Keep me signed in until Loggged out

protected void Page_Load(object sender, EventArgs e)
{
    HttpCookie CookieName = Request.Cookies["username"];

    if (!IsPostBack)
    {
        if (CookieName != null)
        {
            LoginDiv.Visible = false;
            Label3.Visible = true;
            Label3.Text = CookieName.Value;
            Button2.Visible = true;

        }
        else
        {
            LoginDiv.Visible = true;
            Label3.Visible = false;
            Button2.Visible = false;

        }
    }
}
protected void Button1_Click(object sender, EventArgs e)
{
    //login
    if (CheckBox1.Checked ==true)
    {
        HttpCookie CookieName = new HttpCookie("username");
        CookieName.Value = TextBox1.Text;
        CookieName.Expires = DateTime.Now.AddDays(90);
        Response.Cookies.Add(CookieName);
        LoginDiv.Visible = false;
        Label3.Visible = true;
        Button2.Visible = true;
        Label3.Text = CookieName.Value;
    }
}
//log out
protected void Button2_Click(object sender, EventArgs e)
{
    HttpCookie CookieName = Request.Cookies["username"];
    CookieName.Expires.AddMilliseconds(1);
    CookieName.Value = null;
    Response.Cookies.Add(CookieName);
    LoginDiv.Visible = true;
    Label3.Visible = false;
    Button2.Visible = false;
}

<div id="LoginDiv" runat="server">
       <asp:Label ID="Label1" runat="server" Text="user name"></asp:Label>
       <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
       <br />
       <asp:Label ID="Label2" runat="server" Text="Password"></asp:Label>
       <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
       <asp:CheckBox ID="CheckBox1" runat="server" Text="Remember me" />
       <br />
       <asp:Button ID="Button1" runat="server" Text="Login" onclick="Button1_Click" />

   </div>
   <div>
   <asp:Label ID="Label3" runat="server" Text="Label" Visible="False"></asp:Label>

   <asp:Button ID="Button2" runat="server" onclick="Button2_Click"
       Text="log out" />
           </div>