The issue is whether you kept the old HSA, or whether you closed it and opened a new one. You can only pay for expenses that were incurred after the HSA was opened. If the old account from 2015 is still open, you can deposit money into it and then use it to pay the expenses. But if you closed that account and opened a new account, you can't pay the 2016 expenses with an account opened in 2017.
