Turns out the clock stopped every night at the time when backups were running and thus the VM was paused (saved, or 'managedsaved' if someone uses libvirt) for a minute. Not sure why, though; while I was testing pause/resume the clock didn't stop, it just failed to get synced by ntpd(8). Maybe over time the drift was too much?
Anyway, the rather curious thing was ntpd(8) not syncing the clock properly after resume, so I ended up giving the 'trusted' option to the server I'm using here. Strangely, it still took quite some time [1], but in the end it managed to sync - so I guess this should work in the long run. [1] Jan 30 11:50:33 ntpd[83421]: peer 148.6.0.1 now valid Jan 30 11:54:37 ntpd[4758]: adjusting local clock by 23.831836s Jan 30 11:54:37 ntpd[83421]: clock is now synced Jan 30 11:54:37 ntpd[83421]: constraint reply from 9.9.9.9: offset 23.653130 Jan 30 11:57:48 ntpd[4758]: adjusting local clock by 22.879877s Jan 30 11:57:48 ntpd[83421]: clock is now unsynced Jan 30 12:01:34 ntpd[4758]: adjusting local clock by 21.754396s Jan 30 12:04:51 ntpd[4758]: adjusting local clock by 20.774539s Jan 30 12:08:33 ntpd[4758]: adjusting local clock by 19.670413s Jan 30 12:12:43 ntpd[4758]: adjusting local clock by 18.426017s Jan 30 12:17:04 ntpd[4758]: adjusting local clock by 17.127167s Jan 30 12:21:19 ntpd[4758]: adjusting local clock by 15.857846s Jan 30 12:21:53 ntpd[4758]: adjusting local clock by 15.688043s Jan 30 12:25:30 ntpd[4758]: adjusting local clock by 14.613690s Jan 30 12:29:49 ntpd[4758]: adjusting local clock by 13.323883s Jan 30 12:33:32 ntpd[4758]: adjusting local clock by 12.204646s Jan 30 12:34:06 ntpd[4758]: adjusting local clock by 12.036162s Jan 30 12:35:10 ntpd[4758]: adjusting local clock by 11.712658s Jan 30 12:36:13 ntpd[4758]: adjusting local clock by 11.412870s Jan 30 12:39:55 ntpd[4758]: adjusting local clock by 10.308062s Jan 30 12:43:34 ntpd[4758]: adjusting local clock by 9.208613s Jan 30 12:44:07 ntpd[4758]: adjusting local clock by 9.048595s Jan 30 12:47:48 ntpd[4758]: adjusting local clock by 7.950845s Jan 30 12:49:27 ntpd[4758]: adjusting local clock by 7.460912s Jan 30 12:53:08 ntpd[4758]: adjusting local clock by 6.360250s Jan 30 12:56:22 ntpd[4758]: adjusting local clock by 5.385971s Jan 30 12:56:53 ntpd[4758]: adjusting local clock by 5.241883s Jan 30 13:01:13 ntpd[4758]: adjusting local clock by 3.951414s Jan 30 13:04:22 ntpd[4758]: adjusting local clock by 3.009970s Jan 30 13:07:05 ntpd[4758]: adjusting local clock by 2.201024s Jan 30 13:11:18 ntpd[4758]: adjusting local clock by 0.937320s Jan 30 13:12:22 ntpd[4758]: adjusting local clock by 0.613777s Jan 30 13:13:27 ntpd[4758]: adjusting local clock by 0.285335s Jan 30 13:14:32 ntpd[83421]: clock is now synced
