error:Selecting 2 dates from a calender

Hi
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)
        {
            ...