Fortunately, this turns out to be simple. The hardest part is finding
main.inc.php for your particular Roundcube installation. If you
have used Virtualmin's script installers,
it will probably be /home/yourdomain/public_html/roundcube/config/main.inc.php
... your mileage may vary.
Only one line has to e changed, from :
$rcmail_config['date_long'] = 'd.m.Y H:i';
$rcmail_config['date_long'] = 'Y-m-d H:i';
I have not yet found a way to change from 24-hour to 12-hour am/pm times, though.