IMHO, You are over-thinking/complicating this.
When I was working at Nvidia, I worked on a multi-boot OS image that had every current flavor of Windows/Linux that the GPUs needed to be tested on behind GRUB (the old GRUB, where you could tell it which OS to boot to without running shell scripts). We "fixed" this same problem by telling Windows to expect UTC BIOS clock and not to do network time syncs (yes this works even with Windows 10, if you get the QWORD/DWORD sizes right). Clock drift in Windows is negligible even after a few days, but you can either suspend or reboot triggering a new read from HwRTC to clear that up.
There is a lot of information on how to set the Windows registry on the interwebs, and Win8+ has a UTC option on the time control panel now. One of the trickiest issues we ran into is some Linux flavors using an offset from the hardware clock rather than reading and writing to the clock directly - you need to understand how all of the time daemons do their thing on your chosen OSes.
-Tim
Date: Sat, 10 Mar 2018 14:07:22 -0500 From: DickPan@protonmail.com To: "coreboot@coreboot.org" coreboot@coreboot.org Subject: [coreboot] Can coreboot automatically detects Windows/Linux and provide local/UTC time accordingly? Message-ID: CilhuYxnQ6aE6Uxgs7kM5OcY6Wu_pKHCSFPJ_uHDfyMWgpD54ibZTJ4gswQ4mspaARHlQ9XTSh-piQs96JEf_q3XPNdjCiVrRr8PDjpEQ7I=@protonmail.com
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I use both Windows and Linux, try different versions and distributions, and often do clean install of these systems. An problem I faced is the default install of Windows takes the time provided by the board as local time, while Linux takes it as UTC time. After every installation, the first thing I have to do is to configure it.
So instead of configure it at the system level, can this be done at the coreboot/seabios level? Can coreboot or seabios automatically detect the system it is running? Can it automatically provides local time if it detected Windows, while provides utc time when it detected Linux?