gistfile1.cpp

// widget.h:
#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QScopedPointer>
#include <QDate>

namespace Ui { class Widget; }

class Widget: public QWidget
{
	Q_OBJECT

private:
	QScopedPointer<Ui::Widget> ui;
private slots:
	void compute(QDate = QDate());
public:
	Widget();
	virtual ~Widget();
};

#endif  //WIDGET_H

// widget.cpp:
#include "widget.h"
#include "ui_widget.h"

Widget::Widget()
  :ui(new Ui::Widget)
{
	ui->setupUi(this);
	compute();
}

Widget::~Widget() {}

void Widget::compute(QDate)
{
	auto days = qAbs(ui->date1->date().daysTo(ui->date2->date()));
	ui->lResult->setText(
	      QString("There is %1 day(s) between %2 and %3").arg(days)
	      .arg(ui->date1->date().toString(), ui->date2->date().toString()));
}

// main.cpp:
#include "widget.h"
#include <QApplication>

int main(int argc, char *argv[])
{
	QApplication a(argc, argv);
	Widget w;
	w.show();
	return a.exec();
}