Сравнение времени по UTC

using System;
 
internal sealed class Program {
  static void Main() {
    TimeSpan ts = DateTime.Now - DateTime.UtcNow;
    Console.WriteLine(
      "Days   : {0}\nHours  : {1}\nMinutes: {2}\nSeconds: {3}",
      ts.Days, ts.Hours, ts.Minutes, ts.Seconds
    );
  }
}

Process p = new Process();
p.StartInfo.Arguments = "/resync";
p.StartInfo.CreateNoWindow = true;
p.StartInfo.FileName = "w32tm.exe";
p.StartInfo.UseShellExecute = false;
p.Start();
p.WaitForExit();

using System.Net;
using System.Net.Sockets;
// ...
public static DateTime GetNetworkTimeUtc(string ntpServer = "time.nist.gov")
{
    // 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;
 
    // NTP работает через UDP и использует порт 123
    var ipEndPoint = new IPEndPoint(addresses[0], 123);
    using (var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp))
    {
        socket.Connect(ipEndPoint);
 
        //Stops code hang if NTP is blocked
        socket.ReceiveTimeout = 3000;
 
        socket.Send(ntpData);
        socket.Receive(ntpData);
    }
 
    //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);
    return (new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc)).AddMilliseconds((long)milliseconds);
}
 
// Convert From big-endian to little-endian
static uint SwapEndianness(ulong x)
{
    return (uint)(((x & 0x000000ff) << 24) +
                   ((x & 0x0000ff00) << 8) +
                   ((x & 0x00ff0000) >> 8) +
                   ((x & 0xff000000) >> 24));
}