Okay, so conclusion on this topic is either / or any combination:
- Install a RTC or use a board which has a builtin battery buffered clock
- Add a custom
server=/whatever.ntp/whatever.no-DNSSEC.server - Use the IP address of a local NTP server instead (may be your router) but ensure this server is not using your Pi-hole as DNS or gets the time from a reliable non-Internet source (DCF77, GNSS, etc.)
However, don't
- Hard-code the IP address of your favorite NTP server as this can cause a lot of unrelated trouble if their IP changes
Did I forget something? If so, I can edit this into here.