I haven’t fixed the time reset problem with my server yet, but at least I now know what is happening. The BIOS time is bad.
Doing my detective work, I searched the internet on ntpd, then checked the system’s logs to see that the time reset happened when the machine rebooted. It became clear that on reboot, the system sets initial time from the BIOS clock, which in turn indicates the BIOS time is WRONG.
Thinking back to the two times the system time was reset recently, I remembered there was a power outage in Feb. The UPS should handle things, but then this week (the other time reset event) I had to change batteries in the UPS as I was getting the “bad battery” light. Of course I powered it off trying to turn off the alarm while I obtained new batteries…
So the problem is now known. Unfortunately, the only way to fix things is to connect a keyboard and monitor to the server and then reboot it and manually fix the BIOS time. That’s not fun, but it will hold until the next machine reboot, which should not be soon given the new UPS batteries.