Medicare gives you a seven-month window built around your 65th birthday. Sign up in the right part of it and coverage starts on time with no penalty; sign up late and you can pay more every month for the rest of your life. This works it out for you.
How the window works
Your Initial Enrollment Period runs for seven months: the three months before the month you turn 65, your birthday month, and the three months after. If you sign up in the first three months, coverage starts the first day of the month you turn 65. If you wait until your birthday month or later, coverage starts the first day of the following month — which can leave you uncovered for weeks.
One quirk catches people out: if you were born on the first of a month, Medicare treats you as turning 65 in the previous month, so your whole window shifts a month earlier.
What waiting costs
If you miss the window entirely and don't have other creditable coverage, Part B adds 10% for each full year you could have had it, and that increase lasts as long as you have Medicare. Part D adds 1% of the national base beneficiary premium for each month you went without drug coverage, also for as long as you have it.
These penalties are the single most expensive avoidable mistake in Medicare, and they are permanent. That is why the window matters more than almost anything else on this site.
When the normal rules don't apply
- Still working past 65 with coverage from an employer with 20 or more employees? You may be able to delay Part B without any penalty — see Working Past 65.
- Lost other coverage? A Special Enrollment Period may open outside these dates — see Enrollment & Deadlines.
- Missed everything? General Enrollment runs January 1 to March 31 each year.
Next steps
Once you know your dates, the Turning 65 walkthrough covers the decisions you'll actually make, and Getting Help Paying explains the programs that cover premiums if your income is limited. Free one-on-one help is available from your SHIP counselor.
This tool does the date arithmetic from the rules published by Medicare. It is general information, not advice about your situation, and it cannot see your work history or other coverage. Always confirm with Medicare or your SHIP before acting.