It's a Qt bug. It reproduces in C++ under both Qt 5.2.0 and Qt 4.8.5. Please file a bug report.
#include <QApplication>
#include <QDateEdit>
#include <QDate>
#include <QHBoxLayout>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QWidget w;
QHBoxLayout * layout = new QHBoxLayout(&w);
QDateEdit edit;
edit.setDate(QDate::currentDate());
edit.setCalendarPopup(true);
edit.setDisplayFormat("MM/dd/yyyy");
layout->addWidget(&edit);
w.show();
return a.exec();
}