asp:LoginStatus automatically log out after some time

hi

I have asp:LoginStatus on my master page

and in my login.aspx.cs file I have logiIn button, which have the selected event handler

protected void btLogIn_Click(object sender, EventArgs e)
        {
            int chk = _dbobj.CheckingIfUserExists(tbUserName_login.Text, tbPassword_login.Text);
            if (chk != 0)
            {
                Response.Cookies["UserName"].Value = tbUserName_login.Text.Trim();
                lbUserNameValidation_logIn.Text = "";
                Response.Redirect("Home.aspx", false);

                if (chbRemember_login.Checked)
                {
                    Response.Cookies["UserName"].Expires = DateTime.Now.AddDays(1);
                    System.Web.Security.FormsAuthentication.RedirectFromLoginPage(tbUserName_login.Text, true);
                }
                else
                {
                    Response.Cookies["UserName"].Expires = DateTime.Now.AddDays(-1);
                    System.Web.Security.FormsAuthentication.RedirectFromLoginPage(tbUserName_login.Text, false);
                }

            }
            else
               ............

When I log in without checking remember box, it remembers anyway, and doesn't set logged out after closing and opening the page.

What to do. Please help