Codes will not be saved will only if the amount entered with the code is more than the product price or if the code,discount pair is not entered correct. Like if rather than entering
or if you missed the comma (,)
Date is not for discount codes, it's for the product expiration, say if you want the product to be unavailable after a set date and time (like an event ticket).
URL generated codes are not related to discount. Please see help page for this.