What is the HSA testing period?
Under the testing period, if you use the last-month rule, you must also remain an eligible individual (retain your same coverage under the CDHP or other high-deductible health plan) for the following 12 months. If you fail to remain an eligible individual (i.e., if you change coverage from the CDHP to the EPO or HMO or enroll in Medicare, etc.) any “extra” contributions you made as a result of the last-month rule must be included in your gross income. Also, a 20% additional tax applies to this amount. Your excess contributions are determined by the contribution limit divided by 12 months, compared to your time eligible.