When Your Paychecks Stop, an 8-Month Medicare Clock Starts — and Most People Read It Wrong

A 64-year-old retires in March. Her employer’s group health coverage ends with her last paycheck, so she elects COBRA for 18 months to bridge the gap. Nearly a year later, she learns something no one clearly explained when she left work: Medicare’s 8-month Special Enrollment Period generally starts when active employment or active employer coverage ends, not when COBRA ends. By the time she discovers the rule, much of that enrollment window has already passed.

This is one of the most common Medicare timing mistakes retirees make. If you have employer or union coverage tied to active work, or your spouse does, understanding when the Special Enrollment Period begins can help you avoid coverage gaps and potential late-enrollment penalties. If you are already enrolled in Part B or do not have employer coverage to lose, this issue generally does not apply.

Where the 8-Month Clock Actually Starts

The Part B Special Enrollment Period gives you eight months to sign up without the late enrollment penalty after one specific event: the end of current active employment coverage, either yours or your spouse’s. The clock starts the month after employment ends or the month after the group health plan ends, whichever comes first. It runs continuously from there.

COBRA does not extend it. Retiree coverage does not extend it. Severance medical benefits do not extend it. The Social Security Administration treats all three as non-active coverage, so they do not qualify a person for the SEP no matter how comprehensive they are. A retiree who waits out an 18-month COBRA run and applies for Part B in month 19 has already missed the window by ten months.

The SEP also has a size gate most people never hear about: the employer must have had 20 or more employees for the active-employment coverage to count as primary to Medicare. If you worked for a smaller employer past 65 and delayed Part B because your group plan was “good enough,” Medicare considers itself the primary payer for the years you were eligible. The penalty clock has been running since the month you turned 65, not the month you retired.

What the Penalty Actually Costs

The Part B late enrollment penalty is 10% of the standard premium for every full 12-month period you were eligible but not enrolled. It attaches to every monthly premium for the rest of your life, and the base it rides on goes up almost every January.

Using the 2026 standard Part B premium of $202.90 per month, the math is straightforward. A beneficiary who incurs a 10% late-enrollment penalty would pay about $20 more per month, or roughly $240 more per year, based on current premiums. A 20% penalty would double that amount. Because the penalty is calculated as a percentage of the standard Part B premium, the dollar amount generally rises over time as Medicare premiums increase.

The penalty is also separate from IRMAA. Beneficiaries subject to income-related surcharges pay those amounts in addition to any late-enrollment penalty. For higher-income retirees, the combined effect can make delaying Part B considerably more expensive than expected.

Three Actions to Take This Week

• Anchor the date. Find the last day your active employer coverage was in force, not the last day of COBRA. Count forward eight months. If that date is in the future, you still have a clean window. If it is in the past, stop waiting and file for Part B immediately to stop additional penalty months from accruing.
• File form CMS-40B and CMS-L564 together. CMS-40B is the Part B application; CMS-L564 is the employer verification of your group coverage dates. Submitting both at once documents the SEP trigger and prevents Social Security from defaulting you into the General Enrollment Period, which carries the penalty automatically.
• Confirm the employer-size rule before relying on it. If you stayed on a group plan past 65 at an employer with fewer than 20 employees, call 1-800-MEDICARE and ask whether your Part B clock has already been running. Finding out now is cheaper than finding out at 72.

Source note: Premium and deductible figures reflect the CMS 2026 plan-year fact sheet released November 14, 2025. SEP and penalty rules are evergreen as of June 2026.