DateTime dt = DateTime.Now;
switch (dropSaveDate.SelectedValue)
{
case "2小时":
dt = DateTime.Now.AddMinutes(120);//二小时过期
                        break;
case"一天":
dt = DateTime.Now.AddDays(1);
break;
case"一周":
dt = DateTime.Now.AddDays(7);
break;
case"一月":
dt = DateTime.Now.AddMonths(1);
break;
default:
break;
}
// 创建身份验证票证
                FormsAuthenticationTicket authTicket = new FormsAuthenticationTicket
(
1, // version
                                                            gUserId, // user name
                                                            DateTime.Now, // 开始时间
                                                            dt,// 过期时间为一
//天:DateTime.Now.AddDays(1)
                                                            true, // 持久性，若为True，需设置
//Cookies的Expires属性,若为False，则关闭浏览器就会失效
                                                           ""); // User data

string encryptedTicket = FormsAuthentication.Encrypt(authTicket);
// 创建Cookie
                HttpCookie authCookie = new HttpCookie(FormsAuthentication.FormsCookieName, encryptedTicket);
authCookie.Expires = authTicket.Expiration;
Response.Cookies.Add(authCookie);