HDHP coverage. How do I know what months I had HDHP?

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An HDHP is a "High Deductible Health Plan". You are required to have one in order to be eligible for an HSA. The HDHP is your health insurance, and the HSA is a way to pay for the insurance plan's deductible with tax-free dollars.

Your employer likely enrolled you in the HDHP at the same time that the HSA began. If there is some doubt, contact your HR department to confirm the start date.