How to get timezone datetime correct if PC local time is wrong

Question

How can I get datetime with timezone info (GMT/UTC +7)?

Let's say in real time it was 06:00 PM and then someone alters the local PC time to 01:00 PM. How do I get datetime 06:00 PM?

I have tried this:

System.Globalization.CultureInfo.CurrentCulture.ClearCachedData();
DateTime utcTime = DateTime.UtcNow;
TimeZoneInfo tzi = TimeZoneInfo.FindSystemTimeZoneById("SE Asia Standard Time");
DateTime localTime = TimeZoneInfo.ConvertTimeFromUtc(utcTime, tzi);
Console.WriteLine(localTime);

But I still got 01:00 PM.

Answer 1

Just contact NTP server and take it's time. Code found on stackoverflow. You need internet connection to do that of course.

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

    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);
        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.ToLocalTime(); }

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

Answer 2

If you don't want to trust the information on the user's PC at all, then you will need:

1. An alternate source of time zone data
2. A network time source, such as via NTP

You can achieve the first with Noda Time, and the second with my NodaTime.NetworkClock add-on. Internally, it uses similar NTP code as that given by Martin's answer.

public static DateTime GetRealTimeInZone(string timeZoneId)
{
    var clock = NetworkClock.Instance;
    var now = clock.GetCurrentInstant();
    var tz = DateTimeZoneProviders.Tzdb[timeZoneId];
    return now.InZone(tz).ToDateTimeUnspecified();
}

Usage:

DateTime dt = GetRealTimeInZone("Asia/Bangkok");

Or, if you do trust the local time zone setting, just not the clock, then:

public static DateTime GetRealTimeInZone()
{
    var clock = NetworkClock.Instance;
    var now = clock.GetCurrentInstant();
    var tz = DateTimeZoneProviders.Tzdb.GetSystemDefault();
    return now.InZone(tz).ToDateTimeUnspecified();
}

Usage:

DateTime dt = GetRealTimeInZone();