Here You Can Use Update Panel, Button and Lable....

Timer1.Enabled = True

If lbl1.text >=0 Then
     lbl1.text = CInt(lbl1.text) - 1
Else
     Timer1.Enabled = False
End If

protected void Lable_Click(object sender, EventArgs e)
    {

                   (Label)sender).Text=  (Convert.ToInt32(((Label)sender).Text)==0?0:Convert.ToInt32(((Label)sender).Text)-1).toString();
}

protected void btn_click(object sender, EventArgs e)
{
  int num = 0;
  int.TryParse(lbl.text,out num)
  if(num > 0)
  {
    lbl.text = num -1;
  }

}