It's typical for a manufacturer to have a fixed number of days for lot expiration; e.g. this product is good for 90 days. Add to Lot/Serial Classes a Lot Expiration Days (integer), add to stock items the same field and default from Lot/Serial Class. When reporting Inventory receiptsdefault the Lot Expiration Date as Today()+LotExpirationDays.
You could do the same for purchase receipts but not recommended since you would want the operator to enter the date from the product information supplied by the vendor. Maybe an option added to lot/serial classes to exclude purchase receipts.