understanding NTP in java

[Mandar Khire]

Hi,
By reading how to make my java app get global time from some online clock?
I write code which fetch Date & Time from ntp server.
code is as follows:-

import java.io.IOException;
import java.net.InetAddress;
import java.text.SimpleDateFormat;

import org.apache.commons.net.ntp.NTPUDPClient;
import org.apache.commons.net.ntp.TimeInfo;
public class NtpTesting {

	public static void main(String[] args) {
		try {
			String time_SERVER = "time-a.nist.gov";
			String ntpDatetime = null;
			InetAddress inetAddress = null;
			TimeInfo timeInfo = null;
			NTPUDPClient timeClient = new NTPUDPClient();
			inetAddress = InetAddress.getByName(time_SERVER);
			if(timeClient != null){
				if(inetAddress != null){            			 
					timeInfo = timeClient.getTime(inetAddress);
					if(timeInfo != null){
						long returnTime = timeInfo.getReturnTime();
						ntpDatetime = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss:SSS").format(new java.util.Date(returnTime));
						String systemDatetime = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss:SSS").format(new java.util.Date());
						System.out.println("Time from NTP  = "+ ntpDatetime);
						System.out.println("systemDatetime = "+systemDatetime);
					}
				}
			}
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
}

When i test it with 2 different conditions in computer which has Centos.
1. Service ntpd working.
2. Service ntpd stopped. change system date 1 year before (2014).

Both test i got same result.
Is there any way to get exception or error when ntpd manually stopped & this program run?

When i stop internet & run this program, at 40 sec i got expected exception as below:-

java.net.UnknownHostException: time-a.nist.gov: unknown error
	at java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
	at java.net.InetAddress$2.lookupAllHostAddr(InetAddress.java:907)
	at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1302)
	at java.net.InetAddress.getAllByName0(InetAddress.java:1255)
	at java.net.InetAddress.getAllByName(InetAddress.java:1171)
	at java.net.InetAddress.getAllByName(InetAddress.java:1105)
	at java.net.InetAddress.getByName(InetAddress.java:1055)
	at uuid_example.NtpTesting.main(NtpTesting.java:21)

[Stephan van Hulst]

Why would your application have to throw an exception when ntpd is not running?

[Mandar Khire]

Why would your application have to throw an exception when ntpd is not running?

As i wrote 'When i stop internet & run this program, at 40 sec i got expected exception'.
For testing i connect my pc to small basic network switch & in that attached internet Ethernet cable. When i remove internet cable. My pc cannot connect to Google etc website.
but 'Network connection' show me no problem.
So after that i run this program, & got exception.

My question is Internet working fine but i stopped ntp service from service manager or from system datetime configuration.
Then why not my program give me error. It show me datetime same as system datetime.

I read about NTPUDPClient as below:-

/***
* The NTPUDPClient class is a UDP implementation of a client for the
* Network Time Protocol (NTP) described in RFC 1305 as well as the
* Simple Network Time Protocol (SNTP) in RFC-2030. To use the class,
* merely open a local datagram socket with open
* and call getTime to retrieve the time. Then call
* close
* to close the connection properly.
* Successive calls to getTime are permitted
* without re-establishing a connection. That is because UDP is a
* connectionless protocol and the Network Time Protocol is stateless.
*
* @author Jason Mathews, MITRE Corp
* @version $Revision: 1299238 $
***/

Is there any way that use TCP or something for NTP so when actually when we try to connect & it cant reach there, then program show some error or exception?

[Stephan van Hulst]

You should probably Google what ntpd is and figure out why it's not related to how your application operates.

[Tim Holloway]

NTP isn't nearly as simple as you might think. Among other things, it's designed to accept time inputs from multiple sources. Typically several Internet timeservers or server pools, but also locally-attached GPS or atomic clocks, WWVB receivers, and so forth. NTP then takes up all this information and attempts to construct a consensus as to what the actual local time is. Subject to some blurring since the system it's running on has internal latencies.

You don't talk to NTP directly. The NTP daemon adjusts the system clock, so the ordinary system time functions are what you use to get the time, whether NTPD is running or not.

Microsoft Windows LAN users have also been able to have their system clocks adjusted by the Microsoft LAN itself when you login to the LAN.

[Mandar Khire]

Thanks Tim Holloway & Stephan van Hulst,

You should probably Google what ntpd is and figure out why it's not related to how your application operates.

I search about NTP & SNTP, & find out my query will be solve by SNTP(might be).

