Health savings accounts look like a savings product and behave like a tax instrument. Eligibility is tested on the first day of every month, so the annual limit is a sum of twelfths at whichever coverage tier applied in each month — and the last-month rule can lift it to the full annual figure in exchange for a testing period that can claw the difference back a year later. This models that arithmetic, and the card-swipe substitution decisions a custodian has to make in real time.