QDate::currentDate().daysTo( todo->dtDue().date() ) returns negative number if the due date is invalid, thus the summary displays 2,451,561 days due: <a href="/issues/2008-04-13/files/todonoduedateed3.png">http://img515.imageshack.us/my.php?image=todonoduedateed3.png</a>;

After the fix (todosummarywidget.patch), we do not display invalid dates but we still show the todo items:
<a href="/issues/2008-04-13/files/todonoduedatefixpp3.png">http://img209.imageshack.us/my.php?image=todonoduedatefixpp3.png</a>;

(initially reported as <a href="https://www.intevation.de/roundup/kolab/issue2602">https://www.intevation.de/roundup/kolab/issue2602</a>;)