How to query query an NTP Time Server in C#.

using System;
using System.Diagnostics;
using System.Net;
using System.Net.Sockets;
using System.Threading.Tasks;

namespace CheckTimeServer.Console
{
    public static class Program
    {
        public static async Task Main(string[] args)
        {
            //
            // Example usage:
            //

            var networkTime = await GetNetworkTime();
            if (networkTime.HasValue)
            {
                var localZone = TimeZoneInfo.Local;

                System.Console.WriteLine("Network time: {0} ({1})", networkTime.Value,
                    localZone.IsDaylightSavingTime(networkTime.Value)
                        ? localZone.DaylightName
                        : localZone.StandardName);
            }
            else
            {
                System.Console.WriteLine("Could not retrieve network time.");
            }

            System.Console.ReadKey();
        }

        private static async Task<DateTime?> GetNetworkTime(string ntpServer = "pool.ntp.org")
        {
            // https://stackoverflow.com/questions/1193955/how-to-query-an-ntp-server-using-c

            if (ntpServer == null)
            {
                throw new ArgumentNullException(nameof(ntpServer));
            }

            try
            {
                const int daysTo1900 = 1900 * 365 + 95; // 95 = offset for leap-years etc.
                const long ticksPerSecond = 10000000L;
                const long ticksPerDay = 24 * 60 * 60 * ticksPerSecond;
                const long ticksTo1900 = daysTo1900 * ticksPerDay;

                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[0], 123);
                // ReSharper disable once RedundantAssignment

                var pingDuration = Stopwatch.GetTimestamp(); // temp access (JIT-Compiler need some time at first call)

                using (var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp))
                {
                    await socket.ConnectAsync(ipEndPoint);
                    socket.ReceiveTimeout = 5000;
                    socket.Send(ntpData);
                    pingDuration = Stopwatch.GetTimestamp(); // after Send-Method to reduce WinSocket API-Call time

                    socket.Receive(ntpData);
                    pingDuration = Stopwatch.GetTimestamp() - pingDuration;
                }

                var pingTicks = pingDuration * ticksPerSecond / Stopwatch.Frequency;

                // optional: display response-time
                // Console.WriteLine("{0:N2} ms", new TimeSpan(pingTicks).TotalMilliseconds);

                var intPart = (long) ntpData[40] << 24 | (long) ntpData[41] << 16 | (long) ntpData[42] << 8 | ntpData[43];
                var fractPart = (long) ntpData[44] << 24 | (long) ntpData[45] << 16 | (long) ntpData[46] << 8 | ntpData[47];
                var netTicks = intPart * ticksPerSecond + (fractPart * ticksPerSecond >> 32);

                var networkDateTime = new DateTime(ticksTo1900 + netTicks + pingTicks / 2);

                return networkDateTime.ToLocalTime(); // without ToLocalTime() = faster
            }
            catch
            {
                // fail
                return null;
            }
        }
    }
}