total annual cost, including premiums and out-of-pocket. Avoid gaps by enrolling during the IEP or a qualifying SEP.

  • Medigap (Supplement): If you plan to keep Original Medicare (Parts A and B), compare Medigap policies in your state. You usually get guaranteed-issue rights only during the Medigap Open Enrollment Period which begins the month you turn 65 and have Part B.

For help comparing supplemental options, see our beginner’s guide to Medigap Choosing the Right Medicare Supplement: A Beginner’s Guide.

Documentation and record-keeping (be audit-ready)

  • Save copies of enrollment confirmations from Social Security or Medicare.
  • Get written statements from your employer confirming dates of health coverage and whether the coverage was creditable for Part D.
  • Keep correspondence from Medicare and plan notices in a single file (digital or paper).
  • If you apply for a SEP, keep all supporting documents (termination letters, proof of move, etc.).

These records are often what resolves disputes or proves eligibility for a SEP.

Common mistakes to avoid

  • Assuming automatic enrollment—only those already receiving Social Security benefits are automatically enrolled in Parts A and B in many cases.
  • Failing to get HR confirmation that employer coverage is creditable for Part D.
  • Choosing a plan based solely on premiums—review formularies, networks, and the plan’s star ratings.
  • Missing the AEP and then being stuck with a plan that no longer fits your needs.

For additional pitfalls and how to avoid them, see our related glossary post on common enrollment mistakes (https://finhelp.io/glossary/medicare-enrollment-mistakes-that-can-cost-you/).

Sample 6‑month action plan if you turn 65 soon

  • 6 months before: Research Medicare basics and review employer benefits. Schedule a call with HR to confirm coordination of benefits.
  • 3 months before: Finalize the decision on Part B with input from HR or a benefits counselor; prepare documents to enroll.
  • The month you turn 65: Submit enrollment for Part A and, if choosing, Part B and Part D.
  • 1–3 months after: Confirm enrollment letters have arrived; join a Medigap open enrollment window if you plan to buy a supplement.

Frequently asked operational questions

  • What if I miss the IEP? Immediately check if you qualify for a SEP (for example, loss of employer coverage). If not, you may enroll during a General Enrollment Period (Jan 1–Mar 31) but face penalties and delayed coverage.

  • Can I delay Part B if I have employer coverage? Often yes if you have qualifying employer coverage; get written proof from your employer to avoid penalties later.

  • How do I prove creditable prescription coverage? Ask your plan sponsor or insurer for a certificate that lists coverage dates and confirms creditable coverage for Medicare Part D.

Actionable next steps (one-page checklist you can print)

  • Confirm eligibility and whether Social Security enrollment is automatic.
  • Calendar the IEP and AEP dates; set reminders one month and one week before deadlines.
  • Request written documentation from your employer about coverage dates and creditable prescription coverage.
  • Compare Part D formularies and Part C networks if you expect regular medication or specialist care.
  • Keep all enrollment confirmations and HR letters in a single, easy-to-find file.

Professional note from the author

In my practice advising clients on retirement and healthcare, the single most common cause of penalties or gaps is delayed documentation—clients assumed employer HR handled everything. Getting written confirmation and marking your IEP/AEP on multiple calendars prevents nearly all problems.

Additional FinHelp guides referenced above:

Professional disclaimer

This article is educational only and does not replace personalized advice from a licensed Medicare counselor, insurance agent, or financial planner. Rules and premiums can change—always verify specifics with Medicare.gov or a qualified advisor before acting.