We have ClearOS 7 Business. and are using the version of Roundcube purchased from the marketplace.
It was working correctly except that the Date field was blank. This was both in the list of emails (the upper pane, in default configuration) and in the individual email (the lower pane).

A quick web search turned up that the cause for this is needing to manually edit the php.ini file to add a timezone:

https://help.directadmin.com/item.php?id=476

The first time I searched for php.ini on our system, I found:
/usr/clearos/sandbox/etc/php.ini
I duly edited that file, adding a timezone, and restarted the system (not knowing exactly which service I would need to restart to make the change take effect).
Frustratingly, the Date fields were still blank.
My next thought, which turned out to be correct, is that there's more than one copy of php.ini on the system, and the one I edited was the wrong one.
It turns out that there's also /etc/php.ini
Editing that version fixed the problem.