[REG 5->6] QDateEdit defaults to 20th century

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();
}