import java.net.*;
import java.util.*;

public class UDPTimeClient {
  
  public final static int PORT = 37;
  public final static String DEFAULT_HOST = "time.nist.gov";
  
  public static void main(String[] args) {
    
    InetAddress host;
    try {
      if (args.length > 0) {
        host = InetAddress.getByName(args[0]);
      } else {
        host = InetAddress.getByName(DEFAULT_HOST);
      } 
    } catch (RuntimeException | UnknownHostException ex) {
      System.out.println("Usage: java UDPTimeClient [host]");
      return;
    }

    UDPPoke poker = new UDPPoke(host, PORT);
    byte[] response = poker.poke();
    if (response == null) {
      System.out.println("No response within allotted time");
      return;
    } else if (response.length != 4) {
      System.out.println("Unrecognized response format");
      return;         
    }
    
    // The time protocol sets the epoch at 1900,
    // the Java Date class at 1970. This number 
    // converts between them.
  
    long differenceBetweenEpochs = 2208988800L;

    long secondsSince1900 = 0;
    for (int i = 0; i < 4; i++) {
      secondsSince1900 
          = (secondsSince1900 << 8) | (response[i] & 0x000000FF);
    }

    long secondsSince1970 
        = secondsSince1900 - differenceBetweenEpochs;       
    long msSince1970 = secondsSince1970 * 1000;
    Date time = new Date(msSince1970);
    
    System.out.println(time);
  } 
}