Updated August 1, 2026. Quick answer: if you enrol in Medicare after your initial window, Part A is backdated up to 6 months — and your HSA contribution limit was zero for every one of those months. The money you put in during a period you did not yet know was covered becomes an excess contribution, taxed at 6% a year for as long as it stays in the account. Nothing warns you. The rule is not obscure — it is just never mentioned at the moment it matters, which is six months before you do anything.
The rule, in the two sources that govern it
Medicare backdates. CMS states the retroactive rule directly:
“Premium-free HI for the aged begins with the month in which the individual attains age 65, provided he or she files an application for HI or for cash benefits and HI within 6 months of the month in which he or she attains age 65. If the application is filed later than that, HI entitlement can be retroactive for only 6 months.”
CMS Pub. 100-01, Medicare General Information, Eligibility, and Entitlement Manual, ch. 2 § 10.2
Medicare’s own consumer wording is blunter: “Your Part A coverage starts 6 months back from when you sign up or when you apply for benefits from Social Security (or the Railroad Retirement Board). Coverage can’t start earlier than the month you turned 65.” Note the floor — the backdating cannot reach earlier than the month you turned 65, which is the one piece of mercy in the design.
The IRS treats those months as covered. Publication 969 addresses the retroactive case by name:
“Beginning with the first month you are enrolled in Medicare, your contribution limit is zero. This rule applies to periods of retroactive Medicare coverage. So if you delayed applying for Medicare and later your enrollment is backdated, any contributions to your HSA made during the period of retroactive coverage are considered excess.”
IRS Publication 969
Read those two together and the trap is fully specified. Medicare decides retroactively that you were covered; the IRS decides retroactively that you were never allowed to contribute. Neither agency has to talk to the other, and neither one writes to you.
What it costs
“Generally, you must pay a 6% excise tax on excess contributions. See Form 5329 … The excise tax applies to each tax year the excess contribution remains in the account.”
The phrase that does the damage is each tax year the excess contribution remains in the account. This is not a one-off penalty. Left alone, it is an annuity running against you, and it compounds quietly because nothing on your statement is labelled “excess”.
When to stop contributing
Six months before you apply — not the month you apply, and not the month coverage starts. If you are contributing monthly, that means the last safe month is the seventh month back from your application:
| If you enrol then | Part A can start as far back as | Last safe HSA contribution month |
|---|---|---|
| You apply in January | the previous July | June of the previous year |
| You apply in April | the previous October | September of the previous year |
| You apply in July | the previous January | December of the previous year |
| You apply in October | the previous April | March of that year |
Assumes enrolment after the initial window, so the full six-month backdating applies, and that you turned 65 more than six months earlier. If you are enrolling within your initial window your coverage start is set by that window instead, and there is nothing retroactive to plan around.
The employer contribution counts too. If your employer funds part of your HSA on a payroll cycle, those deposits land in the lookback window exactly as your own do, and they are excess on the same terms. Stopping your own contribution while payroll keeps going is a common half-fix.
If you are already caught
There is a clean exit, and it has a deadline:
“You withdraw the excess contributions by the due date, including extensions, of your tax return for the year the contributions were made. You withdraw any income earned on the withdrawn contributions and include the earnings in “Other income” on your tax return for the year you withdraw the contributions and earnings.”
Two obligations, not one. You withdraw the excess and the earnings attributable to it, and the earnings go into your income for the year you take them out. Do it by the due date of the return for the year the contributions were made, extensions included, and the 6% excise does not apply. Miss that date and the excise starts, and repeats.
Ask the custodian specifically for a return of excess contribution, not an ordinary distribution. They are different transactions, reported differently, and only one of them solves this.
The related trap: the last-month rule
“Under the last-month rule, if you are an eligible individual on the first day of the last month of your tax year (December 1 for most taxpayers), you are considered an eligible individual for the entire year.” It lets someone eligible on 1 December fund a full year. But it comes with a testing period, and Medicare enrolment breaks it:
“If you fail to remain an eligible individual during the testing period, for reasons other than death or becoming disabled, you will have to include in income the total contributions made to your HSA that wouldn’t have been made except for the last-month rule. … This amount is also subject to a 10% additional tax.”
So the person who used the last-month rule in the year before they enrolled has a second, separate exposure: the contributions that only the rule permitted come back into income, plus a 10% additional tax. Anyone approaching 65 who has used it should count both.
What you can still do with the account
Enrolling ends contributions. It does not touch the balance, and after 65 the account becomes a genuinely good way to pay Medicare premiums — with one exclusion that catches almost everybody: Medigap does not qualify, and Part B, Part D and Advantage do.
If you are working past 65 and this is the reason you were delaying Medicare, the delay may be entirely legitimate — it depends on your employer’s size, and delaying safely is what keeps the backdating from ever applying to you. For the dates themselves, your enrolment window; and if the delay was not protected, the Part B penalty is the other side of the same decision.
Retroactive-entitlement rule from CMS Pub. 100-01 ch. 2 § 10.2 and medicare.gov; HSA treatment, excise, correction, last-month rule and testing period from IRS Publication 969. All read August 1, 2026, and quoted rather than paraphrased because the wording is what decides the outcome. General information, not tax advice.