When does my Medicare Initial Enrollment Period start and end?

The basic rule, counted in months

Medicare.gov describes the Initial Enrollment Period as a 7-month window that starts 3 months before you turn 65 and ends 3 months after the month you turn 65. Social Security's Medicare booklet says the same thing in different words: a seven-month period that begins three months before the month you turn 65, includes that month, and ends three months after it.

The window is counted in whole calendar months, not in days from your birthday. Take someone born on March 14, 1962. They turn 65 on March 14, 2027, so March 2027 is the birthday month. Count back three months and the window opens on December 1, 2026. Count forward three and it closes on June 30, 2027. The exact day in March does not move the window, with one exception covered below.

Notice that this example crosses a year boundary. Anyone whose 65th birthday falls between January and March has a window that opens in the previous calendar year, which is an easy thing to miss if you only start thinking about Medicare in your birthday year.

If your birthday is on the 1st of the month

Medicare.gov has a separate note for this case: your 7-month period starts earlier. It starts 4 months before you turn 65 and ends 2 months after the month you turn 65, and coverage generally starts on the first day of the month before you turn 65.

Medicare.gov's own example is a June 1 birthday. The Initial Enrollment Period runs from February to August, and if that person enrolls in February, March or April, coverage starts May 1. In effect the whole window is calculated as if the birthday were in May.

Social Security uses the same convention elsewhere. Its retirement planner says that if you were born on the 1st of the month, your full retirement age is figured as if your birthday was in the previous month, and its Medicare booklet says that for people enrolled automatically, Part A and Part B start on the first day of the prior month when the birthday is on the 1st.

When coverage actually starts

Being inside the window and being covered are two different things. Medicare.gov says coverage always starts on the first of a month, and the start month depends on when in the window you enroll.

If you qualify for premium-free Part A, Part A starts the month you turn 65 (or the month before, for a birthday on the 1st). For Part B, Medicare.gov's table has two rows. Enroll before the month you turn 65 and coverage starts the month you turn 65. Enroll in the month you turn 65 or in the three months after, and coverage starts the next month.

For the March 14 birthday above, enrolling in December, January or February means Part B starts March 1, 2027. Enrolling in April means it starts May 1. The later months are still inside the window and carry no late penalty, but they leave a shorter or longer stretch before coverage begins, which matters if other health insurance is ending.

Who is enrolled automatically

According to Social Security's Medicare booklet, if you are already getting Social Security or Railroad Retirement Board benefits, you are enrolled in both Part A and Part B automatically, starting the first day of the month you turn 65. You do not need to apply, although Part B is voluntary and has a monthly premium.

If you are approaching 65 and not getting benefits yet, the same booklet says to contact Social Security about three months before your 65th birthday, and to enroll even if you do not plan to retire at 65. The exception is coverage through a current employer's group health plan, yours or your spouse's. Social Security says you can delay Part B in that case, and a Special Enrollment Period lets you enroll later without the late penalty.

What happens if the window passes

Without a Special Enrollment Period, the next chance is the General Enrollment Period, January 1 to March 31 each year. Medicare.gov says coverage then starts the month after you enroll.

The bigger cost is the Part B late enrollment penalty. Medicare.gov puts it at an extra 10% for each full 12-month period you could have had Part B but did not, added to your monthly premium and usually charged for as long as you have Part B. Its 2026 example: someone who waited two full years pays a 20% penalty, so the standard $202.90 premium becomes $243.50 a month. Medicare.gov also notes that you generally will not owe this penalty if you qualify for a Special Enrollment Period or enroll in a Medicare Savings Program.

Drug coverage has its own rule. Medicare.gov says the Part D penalty is 1% for each month (12% a year) if you go 63 days or more without creditable drug coverage after you first get Medicare.

Common mistakes when counting the window

Counting from your retirement date. The window is tied to your 65th birthday, not to your last day at work or your Social Security claiming age. Full retirement age is 67 for anyone born in 1960 or later, but Social Security's own planner says that if you delay benefits past 65 you should still apply for Medicare within 3 months of your 65th birthday.

Forgetting the 1st-of-the-month rule. A June 1 birthday puts the window in February to August, not March to September. Get this wrong and you think you have a month you do not have.

Treating COBRA or retiree coverage like a job-based plan. Social Security's booklet states that COBRA and retiree health coverage do not count as current employer coverage, and Medicare.gov lists the end of COBRA or retiree coverage among situations that do not qualify for a Special Enrollment Period.

If you would rather not count months by hand, RetireRunway works the window out from the birth date you enter, applies the 1st-of-the-month rule, and shows the seven months beside your retirement countdown with links to Medicare.gov, Social Security and SHIP counseling.

This guide summarizes Medicare.gov and Social Security publications as of 2026. It is not insurance or legal advice and cannot confirm your eligibility. Social Security (1-800-772-1213) and Medicare (1-800-MEDICARE) confirm your enrollment dates, and your State Health Insurance Assistance Program (SHIP) offers free, unbiased counseling.

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