Although balances accrue within a benefit year, they don't carry into a new year. So, to have access to the available balance for a certain year, you'll need to have eligible expenses during that year. For example, 2017 benefit amounts can only reimburse 2017 expenses. You won't have access to the previous year's balance for any expenses incurred during the new year.
At the end of the benefit year, you'll have 90-days to submit any expenses from the previous benefit year that you might've forgotten. For example, you'll have until the end of March 2018 to submit any remaining 2017 expenses in order to take full advantage of your remaining balance from 2017.