Yes, you can reimburse yourself from your HSA for a past medical expense, even if you didn't have funds in the account at the time the expense was incurred, as long as you have the receipts and the account was open when the expense occurred
. There is no time limit for requesting reimbursement, but you must keep the receipts to prove the expense was qualified. At tax time you will get a form documenting your withdrawal and then you just report that the withdrawal was for a qualified medical expense.
