Related discussion comes from here: https://chromium-review.googlesource.com/#/c/chromiumos/platform/ec/+/666985/

To support different on-ec-chip rtc easier, in kernel rtc-cros-ec driver we want to add extra params for Y/M/D/H/M/S, in additions to "seconds since epoch".

Vincent said:
"For RTC_SET host fills in both, for RTC_GET, EC puts a special value in the 'time' field (aka seconds since epoch) e.g 0xFFFFFFFF if it uses the splitted format, so it's backward compatible with the existing command.
On the kernel side, it's probably a 5-line change given the kernel has all the primitives in rtc-core."