You don't talk to NTP directly. The NTP daemon adjusts the system clock, so the ordinary system time functions are what you use to get the time, whether NTPD is running or not.

In Centos whenever i edit Adjust Date & time by GUI(by right click on clock shown on Desktop), I see 'service ntpd status' as stopped. example I try to set date as 1-Jan-1980, i have to stop NTP then i can change date.
When i select NTP, service ntpd status shows running. So i thought there is something connection between system clock & service ntpd.
My actual aim is
To check system time is really sync with internet or not, before my program start doing actual functionality.
I found one more code which use SNTP. & might be by this i solve lots of possibilities.
2 programs, MySNTP.java & NtpMessage.java

When i connect my pc to network switch which has Internet connection. & Then test this program.
1. Stop ntpd service but internet working fine.
2. Internet connection removed from switch but PC network & ntpd service is fine.

When i test 1st condition, I got result as

Time difference is more than 1 min means Might be System time is not sync.

When i test 2nd condition i got result

java.net.UnknownHostException: time-a.nist.gov: unknown error
	at java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
	at java.net.InetAddress$2.lookupAllHostAddr(InetAddress.java:907)
	at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1302)
	at java.net.InetAddress.getAllByName0(InetAddress.java:1255)
	at java.net.InetAddress.getAllByName(InetAddress.java:1171)
	at java.net.InetAddress.getAllByName(InetAddress.java:1105)
	at java.net.InetAddress.getByName(InetAddress.java:1055)
	at .MySNTP.getNTPDate(MySNTP.java:75)
	at .MySNTP.main(MySNTP.java:40)

My question is
1. All my exercise of checking my pc is sync with internet or not, is right way or wrong way?

For reference both codes are as follows:-

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class MySNTP {
	static String checkDynamic_date = "";
	static String checkNTP_date = "";
	private static String sD1;
	private static String sD2;
	static SimpleDateFormat df=new SimpleDateFormat("dd/MM/yyyy hh:mm:ss:SSS");
	private static final int PORT = 123;
	public static void main(String[] args) {
		static String checkDynamic_date = "";
	static String checkNTP_date = "";
	private static String sD1;
	private static String sD2;
	static SimpleDateFormat df=new SimpleDateFormat("dd/MM/yyyy hh:mm:ss:SSS");
	private static final int PORT = 123;
	public static void main(String[] args) {
		Process p1;
		String s1;
		String line2 = "";
		try {
			p1 = Runtime.getRuntime().exec("service ntpd status");
			BufferedReader br = new BufferedReader(
					new InputStreamReader(p1.getInputStream()));
			while ((s1 = br.readLine()) != null){
				line2 = s1;
			}
			p1.waitFor();
			p1.destroy();
		} catch (Exception e) {
			e.printStackTrace();
		}

//		if(line2.equals("ntpd is stopped")){
//			System.out.println("ntpd is stopped.");
//		}else{
			checkNTP_date = getNTPDate();
			checkDynamic_date = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss:SSS").format(new java.util.Date());
			if(checkNTP_date != null){
				if(!checkNTP_date.isEmpty() && !checkDynamic_date.isEmpty()){
					dateDiff(checkDynamic_date,checkNTP_date);
				}else{
					if(checkNTP_date.isEmpty()){
						System.out.println("checkNTP_date.isEmpty()");
					}
					if(!checkDynamic_date.isEmpty()){
						System.out.println("checkDynamic_date.isEmpty()");
					}
				}
				long diff=getDifferenceInMinutes();
				System.out.println("Time difference (abs): " + diff);
				if(diff < 999){
					System.out.println("Time difference is less than 1 sec ");
				}else{
					System.out.println("Time difference is more than 1 sec ");
					if(diff < 60000){
						System.out.println("Time difference is less than 1 min ");
					}else{
						System.out.println("Time difference is more than 1 min means Might be System time is not sync.");
					}
				}
			}else{
				System.out.println("checkNTP_date = null.");
			}
//		}
	}
	private static String getNTPDate() {
		InetAddress address = null;
		DatagramSocket socket = null;
		String sntpDateTime = null;
		try {
			final String serverName = "time-a.nist.gov";
			socket = new DatagramSocket();
			if(socket != null){
				address = InetAddress.getByName(serverName);
				if(address != null){						
					final byte[] buffer = new NtpMessage().toByteArray();
					if(buffer != null){
						DatagramPacket packet = new DatagramPacket(buffer, buffer.length, address, PORT);
						if(packet != null){
							socket.send(packet); 
							packet = new DatagramPacket(buffer, buffer.length);
							socket.receive(packet);
							final NtpMessage msg = new NtpMessage(packet.getData());
							socket.close();
							sntpDateTime = NtpMessage.timestampToString(msg.transmitTimestamp);
						}
					}
				}
			}

