problem using asp.net ajax

thanks every body for their reply

also change your button click event to the following:

    protected void Button1_Click(object sender, EventArgs e)
    {
        DateTime date = Convert.ToDateTime(tb_select_date.Text);
        CalendarExtender1.SelectedDate = date;      // add this line
    }

set ReadOnly=False for the textbox.

page load event code is here
if (!IsPostBack)
        {            
            CalendarExtender1.SelectedDate = DateTime.Now.Date;
            DateTime date = DateTime.Now.Date;
            tb_select_date.Text = date.Date.ToShortDateString();
            GenerateEmployeeList(date);
        } 

and button click code is this
protected void Button1_Click(object sender, EventArgs e)
    {
        DateTime date = Convert.ToDateTime(tb_select_date.Text);
        GenerateEmployeeList(date);
    }

html page
        
                            
                 
        
                 
         
                         
                                 
         

this html page code is enclose in a contentplaceholder master page

Are you setting the default date in the Page_Load() event?  If so, you need to check the IsPostBack flag before setting the date:

Page_Load()
{
   if (!IsPostBack)
   {
       DateControl.SelectedDate = DateTime.Today;
    }
}