Qt Code: 
  1. class StopWatch : public QWidget
  2. {
  3.     Q_OBJECT
  4. public:
  5.     explicit StopWatch(QWidget * parent = 0)
  6.         : QWidget(parent)
  7.         , mRunning(false)
  8.         , mStartTime()
  9.         , mLabel(new QLabel("0:0:0:0"))
  10.     {
  11.         QHBoxLayout * hBoxLayout = new QHBoxLayout(this);
  12.         QPushButton * startButton = new QPushButton("Start");
  13.         QPushButton * stopButton = new QPushButton("Stop");
  14.  
  15.         hBoxLayout->addWidget(startButton);
  16.         hBoxLayout->addWidget(stopButton);
  17.         hBoxLayout->addWidget(mLabel);
  18.  
  19.         connect(startButton, SIGNAL(clicked()), SLOT(start()));
  20.         connect(stopButton, SIGNAL(clicked()), SLOT(stop()));
  21.  
  22.         startTimer(0);
  23.     }
  24.  
  25. public slots:
  26.     void start(void)
  27.     {
  28.         mStartTime = QDateTime::currentDateTime();
  29.         mRunning = true;
  30.     }
  31.  
  32.     void stop(void)
  33.     {
  34.         mRunning = false;
  35.     }
  36.  
  37. protected:
  38.     void timerEvent(QTimerEvent *)
  39.     {
  40.         if(mRunning)
  41.         {
  42.             qint64 ms = mStartTime.msecsTo(QDateTime::currentDateTime());
  43.             int h = ms / 1000 / 60 / 60;
  44.             int m = (ms / 1000 / 60) - (h * 60);
  45.             int s = (ms / 1000) - (m * 60);
  46.             ms = ms - (s * 1000);
  47.             QString diff = QString("%1:%2:%3:%4").arg(h).arg(m).arg(s).arg(ms);
  48.             mLabel->setText(diff);
  49.         }
  50.     }
  51.  
  52. private:
  53.     bool mRunning;
  54.     QDateTime mStartTime;
  55.     QLabel * mLabel;
  56. };
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