connect(ui->QCalenderWidgetAfleverdatum, SIGNAL(selectionChanged()),mapper,SLOT(submit()));
but the it is submitting NULL to the database

Mapper->addMapping(ui->QCalenderWidgetAfleverdatum ,orderGegevens->record().indexOf("leverdatum"), "selectedDate");
i'm getting : QODBCResult::exec: unable to bind variable: "[Microsoft][ODBC SQL Server Driver]De optionele voorziening is niet geïmplementeerd"
I don't think it is a problem with QCalendarWidget/QDataWidgetMapper, it may be somthing to do with ODBC driver, check if removing the QCalendarWidget is submitting the other data properly.