When I got access to the platform OS and found the timesyncd.conf file, I see that it’s hardwired to look for an external timeserver, and I could find no supported way (i.e. exposed as a setting to the user) of changing that (i.e. if an update happens, presumably any behind-the-scenes changes I made to that setting would get set back to whatever the distro came with.
99% of the time that would be fine, however…
The use-case I’m trying to solve is:
When we are away on an extended travel, if all power goes out (for longer than my UPS can carry it) including Internet service, and power is restored but not Internet. So even my border router (which does supply my LAN with an NTP time source) won’t know what time it really is.
And the time/sunrise/sunset-driven automations would start triggering at really odd times (until Internet returned). In more than one case that has taken a few days.
Clearly, the proper solution for me is to buy a platform that includes a built-in battery-backed clock, because it looks like adding my I2C RTC board to HAOS on the RaspberryPi isn’t the proper solution (which I could do, except for the above unsupported-change problem).