textbox is not reading the new selection of calendar

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;

namespace WebApplication1
{
    public partial class WebForm1 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                DateTime today = DateTime.Today;
                Calendar1.TodaysDate = today;
                Calendar1.SelectedDate = Calendar1.TodaysDate;
                TextBox1.Text = Calendar1.SelectedDate.Date.ToString();
                Calendar1.Visible = false;
            }
        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            Calendar1.Visible = true;
        }

        protected void Calendar1_SelectionChanged(object sender, EventArgs e)
        {
            if (Calendar1.Visible == true)
            {
                TextBox1.Text = Calendar1.SelectedDate.Date.ToString();
                Calendar1.Visible = false;
            }
            else
            {
                Calendar1.Visible = true;
            }
        }
    }
}