Web Forms :: Error:Selecting 2 Dates From A Calender

I wanted to select 2 dates from a Calendar and display the selected dates and their difference between the two selected dates in labels. I put a calender in .aspx file and in coding:

public static int counter;
protected void Page_Load(object sender, EventArgs e)
{
}
protected void Calendar1_SelectionChanged(object sender, EventArgs e)
{
myLabel.Text = "";
//Label1.Text = "You selected: <br />";
//Now, we assign the variable value in the text box
foreach (DateTime dt in Calendar1.SelectedDates)
{
counter++;
Calendar1.SelectedDates.Add(dt);
Label1.Text += dt.ToShortDateString() + "<br />";
myLabel.Text = counter.ToString() + "days";
}
}

The problem is i cant not stop selecting dates after selecting 2 randomly dates.