C#

System.Drawing.Pen myPen = new System.Drawing.Pen(System.Drawing.Color.Red, 5);
System.Drawing.Pen whitePen = new System.Drawing.Pen(System.Drawing.Color.White, 5); // define the erasing pen
g.DrawRectangle(whitePen, myRec); // erase the old rectangle
myRec.Width = (int)width;  // adjunst the new rectangle coordinates
myRec.Height = (int)height;
g.DrawRectangle(myPen, myRec); // draw the new rectangle
InvalidateVisual();

Graphics g = Graphics.FromHwnd(new System.Windows.Interop.WindowInteropHelper(this).Handle);

private System.Windows.Shapes.Rectangle GetRectangle(double left, double top)
{
  System.Windows.Shapes.Rectangle myRect = new System.Windows.Shapes.Rectangle();
  myRect.Stroke = System.Windows.Media.Brushes.Black;
  myRect.Fill = System.Windows.Media.Brushes.SkyBlue;  
  myRect.Height = 0;
  myRect.Width = 0;
  // Add the rectangle to the canvas.
  canvas.Children.Add(myRect);
  canvas.SetLeft(myRect, left);
  canvas.SetTop(myRect, top);
  return myRect;
}

private System.Windows.Shapes.Rectangle myRec;
private void Window_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
  flg = true;
  firstPoint = e.GetPosition(mainwin);
  myRec = GetRectangle(firstPoint.X, firstPoint.y);
}

private void mainwin_MouseMove(object sender, MouseEventArgs e)
{
  if (!flg) return;
  System.Windows.Point currentPoint = e.GetPosition(mainwin);
  double width = currentPoint.X - firstPoint.X;
  double height = currentPoint.Y - firstPoint.Y;

  if (width <= 0 || height <= 0) return;

  myRec.Width = width;
  myRec.Height = height;
}

class bleh
{
  List<DateTime> times;
  DateTime next;
  bool quit = false;

  void main()
  {
    while(!quit)
    {
      bool resting = false;
      //Search through your list and find the next time that will be reached
      next = ...;

      do
      {
        resting = false;
        TimeSpan diff = next - DateTime.Now;

        //If the time difference between now and the next event is greater
        // than some tolerable value, sleep for a while
        if(diff > someVal)
        {
          resting = true;
          sleep;
        }
      }
      while(resting);

      while( true )
      {
        if(DateTime.Now >= next)
        {
          fire event;
          break;
        }
      }
    }
  }
}