C# v3微信 access token 过期处理的问题

         //记录access token 申请时的时间
         private static DateTime GetAccessToken_Time;  
         /// <summary>
         /// 过期时间为7200秒
         /// </summary>
         private static int Expires_Period = ;

 　　　　　　private static string mAccessToken;
         /// <summary>
         /// 获取access token
         /// </summary>
         /// <returns></returns>
         private static string GetAccessToken()
         {
             if (string.IsNullOrEmpty(mAccessToken) || HasExpired())
             {
                 string Url = "https://api.weixin.qq.com/cgi-bin/token";
                 string RequestPara = "grant_type=client_credential&appid=" + APPID + "&secret=" + APPSECRET;
                 string result = WebRequestHelper.GetData(RequestPara, Url);
                 if (!result.Equals(""))
                 {
                     GetAccessToken_Time = DateTime.Now;
                     accesstoken token = JsonFormat.Deserialize<accesstoken>(result);
                     Expires_Period = Convert.ToInt32(token.expires_in);
                     return token.access_token;
                 }
                 else
                 {
                     GetAccessToken_Time = DateTime.MinValue;
                 }
             }
             return null;
         }

         /// <summary>
         /// 判断Access_token是否过期
         /// </summary>
         /// <returns>bool</returns>
         private static bool HasExpired()
         {
             if (GetAccessToken_Time != null)
             {
                 //过期时间，允许有一定的误差，一分钟。获取时间消耗
                 if (DateTime.Now > GetAccessToken_Time.AddSeconds(Expires_Period).AddSeconds(-))
                 {
                     return true;
                 }
             }
             return false;
         }