NTP: GetNetworkTime

gistfile1.cs

        public static DateTime GetNetworkTime()
        {
            //default Windows time server
            const string ntpServer = "time.windows.com";

            // NTP message size - 16 bytes of the digest (RFC 2030)
            var ntpData = new byte[48];

            //Setting the Leap Indicator, Version Number and Mode values
            ntpData[0] = 0x1B; //LI = 0 (no warning), VN = 3 (IPv4 only), Mode = 3 (Client Mode)

            var addresses = Dns.GetHostEntry(ntpServer).AddressList;

            //The UDP port number assigned to NTP is 123
            var ipEndPoint = new IPEndPoint(addresses[0], 123);
            //NTP uses UDP
            var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);

            socket.Connect(ipEndPoint);

            socket.Send(ntpData);
            socket.Receive(ntpData);
            socket.Close();

            //Offset to get to the "Transmit Timestamp" field (time at which the reply 
            //departed the server for the client, in 64-bit timestamp format."
            const byte serverReplyTime = 40;

            //Get the seconds part
            ulong intPart = BitConverter.ToUInt32(ntpData, serverReplyTime);

            //Get the seconds fraction
            ulong fractPart = BitConverter.ToUInt32(ntpData, serverReplyTime + 4);

            //Convert From big-endian to little-endian
            intPart = SwapEndianness(intPart);
            fractPart = SwapEndianness(fractPart);

            var milliseconds = (intPart * 1000) + ((fractPart * 1000) / 0x100000000L);

            //**UTC** time
            var networkDateTime = (new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc)).AddMilliseconds((long)milliseconds);

            return networkDateTime;
        }

        // stackoverflow.com/a/3294698/162671
        static uint SwapEndianness(ulong x)
        {
            return (uint)(((x & 0x000000ff) << 24) +
                           ((x & 0x0000ff00) << 8) +
                           ((x & 0x00ff0000) >> 8) +
                           ((x & 0xff000000) >> 24));
        }