A "standard PC" is not in fact required to have a century register. One can obtain from ACPI information where it is if the hardware has one but this is an optional feature and PCs are not required to have a valid register number in their where-the-century-register-is fields. Indeed, the entire MC146818 compatibility system is optional as far as modern ACPI is concerned. There's an FADT flag that signals that it is not there.