Medicare Enrollment Periods for Federal Employees: IEP, SEP, and GEP Explained

The Three Medicare Enrollment Windows

Federal employees face the same Medicare enrollment periods as everyone else, but the interaction with FEHB creates specific timing considerations that most general Medicare guides don't address. Here's how each window works for federal workers and retirees.

Initial Enrollment Period (IEP)

The IEP is a 7-month window centered on your 65th birthday month: it starts 3 months before your birthday month, includes your birthday month, and ends 3 months after.

If you're already retired when you turn 65: This is your primary window for enrolling in Medicare Part B without penalty. FEHB continues as retiree coverage regardless of whether you add Part B, but declining Part B during your IEP starts the late enrollment penalty clock.

If you're still working at 65: You can enroll in Part A (free for most) during your IEP, but you can safely skip Part B. Your active FEHB plan is primary coverage, so there's no financial benefit to paying the Part B premium while you're still employed. You'll use the Special Enrollment Period when you eventually retire.

When coverage starts depends on when during the IEP you enroll:

• Enroll in the 3 months before your birthday month: coverage starts the 1st of your birthday month
• Enroll during your birthday month or the 3 months after: coverage starts the 1st of the following month

Enrolling early in your IEP avoids any gap between turning 65 and coverage starting.

Special Enrollment Period (SEP)

The SEP exists specifically for people who delayed Part B because they had group health coverage based on active employment. For federal employees who work past 65 with FEHB, this is the enrollment window that lets them avoid the late penalty.

The SEP lasts 8 months, starting from the month your active employment ends — not the month your FEHB coverage ends. This distinction matters because FEHB continues into retirement: your employment ends, but your coverage doesn't.

To use the SEP, submit:

• Form CMS-40B (Application for Medicare Part B)
• Form CMS-L564 (Request for Employment Information), completed by your agency HR or OPM, certifying your dates of active employment and FEHB coverage

Submit both to your local Social Security office. Coverage starts the first day of the month after the SSA processes your enrollment — no waiting period during an SEP, unlike the GEP.

The 8-month clock is strict. If you retire at 67 in March 2026, your SEP runs through October 2026. Miss it, and you fall to the GEP with its penalty and delayed start.

General Enrollment Period (GEP)

The GEP runs January 1 through March 31 each year. Coverage starts the first day of the month after you enroll. The GEP is the fallback for anyone who missed both their IEP and their SEP.

For federal retirees, the GEP comes with two penalties:

1. The late enrollment penalty: 10% added to your Part B premium for every full 12-month period you were eligible but not enrolled. This is permanent.
2. The coverage gap: You enroll in January–March and coverage starts the following month. That's still a stretch without Part B if you needed it immediately, during which FEHB remains your only coverage with standard (non-wraparound) cost-sharing.

The GEP is almost always the worst option. Proper planning avoids it entirely.

Part A Enrollment: Simpler but With an HSA Trap

Medicare Part A enrollment is more straightforward. Most federal employees with 40+ quarters of Social Security-covered earnings qualify for premium-free Part A. You can enroll at 65 during your IEP regardless of whether you're still working.

Part A doesn't require you to leave your job. It doesn't conflict with FEHB (since Part A covers hospital care while FEHB covers everything, there's no downside to having both). And there's no late enrollment penalty for Part A if you qualify for premium-free coverage.

The HSA exception: If you're contributing to a Health Savings Account through an FEHB High Deductible Health Plan, you must stop HSA contributions six months before enrolling in Medicare Part A. Why six months? Medicare Part A can be retroactive up to six months, and the IRS treats HSA contributions as impermissible if you had any Medicare coverage during the contribution period.

If you're 64.5 and still contributing to an HSA, stop contributions by the month that's six months before your Part A enrollment date. Excess contributions face a 6% IRS penalty per year until corrected.

Which Window Applies to You

Scenario 1 — Retiring before 65: You'll use the IEP when you turn 65. No SEP applies because you won't have active employment-based coverage at 65.

Scenario 2 — Retiring at or after 65: You'll use the SEP. Start the process during your last months of active employment: have HR prepare the CMS-L564, download CMS-40B, and submit both to SSA within the 8-month window after your separation date.

Scenario 3 — Already retired, past 65, no Part B: If you declined Part B during your IEP and your SEP has expired, the GEP (January–March) is your only option. The late penalty applies.

Scenario 4 — Working past 65 and retiring soon: Coordinate the timing. Some retirees submit their CMS-40B during the last month of active employment so Part B activates immediately after separation, avoiding any gap where FEHB alone bears full primary-payer costs.

The FEHB & Medicare Coordination Guide includes a month-by-month enrollment timeline that maps these windows against your retirement date, FEHB continuation, and IRMAA lookback periods.

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