if you try the following code and enter 1.1.12, you'd expect, according the to docs that it would default to 21th century... But it does to 20th. And that is a regression against Qt5.

#include <QDateEdit>
#include <QHBoxLayout>
#include <QApplication>

int main(int argc, char **argv) {
    QApplication app(argc, argv);

    QWidget w;
    QHBoxLayout layout(&w);
    QDateEdit edit(&w);
    edit.setDisplayFormat("dd.MM.yy");

    QObject::connect(&edit, &QDateEdit::dateChanged, [](const QDate &value) {
        qDebug() << "changed value" << value;
    });
    QDateTimeEdit edit2(&w);
    layout.addWidget(&edit);
    layout.addWidget(&edit2);

    w.show();

    return app.exec();
}