NTPClient.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Sockets;
using System.Text;

namespace NTPClient
{
    class NTPClient
    {
        public static DateTime GetNetworkTime(string ntpServer = "a.ntp.br", int receiveTimeout = 3000)
        {
            var ntpData = new byte[48];
            ntpData[0] = 0x1B; //LeapIndicator = 0 (no warning), VersionNum = 3 (IPv4 only), Mode = 3 (Client Mode)

            var addresses = Dns.GetHostEntry(ntpServer).AddressList;
            var ipEndPoint = new IPEndPoint(addresses.First(ip => ip.AddressFamily == AddressFamily.InterNetwork), 123);
            using (var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp))
            {
                socket.Connect(ipEndPoint);
                socket.ReceiveTimeout = receiveTimeout;
                socket.Send(ntpData);
                socket.Receive(ntpData);
                socket.Close();
            }

            ulong intPart = (ulong)ntpData[40] << 24 | (ulong)ntpData[41] << 16 | (ulong)ntpData[42] << 8 | (ulong)ntpData[43];
            ulong fractPart = (ulong)ntpData[44] << 24 | (ulong)ntpData[45] << 16 | (ulong)ntpData[46] << 8 | (ulong)ntpData[47];

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

            var networkDateTime = (new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc)).AddMilliseconds((long)milliseconds);
            return networkDateTime.ToLocalTime();

        }
    }
}