title: How to start a timer at a specific time for a specific time?
body: <p>Here is the scenario:</p>
  
  <p>There is a <code>Textbox</code> in which a user enters the time at which the <code>DispatcherTimer</code> must start. The Timer is set to run for 4 hours always and Timer.Tick event plays a sound. Let's say the timer starts at 04:00:00 (HH:mm:ss), it should run till 8:00:00 and then stop. Then when the user changes the time, let's say, to 09:00:00. The timer should run till 13:00:00 and so on.</p>
  
  <p>How can this be achieved? I can't run a 1 minute timer that keeps checking for the right time because that will be very heavy on the system.</p>
  
  <p>All help is appreciated. Thank you in advance.</p>