here u go. .this is called
private void buStartDate_Click(object sender, System.Web.UI.ImageClickEventArgs e)
{
try
{
System.DateTime.Parse(textboxFromDate.Text,new CultureInfo("en-GB"

);
}
catch
{
//Invalid date set todays date for now
Response.Redirect("Default.aspx"

;
//DateTime Date = DateTime.Today;
//textboxFromDate.Text = Date.Day.ToString() + "/" + Date.Month.ToString() + "/" + Date.Year.ToString();
}
this.Controls.Add(new LiteralControl("<script>"

);
this.Controls.Add(new LiteralControl("document.all[\"textboxFromDate\"].value = window.showModalDialog('Calendar.htm',document.all[\"textboxFromDate\"].value, 'dialogLeft:500px;dialogTop:500px;dialogHeight:210px;dialogWidth:265px;center:No;help:No;scroll:No;resizable:No;status:No;')"

);
this.Controls.Add(new LiteralControl("</script>"

);
}