Hello Richard, you're right, the highlighting of today is currently timezone independent, hence always today in UTC is highlighted in the calendar.
Here the quick fix which you can take in the current module:
in modules/mod_seminarman_calendar/tmpl/default.php add the following codes from the line 71 (before the "for" loop)
$config = JFactory::getConfig();
$offset = $config->get('offset');
$dt = new DateTime("now", new DateTimeZone($offset));
$cdate = $dt->format('d.m.Y, H:i:s');
$current_date = strtotime($cdate); // current datetime in seconds in your timezone