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 permanent 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 then runs continuously, with no pause for anything that follows.

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, meaning none of them qualify a person for the SEP no matter how comprehensive the benefits appear. 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 requirement 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 seemed sufficient, Medicare considers itself the primary payer for the years you were eligible. In that situation, 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 rises almost every January.

The 2026 standard Part B premium is $202.90 per month, a jump of $17.90 (nearly 10%) from $185 in 2025. A beneficiary who incurs a single 10% late-enrollment penalty pays about $20 more per month, or roughly $240 more per year. A 30% penalty, the kind someone earns by delaying three full years, adds approximately $60 per month and compounds to more than $14,000 in extra costs over a 20-year retirement. Because the penalty is calculated as a percentage of the standard premium, the dollar amount climbs each year as Medicare raises its rates.

The penalty is also separate from IRMAA. Beneficiaries subject to income-related surcharges pay those amounts on top of any late-enrollment penalty. For 2026, IRMAA kicks in at $109,000 in modified adjusted gross income for individual filers and $218,000 for married couples filing jointly. For higher-income retirees who also delayed enrollment, the combined effect can be considerably more expensive than either charge alone.

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 still in the future, you have a clean window. If it has already passed, file for Part B immediately to stop additional penalty months from accruing.
• File form CMS-40B and CMS-L564 together, and do it before your last day of work if you can. 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. Tracking down a former employer’s HR department weeks or months after leaving can delay this process and eat into your eight-month window.
• 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 far less costly than finding out at 72.

Source note: The 2026 standard Part B premium of $202.90 and the annual Part B deductible of $283 reflect the CMS plan-year fact sheet released November 14, 2025. SEP and penalty rules are evergreen as of July 2026.

Editor’s note: This update added the 2026 Part B annual deductible of $283, noted the nearly 10% premium increase from $185 in 2025, included a concrete 30-year penalty scenario showing more than $14,000 in lifetime extra costs for a three-year delay, and added the 2026 IRMAA income thresholds of $109,000 for individual filers and $218,000 for joint filers.

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