The ACPI spec describes the FADT->Century field as:

    The RTC CMOS RAM index to the century of data value (hundred and
    thousand year decimals).  If this field contains a zero, then the
    RTC centenary feature is not supported.  If this field has a non-zero
    value, then this field contains an index into RTC RAM space that
    OSPM can use to program the centenary field.

Use this field to decide whether to program the CENTURY register
of the CMOS RTC device.

Reviewed by:	akumar3@isilon.com, dab, vangyzen
MFC after:	1 week
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D33667

MFC after:	1 week
Sponsored by:	Dell EMC Isilon