Accounting for the word "Perpetual" in a date field when there is no end date

I am currently running this formula: =[Contract Expiration Date]@row - [Notify days to renew/cancel]@row to calculate the date when we need to review a contract. It essentially takes the end date of the contract and subtracts the numbers of days specified in the contract if you want to cancel. We have some contracts with no end dates that we designate as "Perpetual" in the Contract Expiration Date field. How do I update my formula to account for if Contract Expiration Date is "Perpetual" then return "Perpetual" and if not, then do the formula above?