The Medicare enrollment timeline: dates, windows and penalties

By Tom, CPA · Updated October 25, 2026

TL;DR: Your Initial Enrollment Period is seven months around your 65th birthday. Miss it without qualifying employer coverage and you face a lifelong Part B penalty of 10% per full 12 months late. COBRA and retiree coverage don't protect you.

Educational only, not tax, legal, or investment advice. Examples are illustrations, not predictions. Rules and limits change; verify current figures and talk to a qualified professional about your situation.

Miss a window, pay forever. Medicare enrollment is one of the few financial deadlines where procrastination earns a permanent surcharge. Early retirees are especially exposed: they've been off employer coverage for years, so the "I'm still working" exception doesn't apply, and COBRA or a retiree plan can lull them into thinking they're covered for Medicare purposes. They aren't.

Here's the timeline, the penalties, and the gotchas, all from Medicare.gov as of Oct 2026.

CPA Warning Label: Medicare penalties last for as long as you have Medicare. That is a very long time to pay for a missed calendar reminder.

Initial Enrollment Period

Medicare.gov describes the Initial Enrollment Period as seven months: the three months before the month you turn 65, your birthday month, and the three months after.

• Sign up in the three months before: coverage starts the first day of your birthday month.
• Sign up in your birthday month or later: coverage starts the first day of the month after you sign up.

Illustration: Marisol turns 65 on July 14. Her window runs April 1 through October 31. If she signs up in May, coverage starts July 1. If she signs up in September, it starts October 1, leaving a gap if her marketplace plan ended earlier. Early retirees should sign up in the three months before their birthday month.

If you're already collecting Social Security before 65, you're generally enrolled in Parts A and B automatically.

General Enrollment Period

Missed your Initial Enrollment Period and don't qualify for a Special Enrollment Period? The General Enrollment Period runs January 1 through March 31 each year. Coverage starts the month after you sign up. You may owe a late enrollment penalty, and you may have a coverage gap in the meantime.

Employer coverage exceptions

If you or your spouse are still working and covered by that employer's group health plan, you can usually delay Part B without penalty and use a Special Enrollment Period later. Medicare.gov says you can sign up any time while you have that coverage, or during the eight months after the coverage or the employment ends, whichever is first.

Important limits:

• COBRA doesn't count. COBRA and retiree health coverage aren't considered coverage based on current employment for this purpose. You won't get a Special Enrollment Period when that coverage ends.
• The eight-month clock starts when employment ends, not when COBRA ends. People who ride COBRA for 18 months past 65 can blow through the window.
• Small employers. If the employer has fewer than 20 employees, Medicare is often primary and you may need Part B right away. Ask the plan.
• Marketplace plans are not employer coverage. Early retirees on marketplace plans should enroll during the Initial Enrollment Period. Once eligible for premium-free Part A, they generally can't keep getting the premium tax credit.

Penalty math

Part B: Medicare.gov says the monthly premium may go up 10% for each full 12-month period you could have had Part B but didn't. You'll usually pay it as long as you have Part B. Medicare.gov's own example: someone who waited two full years pays 20% more, or $243.50 a month in 2026 instead of $202.90.

Part D: If you go 63 or more days in a row without Part D or other creditable drug coverage after your Initial Enrollment Period, a penalty applies. It equals 1% of the national base beneficiary premium ($38.99 in 2026) for each full month you went without. Example: 30 months late is 30% of $38.99, or about $11.70 a month, recalculated each year as the base premium changes.

Premium Part A: Most people get Part A premium-free. If you must buy it and enroll late, the premium may rise 10%, for twice the number of years you could have had it but didn't.

HSA stop rule

Medicare.gov says if you sign up for Part A after turning 65, coverage can go back up to six months, but not before the month you turned 65. That retroactive coverage makes HSA contributions in those months excess contributions. Stop HSA contributions about six months before you apply. Applying for Social Security retirement benefits after 65 triggers Part A automatically, so the same rule applies when you claim. More in HSA-eligible plans for early retirees.

Ending marketplace coverage

HealthCare.gov warns that your marketplace plan won't end automatically when Medicare starts. You must end it yourself. You can report Medicare coverage up to three months ahead. Otherwise you may keep paying premiums, and any premium tax credit for months after you became eligible for premium-free Part A may have to be repaid. Since the repayment caps no longer apply for tax years after 2025, that can be the full amount.

Medigap's one-time window

Your Medigap Open Enrollment Period lasts six months, starting the first month you have Part B and are 65 or older. During that window insurers generally can't use medical underwriting. After it, they often can, so delaying Part B also delays this protection. See Medicare Advantage vs Medigap.

Illustration: the COBRA trap in numbers

Illustration: Glen retires at 64 in June and elects COBRA for 18 months. He turns 65 the following February and doesn't enroll in Part B because he "has coverage." COBRA ends in December, nine months after his Initial Enrollment Period closed. He can't use the employer Special Enrollment Period, because the eight months ran from when employment ended, not from COBRA's end. He signs up in the next General Enrollment Period, January through March, with coverage starting the month after.

His gap: potentially a month or more with no Part B coverage. His penalty: if he was late by less than a full 12 months, no Part B penalty yet, but one more year of delay would mean a permanent 10% increase. Meanwhile, COBRA plans often pay secondary once you're eligible for Medicare, so claims during the gap may be reduced. A small calendar error, a large surprise.

Turning 65 while living abroad or on a spouse's plan

If you're covered by a working spouse's large-employer plan, you can usually delay Part B and use the Special Enrollment Period later. Get the employer to complete the employment verification form when you do sign up; it proves your coverage dates.

Living abroad doesn't change the penalty rules. Medicare generally doesn't cover care outside the U.S., but if you return later and enroll, late penalties can apply. Some expats keep Part B anyway to avoid that. It's a cost-benefit decision worth making on purpose. See health insurance abroad.

Applying

If you're not yet collecting Social Security, you apply for Medicare through Social Security, online at ssa.gov, by phone or at a local office. Save confirmation numbers and screenshots. Watch for your Medicare card, then enroll in a Part D plan, Medicare Advantage plan or Medigap policy within the right windows. Each step has its own deadline, and none of them wait for you.

A 65th-birthday checklist

1. Six months before: stop HSA contributions if you'll apply after 65.
2. Four months before: compare Medigap, Part D and Medicare Advantage options.
3. Three months before: apply for Parts A and B at ssa.gov.
4. Choose drug coverage so you're not uncovered for 63 days.
5. Report Medicare to the marketplace and set the end date.
6. Check whether your income two years earlier triggers IRMAA; see Part B premiums and IRMAA.