@subven said:
Replace your BIOS battery, go into BIOS and set current time, reboot and you should be fine.
This is a dedicated server at Hetzner, not a home server.
But the backup ran fine before and since, so I think perhaps I'll just not worry about it for now.
The bigger completely unrelated issue would appear to be that when I do an integrity check on my rsync backups to Hetzner Storage Box (as opposed to this tarball to Scaleway buckets backup), they fail... I think I'll start a new thread about that.