Let's start with fixing an obvious memory leak. Here "time" is allocated on the heap but never deleted:
Quote Originally Posted by dragon View Post
Qt Code: 
  1. void countdownDialog::displayTime()
  2. {
  3.     QTime *time = new QTime();
  4.     time->setHMS(5,0,0,0);
  5.     lcdNumber->display(time->toString());
  6. }
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Just for the learning purposes, you should start with rewriting it as:
Qt Code: 
  1. void countdownDialog::displayTime()
  2. {
  3.     QTime time;
  4.     time.setHMS(5,0,0,0);
  5.     lcdNumber->display(time.toString());
  6. }
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Now, "time" get automatically destructed while going out of scope.

The next thing is to actually implement some counter functionality. All the above code does is converting the same time (05:00:00) over and over again to a string and setting it to the lcd number widget. You could make the time object a member variable instead and reduce a second every iteration:
Qt Code: 
  1. class countdownDialog
  2. {
  3.     ...
  4. private:
  5.     QTime time; // declare a member variable
  6. }
  7.  
  8. countdownDialog::countdownDialog(QWidget *parent)
  9.    :QDialog(parent)
  10. {
  11.     ...
  12.     // initialize it to anything you want, 
  13.     // for example 5 hours as you did before in displayTime()
  14.     time = QTime(5, 0, 0, 0);
  15. }
  16.  
  17. void countdownDialog::displayTime()
  18. {
  19.     time = time.addSecs(-1); // reduce a second
  20.     lcdNumber->display(time.toString());
  21. }
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