I have switched between ntpd, openntpd, timesyncd and a couple of others I might have forgotten, and the only one that's worked reliably is ntpd, but I was tryi

I have switched between ntpd, openntpd, timesyncd and a couple of others I might have forgotten, and the only one that's worked reliably is ntpd, but I was trying to stay off it, because historically it had quite a few vulnerabilities. I manage hundred of stand-along edge devices, mostly hardened PCs who have to bear up to 50 deg celsius environment from time to time; I've observered 5-10 times that RTCs were wrong by 5-10 hours in some direction after a power-loss, and openntpd fails miserably, timesyncd also not-so-good. ntp does ok. It's been much better since I started updating the rtc every 10 minutes from the system clock - most distros update from rtc on startup and to rtc on shutdown, but if the shutdown is after 6 months due to power loss, you're left with an rtc drift, which -- in harsh environment -- can be significant. I am right now testing chrony, which seems to be a winner, but it's too early to tell.