日本標準時刻を取得する ref: http://qiita.com/yu_ka1984/items/42bd268a6adbe74e5459

file0.cs

    public class TimeService
    {
        /// <summary>
        //  現在時刻を取得します。
        /// </summary>
        /// <returns></returns>
        public async Task<DateTime> GetNow()
        {
            try
            {
                HttpClient client = new HttpClient();
                client.Timeout = TimeSpan.FromSeconds(3);
                var result = await client.GetAsync("https://ntp-a1.nict.go.jp/cgi-bin/json");
                if (result.IsSuccessStatusCode)
                {
                    var time = JsonConvert.DeserializeObject<NitcTime>(await result.Content.ReadAsStringAsync());

                    var now =
                        new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc).AddSeconds(Convert.ToDouble(time.st))
                            .ToLocalTime();

                    return now;
                }
            }
            catch
            {                
            }
            return DateTime.Now;
        }

        public class NitcTime
        {
            public string id { get; set; }
            public decimal it { get; set; }
            public decimal st { get; set; }
            public int leap { get; set; }
            public decimal next { get; set; }
            public int step { get; set; }
        }
    }