Modified from http://support.ntp.org/bin/view/Support/JavaSntpClient Modified to allow user to enter the name of the ntp server (i used north-america.pool.ntp.org), send three requests and print average roundtrip times, and print the last packet's server request time.

gistfile1.java

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.text.DecimalFormat;
import java.util.Scanner;


/**
 * NtpClient - an NTP client for Java.  This program connects to an NTP server
 * and prints the response to the console.
 * 
 * The local clock offset calculation is implemented according to the SNTP
 * algorithm specified in RFC 2030.  
 * 
 * Note that on windows platforms, the curent time-of-day timestamp is limited
 * to an resolution of 10ms and adversely affects the accuracy of the results.
 * 
 * 
 * This code is copyright (c) Adam Buckley 2004
 *
 * This program is free software; you can redistribute it and/or modify it 
 * under the terms of the GNU General Public License as published by the Free 
 * Software Foundation; either version 2 of the License, or (at your option) 
 * any later version.  A HTML version of the GNU General Public License can be
 * seen at http://www.gnu.org/licenses/gpl.html
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT 
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 
 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for 
 * more details.
 *  
 * @author Adam Buckley
 */
public class SntpClient {
    
	// Could make a static field in NtpMessage.java, but this way we edit one file
	private static double mAverageRoundTrip = 0.0;
	private static String thirdRequest = "";
	
	public static void main(String[] args) throws IOException {
		String serverName;
		
//		// Process command-line args
//		if(args.length == 1)
//			serverName = args[0];
//		else {
//			printUsage();
//			return;
//		}
		
		// Allow the user to enter the name of the NTP Server
		serverName = enterNtpServer();
		
		//Send three requests to the server
		for (int i = 0; i < 3; i++) {
			
			// Send request
			DatagramSocket socket = new DatagramSocket();
			InetAddress address = InetAddress.getByName(serverName);
			byte[] buf = new NtpMessage().toByteArray();
			DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 123);
			
			// Set the transmit timestamp *just* before sending the packet
			// ToDo: Does this actually improve performance or not?
			NtpMessage.encodeTimestamp(packet.getData(), 40,
					(System.currentTimeMillis()/1000.0) + 2208988800.0);
			
			socket.send(packet);
			
			
			// Get response
			System.out.println("------------------------------------------------");
			System.out.println("NTP request sent, waiting for response...\n");
			
			packet = new DatagramPacket(buf, buf.length);
			socket.receive(packet);
			
			// Immediately record the incoming timestamp
			double destinationTimestamp =
					(System.currentTimeMillis()/1000.0) + 2208988800.0;
			
			
			// Process response
			NtpMessage msg = new NtpMessage(packet.getData());
			
			// Corrected, according to RFC2030 errata
			double roundTripDelay = (destinationTimestamp-msg.originateTimestamp) -
					(msg.transmitTimestamp-msg.receiveTimestamp);
			
			// Add to accumulating total
			mAverageRoundTrip += roundTripDelay;
			
			double localClockOffset =
					((msg.receiveTimestamp - msg.originateTimestamp) +
					(msg.transmitTimestamp - destinationTimestamp)) / 2;
			
			
			// Display response
			System.out.println("NTP server: " + serverName);
			System.out.println(msg.toString());
			System.out.println("Dest. timestamp:     " +
					NtpMessage.timestampToString(destinationTimestamp));
			System.out.println("Round-trip delay: " +
					new DecimalFormat("0.00").format(roundTripDelay*1000) + " ms");
			System.out.println("Local clock offset: " +
					new DecimalFormat("0.00").format(localClockOffset*1000) + " ms\n\n");
			
			// If last time
			if ( i == 2) {
				thirdRequest = NtpMessage.timestampToString(msg.transmitTimestamp);
			}
			
			socket.close();
		} // End loop
		
		mAverageRoundTrip /= 3; // Take average
		System.out.println("------------------------------------------------");
		System.out.println("Average round trip: " + new DecimalFormat("0.00").format(mAverageRoundTrip * 1000));
		System.out.print("Server received the 3rd request at: " + thirdRequest);
	}
	
	/**
	 * Prints usage
	 */
	static void printUsage() {
		System.out.println(
			"NtpClient - an NTP client for Java.\n" +
			"\n" +
			"This program connects to an NTP server and prints the response to the console.\n" +
			"\n" +
			"\n" +
			"Usage: java NtpClient server\n" +
			"\n" +
			"\n" +
			"This program is copyright (c) Adam Buckley 2004 and distributed under the terms\n" +
			"of the GNU General Public License.  This program is distributed in the hope\n" +
			"that it will be useful, but WITHOUT ANY WARRANTY; without even the implied\n" +
			"warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU\n" +
			"General Public License available at http://www.gnu.org/licenses/gpl.html for\n" +
			"more details.");
	}
	
	private static String enterNtpServer() {
		Scanner reader = new Scanner(System.in);
		System.out.print("Enter name of NTP Server: ");
		return reader.nextLine();
	}
	
}