Based on comment at 2020-07-12 17:14 (don't know time zone, the chat doesn't show it) from JimNva and answer from ritchie58 I searched a bit more and found that, on my two systems, Immunet may be looking on the wrong file or logs to determine the date of last update:
What i think is happening is that ClamAV uses a set of files to the virus database, extension cvd and cld (there may be others). I see bytecode.cvd, main.cvd, windows.cvd and daily.cld. While bytecode.cvd and main.cvd are older, about 4 days, the windows.cvd and daily.cld is newer, from today. Immunet is reporting the modification date from the older files that (I think) are updated less often, instead of the date of one of the two newer.
Also a better place to search for is the file lastupd, modified today and that has the date of the database version on its contents, yesterday: 2020 Jul 20 11:31:24 I beleave (hope) that this is UTC.
If this is it, it will be a lot easier to the devs than it looked!