The date itself

Sign up in March, covered in April: every Medicare start date

Short answer: if you sign up in the three months before your 65th birthday month, coverage starts the first day of your birthday month. If you sign up during your birthday month or in the three months after, coverage starts the first day of the month after you sign up. That second rule changed in 2023 and eliminated the old two and three month waits that used to strand people. Every other window has its own date, and the differences matter when you are timing the end of an employer plan against the start of Medicare.

By the Medicare Decoder team · Updated September 30, 2026 · 4-minute read · Figures are official CMS amounts, checked against CMS

Key takeaways

Sign up before your birthday month: coverage starts the first day of your birthday month.
Sign up during your birthday month or the three months after: coverage starts the first day of the following month.
General Enrollment Period (January 1 to March 31): coverage starts the first day of the month after you sign up.
Annual enrollment changes made October 15 to December 7 always start January 1.
Premium-free Part A can be retroactive up to six months if you enroll after 65, which interacts badly with HSA contributions.

Jump to Your first window Every window Timing against an employer plan The Part A retroactivity trap FAQ

Your first window, and the rule that changed

Your Initial Enrollment Period is seven months long: the three months before your 65th birthday month, that month, and the three months after. Where you sign up inside that window sets your start date. Sign up in the first three months and coverage begins the first day of your birthday month. Sign up in the birthday month or any of the three after, and coverage begins the first day of the month following your enrollment.

That second half is a 2023 improvement worth knowing about, because older articles still describe the previous rule. Before January 1, 2023, enrolling late in the window pushed coverage out by two or three additional months, which left people who signed up in the final month waiting until well after their birthday to actually have coverage. That gap is gone. If your birthday falls on the first of the month, Medicare treats you as turning 65 in the previous month, which shifts the whole schedule one month earlier.

Every window and its start date

When you sign up	Coverage starts
3 months before your birthday month	The first day of your birthday month
Your birthday month, or the 3 months after	The first day of the month after you sign up
General Enrollment Period (Jan 1 to Mar 31)	The first day of the month after you sign up
Special Enrollment Period after employer coverage ends	Usually the first day of the month after you sign up; ask for the specific date
Annual enrollment (Oct 15 to Dec 7), plan changes	January 1
Medicare Advantage Open Enrollment (Jan 1 to Mar 31)	The first day of the month after the plan receives your request
5-star plan switch (Dec 8 to Nov 30)	The first day of the month after you enroll

Coverage always begins on the first of a month. Part A and Part B can have different start dates if you took Part A earlier and delayed Part B.

Timing Medicare against an employer plan

This is where start dates stop being trivia. If you are retiring and your employer coverage ends on, say, June 30, you want Medicare active July 1, which means enrolling in advance rather than in the month you need it. Working backward from the date coverage ends, rather than forward from your last day of work, is the habit that prevents gaps. Two specifics people get wrong:

COBRA does not delay anything. It is not creditable coverage for Part B, so relying on it past 65 accrues a permanent penalty while you believe you are covered. The bridge-coverage article covers this in full.
Your employer-coverage Special Enrollment Period runs eight months from when employment or the group coverage ends, whichever comes first, but drug coverage has a tighter deadline, so do not use the full eight months as a plan. Working past 65 maps the whole sequence.

The Part A retroactivity trap

If you enroll in premium-free Part A after turning 65, Medicare can make it retroactive up to six months (never earlier than your 65th birthday). That sounds harmless and is not, for one group: anyone contributing to a health savings account. HSA contributions are not allowed for any month you have Medicare, including retroactive months, so the retroactivity can turn perfectly legal contributions into excess contributions that require correction. The fix is timing, and it is the entire subject of the HSA article: stop contributing six months before you enroll.

Two practical notes to close. You can check your exact start date in your Medicare account or on your card once it arrives, and your card usually arrives a few weeks after enrollment is processed. And if you want a Medicare Advantage or Part D plan, its coverage generally cannot start before your Part B does, which is why the month-by-month checklist sequences Part B first and the plan second.

Questions people also ask

When does Medicare coverage start if I sign up before my birthday?

If you enroll during the three months before your 65th birthday month, Part B coverage begins the first day of your birthday month. If your birthday is on the first of the month, coverage starts a month earlier, because Medicare treats you as turning 65 in the previous month.

What if I sign up during or after my birthday month?

Coverage starts the first day of the month after you sign up. This changed on January 1, 2023; under the old rule, enrolling in the last three months of your Initial Enrollment Period pushed coverage out by two or three extra months.

When does coverage start if I use the General Enrollment Period?

The first day of the month after you sign up. The General Enrollment Period runs January 1 through March 31, and late enrollment penalties may still apply even though coverage now begins promptly.

When do Medicare plan changes made during open enrollment take effect?

January 1. Changes to Medicare Advantage or Part D plans made between October 15 and December 7 always begin on January 1, and your previous plan ends automatically on December 31.

Can Medicare Part A start retroactively?

Yes. Premium-free Part A can be made retroactive up to six months when you enroll after 65, but never before your 65th birthday. This matters if you contribute to an HSA, because contributions are not allowed for months you have Medicare, including retroactive ones.

Sources for this article

Medicare.govWhen does Medicare coverage start: start dates by enrollment window.
Medicare.govSigning up for Medicare: enrollment periods and how to enroll.
SSA.govSocial Security: signing up for Medicare: the enrollment mechanics and retroactive Part A rules.

Figures checked against these sources on September 30, 2026. We update within days when CMS publishes new amounts.

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