As far as I know, (and I may be wrong) Home Assistant is hard-coded to use Google’s Public NTP service.
If you wanted Home Assistant to use your local NTP server, I imagine you would have to spoof DNS to make it appear as being Google’s NTP.
I was planning on achieving that with this https://github.com/home-assistant/operating-system/blob/329caf6c1c0b6267146bd39199ccf62c387784c0/Documentation/configuration.md#ntp. But then I’m not sure if my local NTP will still be responding after restarting without internet
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I stand corrected, it can be modified; good to know.
Or just add an rtc module to your pi. There are plenty of them, most simply plug onto a part of the gpio port and they typically use a little coin cell to keep the time when the power is out. If it’s only to cover the time of a power / net outage (rather than complete offline operation), even a cheapo rtc for a few bucks should be more than enough.
Ohh, I’m going to look into that, thanks! 
Hi HeyImAlex, there are any configuration win Hassio that must be modified to use the added RTC module connected to the RPi?
Thanks
I don’t think there is, was following this thread but it seems no one has found a solution yet. I saw the home assistant os is based on buildroot and motioneye os is also based on it and does support RTC, this are their instructions, I was thinking maybe enabling developer access to the home assistant os I’d be able to add that same config and hopefully it’d just work 
, I just need to order a RTC module and a raspberry to try that without the risk of breaking my home. Worst case scenario it doesn’t work and I use that new raspberry as a NTP server. I would rather not have my home assistant depend on another server to boot but that’ll be better than what I have now.
I never used Hassio / Home Assistant OS, so I don’t know how straightforward it is (or not). I’m running HA as a venv under Raspbian (well, Raspberry Pi OS now). I have the PCF8523 based RTC, so enabling it was pretty easy (enabling the kernel driver, disable the fake hw clock and enable the real hwclock support). Indeed HA OS seems to be based on Buildroot, so I would assume that it should be possible to add RTC support. But they really locked down the OS, so it might be easier to just have an external NTP.
Alternatively, if you are already setting up a new Pi, you could also install HA either as a Python venv or in a Docker container. And keep full access to the host OS in either case.