Both hassos and Raspberry Pi OS were down for me for several days because the Debian NTP servers were unreachable which caused problems with SSL and so on. The Pi OS config file lists only several Debian NTP servers as fallbacks and none were reachable. When I added another NTP server to the list the Pi OS worked fine. I could not locate the hassos configuration file to make the same change so hassos was down for several days. Adding some NTP servers from other sources to the fallback would add resistance. Google has a free NTP server which would be a good choice.
As to why the NTP servers were unreachable, I don’t know. I normally use Google DNS service which is pretty resilient. I switched to my ISP’s DNS server and continued to find the Debian servers unreachable. Otherwise, my Internet was working fine and other NTP servers worked when I added them to the config file.