I’m writing an application in C# that needs to know if the system time is not fake (only if the network is on). For this reason, I make a class that retrieves the time from a NTP server. The class is written to prevent that the same servers are called too often using a simple circular array. For the data returned by the servers, I invite you to see the RFC-2030.

using System;
using System.IO;
using System.Runtime.Serialization;
using System.Net;
using System.Net.Sockets;


public class NoServerFoundException : System.Exception {
  public NoServerFoundException() : base() { }
  public NoServerFoundException(string message) : base(message) { }
  public NoServerFoundException(string message,
      System.Exception inner) : base(message, inner) { }
  protected NoServerFoundException(SerializationInfo info,
    System.Runtime.Serialization.StreamingContext context) { }
}

class NetworkTime {
  /* For more info, see:
   *  NTP (RFC-2030)
   *  http://tools.ietf.org/html/rfc2030
   */

  private const int requestTimeout = 3000;
  private const int timesForEachServer = 5;
  private const byte offTime = 40; //Transmit Time (see RFC-2030)
  private uint lastSrv;

  //NIST Servers
  public static string[] srvs = {
    "time.nist.gov",
    "pool.ntp.org",
    "europe.pool.ntp.org",
    "asia.pool.ntp.org",
    "oceania.pool.ntp.org",
    "north-america.pool.ntp.org",
    "south-america.pool.ntp.org",
    "africa.pool.ntp.org",
    "ntp1.inrim.it",
    "ntp2.inrim.it"
  };

  public NetworkTime() {
    Random rnd = new Random(DateTime.Now.Millisecond);
    lastSrv = (uint)rnd.Next(0, srvs.Length);
  }

  private IPAddress getServer() {
    lastSrv = (uint)((lastSrv + 1) % srvs.Length);
    IPAddress[] address = Dns.GetHostEntry(srvs[lastSrv]).AddressList;
    if (address == null || address.Length == 0)
      throw new NoServerFoundException("no ip found");
    return address[0];
  }

  public DateTime GetDateTime() { return GetDateTime(false); }
  public DateTime GetDateTime(bool utc) {
    //Examine all servers until we find a server that responds
    for (int st = 0; st < srvs.Length * timesForEachServer; st++) {
      try {
        IPAddress ip = getServer();
        IPEndPoint ipEndP = new IPEndPoint(ip, 123);

        Socket sk = new Socket(AddressFamily.InterNetwork,
                    SocketType.Dgram,
                    ProtocolType.Udp);
        sk.ReceiveTimeout = requestTimeout;

        sk.Connect(ipEndP);

        /* Request
         * VN: 4 = NTP/SNTP version 4
         * Mode: 3 = client
         */
        byte[] data = new byte[48];
        data[0] = 0x23;
        for (int i = 1; i < 48; i++) data[i] = 0;
        sk.Send(data);

        /* Response
         * we read the integer part and fraction part
         * of transmit time (see RFC-2030)
         */
        sk.Receive(data);
        byte[] integerPart = new byte[4];
        integerPart[0] = data[offTime + 3];
        integerPart[1] = data[offTime + 2];
        integerPart[2] = data[offTime + 1];
        integerPart[3] = data[offTime + 0];
        byte[] fractPart = new byte[4];
        fractPart[0] = data[offTime + 7];
        fractPart[1] = data[offTime + 6];
        fractPart[2] = data[offTime + 5];
        fractPart[3] = data[offTime + 4];
        long ms = (long)(
              (ulong)BitConverter.ToUInt32(integerPart, 0) * 1000
             + ((ulong)BitConverter.ToUInt32(fractPart, 0) * 1000)
              / 0x100000000L);
        sk.Close();

        /* DateTime*/
        DateTime date = new DateTime(1900, 1, 1);
        date += TimeSpan.FromTicks(ms * TimeSpan.TicksPerMillisecond);

        return utc ? date : date.ToLocalTime();

      } catch (Exception ex) { }
    }

    throw new NoServerFoundException("no working server has been found");
  }
}