Part B always starts on the first day of a month. Which month depends on when you sign up compared with when your job-based coverage ends. Get it right and you go straight from your job plan to Medicare with no gap.
The short version
| When you sign up | When Part B starts |
|---|---|
| While still covered by job-based coverage from current work (yours or your spouse's) | The month you sign up, or any of the next 3 months. You choose. |
| In the first full month after that coverage ends | The month you sign up, or any of the next 3 months. You choose. |
| In any of the remaining 7 months of your window | The first day of the month after you sign up |
| During your Initial Enrollment Period | Initial Enrollment Period rules (below) |
| In the General Enrollment Period (January 1 to March 31) | The month after you sign up |
Sources: 42 CFR 406.24 and POMS HI 00805.276 for the Special Enrollment Period rows; 42 CFR 407.25 for the others.
The "first full month" matters if your coverage ends in the middle of a month. That month counts as a covered month, so your first full month without coverage is the next one.
A worked example
Say your job-based coverage ends March 31, 2027. March is your last covered month. Your 8-month window runs April through November 2027.
| If you sign up in | Part B can start | Best pick |
|---|---|---|
| December 2026 | December 2026 to March 2027 | Too early to reach April |
| January 2027 | January to April 2027 | April 2027 |
| February 2027 | February to May 2027 | April 2027 |
| April 2027 (first full month after) | April to July 2027 | April 2027 |
| June 2027 | July 1, 2027 only | No choice; April to June has no Part B |
| After November 30, 2027 | Window closed | General Enrollment Period |
Picking April 2027 means no gap, and no paying Part B premiums for months your job plan still covers you. CMS-40B reminds you that you pay premiums for every month you have Part B.
Check your Part B Special Enrollment Period in 2 minutes. Start the free Part B check →
Medicare.gov's simpler version, and why it differs
Medicare.gov describes this more simply. Its Working past 65 page says coverage starts "the month after Social Security ... processes your completed form." Its table says coverage generally starts the month after you sign up, and that you can ask to delay it up to 3 months if you sign up while working or in the first full month after your job coverage ends.
The 07/2025 CMS-40B and Social Security's operating manual (POMS HI 00805.276) also allow the month you enroll. The form has a box called "Choose your coverage start date" where you mark the month you want. Filling it in removes the guesswork. See CMS-40B instructions.
The simple plan
Medicare.gov advises checking when your employer coverage ends and signing up about a month earlier, to avoid a gap. On CMS-40B, choose the first month after your last covered month.
An example from Social Security's manual
Social Security's manual gives this example. Morgan skipped Part B when first eligible because of coverage through a current job. Morgan decided to retire at the end of September 2014, applied for Part B in July 2014, and signed a statement asking for Part B to start in October 2014 (POMS HI 00805.277).
That's the pattern to copy: apply a couple of months ahead and name the month right after your last covered month. Today the request goes in the start-date box on CMS-40B.
If your job ends during your Initial Enrollment Period
Your Initial Enrollment Period is the 7 months around your 65th birthday: the 3 months before the month you turn 65, that month, and the 3 months after. You can't use the Special Enrollment Period until it's over, so its own rules apply (42 CFR 407.25):
- Sign up in the 3 months before the month you turn 65: Part B starts the month you turn 65.
- Sign up in the month you turn 65 or the 3 months after: Part B starts the next month.
Example: you turn 65 in May 2027, so your Initial Enrollment Period is February through August 2027. Your job coverage ends June 30, 2027. If you sign up in June 2027, Part B starts July 1, 2027, right when the job plan stops.
If you miss your window
You'll have to wait for the General Enrollment Period, January 1 to March 31. Part B starts the month after you sign up (42 CFR 407.25). You may also owe a late penalty. See the Part B late enrollment penalty.
Quick tips
- Ask your employer for CMS-L564 early. It's usually the slowest part.
- Choose a start month no more than 3 months after the month you send the form.
- Social Security advises not to cancel any health insurance you have until after your Medicare coverage begins.
- Not sure your coverage qualifies? Start with the Special Enrollment Period rules.