[knot-dns-users] "journal, obsolete exists, file" ...

Hi, I am running Knot 2.5.2-1 on a Debian Jessie, all is good, no worries. I am very pleased with Knot's simplicity and ease of configuration - which are still readable as well! I noticed recently that I am getting knotd[9957]: notice: [$DOMAIN.] journal, obsolete exists, file '/var/lib/knot/zones/$DOMAIN.db' everytime I restart Knot. I get these for all my domains I have confgured, and there is one in particular providing my own .dyn. service :-) - so I am a bit reluctant - just to delete it. But all the .db files have a fairly old timestamp (Feb 2017) and about the same. At that time (Feb 2017) I was running just one authoritative Master instance, nothing fancy. lsof also doesn't report any open files. At that time (Feb 2017) I was running just one authoritative Master instance, nothing else. Can I just delete those files? Cheers Thomas