Chrony and NTP hardening

Hello,

I'm supposed to implement a couple of NTP security requirements as 
suggested by RFC8633.

Specifically, the NTP daemon shall be prevented from accepting dates 
that set the clock to a time earlier than the build date of the system 
or a last-known-good time, which will be saved to a file once a day.

I'm wondering how this could best be achieved with Chrony. My main 
problem is that I can see no way to reliably detect if the time is 
acceptable before Chrony has already synchronized. Moreover, since we 
would also like to use rtcsync, this would mean that then also the RTC 
could be set to the wrong time and we'd therefore have no means to 
recover, and activating rtcsync only afterwards is unfortunately not 
supported.

Is there a better way to achieve this? I somehow doubt that we're the 
first ones with this problem but could not find a solution on the internet.

Best regards,
Bernd

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