The HSA contribution system is built around eligibility tested month by month. The standard math is that for every month you are eligible on the first day of that month, you can contribute one-twelfth of the annual limit. Get HDHP coverage starting October 1, and you get three months of contributions. Get coverage on November 15, you get nothing for November (because eligibility is measured on the first), and you get only December if eligibility holds through that month.
The Last-Month Rule overrides all of that. Be eligible on December 1, and the IRS treats you as eligible for the entire calendar year for contribution purposes. You can fund the full annual limit even if you had no HDHP coverage for the first eleven months. The math is generous in a way that surprises most people the first time they read it.
What does not surprise as easily is the testing period that comes attached to that generosity.
The Last-Month Rule is explained in Publication 969. The mechanic is simple. If you are an eligible individual on December 1 of the contribution year, you are treated as having been an eligible individual for every month of that calendar year. You can contribute the full annual maximum, including any age 55 catch-up amount if you qualify.
The catch is the testing period. To keep the benefit of the Last-Month Rule, you must remain an eligible individual through the testing period, which runs from December 1 of the year you used the rule through December 31 of the following calendar year. That is a thirteen-month commitment. People who read the Last-Month Rule headline without reading the testing period attached to it tend to assume the rule begins and ends with the December 1 eligibility test. The testing period extends thirteen months further.
If you fail to remain eligible at any point during the testing period, the consequences are specific. The portion of your contribution that exceeded what you would have been allowed under the standard months-of-eligibility calculation is included in your gross income for the year you fail the test. On top of that included income, an additional ten percent tax applies to that same amount.
The exceptions to this consequence are narrow. Death and disability are the only events that excuse failure of the testing period.
Reporting happens on Form 8889 in the year the testing period is failed. The income inclusion and the ten percent additional tax both flow through that form.
The contribution itself has a separate set of deadlines worth keeping straight. HSA contributions for a given tax year can be made up to the tax filing deadline of the following year, which is generally April fifteenth. HSA contributions use a fixed original filing-deadline rule. Filing an extension does not extend the HSA contribution deadline.
That is separate from the Last-Month Rule testing-period consequence. A standard excess HSA contribution can generally avoid the six percent excise tax if the excess plus earnings is withdrawn by the tax-filing deadline, including extensions. A failed Last-Month Rule testing period has its own income inclusion and ten percent additional tax treatment. IRS Pub. 969 distinguishes excess contributions and the Last-Month Rule testing period consequence.
The Last-Month Rule deadline is the eligibility test on December 1. There is no extending that date. Either you are HSA-eligible on December 1, or the rule does not apply to that calendar year.
Consider someone uninsured for the first eight months of a calendar year, then starting a new job in September with health benefits beginning October 1. The employer plan is an HSA-qualifying high-deductible health plan, and the worker opens an HSA in October.
Without the Last-Month Rule, this worker would be limited to three-twelfths of the annual contribution limit, based on October, November, and December eligibility.
Using the Last-Month Rule, the worker can contribute the full annual limit. The difference between three-twelfths and the full twelve-twelfths is the amount at risk if the testing period fails.
The testing period runs from December 1 of the contribution year through December 31 of the following calendar year. If the worker changes jobs in March of the following year and the new employer plan is not a qualifying HDHP, the worker has failed the testing period. The portion contributed beyond what the months-of-eligibility rule would have allowed, roughly nine-twelfths of the annual limit in this example, is included in income for the year the failure occurred, and an additional ten percent tax applies to that same amount.
The Last-Month Rule is a real benefit, especially for someone who becomes HSA-eligible late in a calendar year and wants to capture the full annual contribution. Most people who use it will remain eligible through the testing period without issue, because most people do not switch employers, drop HDHP coverage, or enroll in Medicare within a thirteen-month window.
The trap is reserved for those who use the rule and then lose eligibility during the testing period without realizing that part of the contribution may become taxable and subject to an additional ten percent tax. Medicare enrollment is one common cause, because the contributor approaching sixty-five may not connect the Last-Month Rule used the prior December to Medicare timing planned for the following year.
What matters is awareness of the testing period before taking the contribution. The Last-Month Rule rewards those who think one year ahead, which is more thinking than most people do when reading a December enrollment confirmation.
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Disclaimer
This article is for educational and informational purposes only. It is not tax, legal, or financial advice and does not create an advisor–client relationship. Always consult appropriate professionals regarding your specific situation.