Reasonable Expectations re. Expense Reporting [TN]

Our employee handbook is badly worded and doesn't explicitly include a deadline for reporting mileage -- it does say that if you incur expenses on your personal credit card you have 30 days to request reimbursement. Someone just turned in a pile of mileage requests going back to the last fiscal year. The finance department thinks that I (HR) need to make the call on whether or not we're responsible for these expenses. We'll fix the handbook going forward, but if the handbook has our mileage policy and no deadline, are we technically on the hook to pay for this travel, some of which is over a year old?