FormsAuthentication如何添加过期时间

protected void btnLogin_Click(object sender, EventArgs e)
    {
        string userName = "UserName";
        string password = "Password";
        if (txtUserName.Text == userName && txtPassword.Text == password)
        {
            setValidTime(chkRememberMe.Checked, userName);
            Response.Write("<script>top.location='Default.htm';</script>");
        }
        else
        {
            labMsg.Text = "登录失败，请重试";
        }
    }

    /**//**//**//// <summary>
    /// 设置有效时间
    /// </summary>
    protected void setValidTime(bool RememberMe, string strUserName)
    {
        if (RememberMe)
        {
            FormsAuthentication.SetAuthCookie(strUserName, true);
            HttpCookie lcookie = Context.Response.Cookies[FormsAuthentication.FormsCookieName];
            lcookie.Expires = DateTime.Now.AddYears(1);
        }
        else
        {
            FormsAuthentication.SetAuthCookie(strUserName, false);
        }
    }
注销

    protected void btnSignOut_Click(object sender, EventArgs e)
    {
        FormsAuthentication.SignOut();
    }