		} catch (IOException e) {
			e.printStackTrace();
		}

		return sntpDateTime;
	}
	private static void dateDiff(String checkDynamic_date, String time) {
		sD1=checkDynamic_date;
		sD2=time;
	}
	public static long getDifferenceInMinutes (){
		System.out.println("sD1 = " + sD1);
		System.out.println("sD2 = " + sD2);

		Date d1 = null;
		long d2Ms = 0;
		long d1Ms = 0;
		try {
			d1 = df.parse(sD1);
		} catch (ParseException e) {
			e.printStackTrace();
		}
		System.out.println("d1 = " + d1);
		Date d2 = null;
		try {
			d2 = df.parse(sD2);
		} catch (ParseException e) {
			e.printStackTrace();
		}
		System.out.println("d2 = " + d2);
		if(d1 != null){
			d1Ms = d1.getTime();
			System.out.println("d1Ms = " + d1Ms);
		}
if(d2 != null){
	d2Ms = d2.getTime();
	System.out.println("d2Ms = " + d2Ms);	
}

		return Math.abs(d1Ms-d2Ms);
	}
}

import java.text.DecimalFormat;
import java.text.SimpleDateFormat;
 /**
  * This class represents a NTP message, as specified in RFC 2030.  The message
  * format is compatible with all versions of NTP and SNTP.
  *
  8  * This class does not support the optional authentication protocol, and
  9  * ignores the key ID and message digest fields.
 10  * 
 11  * For convenience, this class exposes message values as native Java types, not
 12  * the NTP-specified data formats.  For example, timestamps are
 13  * stored as doubles (as opposed to the NTP unsigned 64-bit fixed point
 14  * format).
 15  * 
 16  * However, the contructor NtpMessage(byte[]) and the method toByteArray()
 17  * allow the import and export of the raw NTP message format.
 18  * 
 19  * 
 20  * Usage example
 21  * 
 22  * // Send message
 23  * DatagramSocket socket = new DatagramSocket();
 24  * InetAddress address = InetAddress.getByName( ntp.cais.rnp.br );
 25  * byte[] buf = new NtpMessage().toByteArray();
 26  * DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 123);
 27  * socket.send(packet);
 28  * 
 29  * // Get response
 30  * socket.receive(packet);
 31  * System.out.println(msg.toString());
 32  * 
 33  *  
 34  * This code is copyright (c) Adam Buckley 2004
 35  *
 36  * This program is free software; you can redistribute it and/or modify it 
 37  * under the terms of the GNU General Public License as published by the Free 
 38  * Software Foundation; either version 2 of the License, or (at your option) 
 39  * any later version.  A HTML version of the GNU General Public License can be
 40  * seen at http://www.gnu.org/licenses/gpl.html
 41  *
 42  * This program is distributed in the hope that it will be useful, but WITHOUT 
 43  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 
 44  * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for 
 45  * more details.
 46  * 
 47  * 
 48  * Comments for member variables are taken from RFC2030 by David Mills,
 49  * University of Delaware.
 50  * 
 51  * Number format conversion code in NtpMessage(byte[] array) and toByteArray()
 52  * inspired by http://www.pps.jussieu.fr/~jch/enseignement/reseaux/
 53  * NTPMessage.java which is copyright (c) 2003 by Juliusz Chroboczek
 54  * 
 55  * @author Adam Buckley
 56  */
  
 public class NtpMessage {
 
    /**
 61     * This is a two-bit code warning of an impending leap second to be
 62     * inserted/deleted in the last minute of the current day.  It's values
 63     * may be as follows:
 64     * 
 65     * Value     Meaning
 66     * -----     -------
 67     * 0         no warning
 68     * 1         last minute has 61 seconds
 69     * 2         last minute has 59 seconds)
 70     * 3         alarm condition (clock not synchronized)
 71     */
    public byte leapIndicator = 0;
 	
 	
    /**
 76     * This value indicates the NTP/SNTP version number.  The version number
 77     * is 3 for Version 3 (IPv4 only) and 4 for Version 4 (IPv4, IPv6 and OSI).
 78     * If necessary to distinguish between IPv4, IPv6 and OSI, the
 79     * encapsulating context must be inspected.
 80     */
    public byte version = 3;
 	
 	
  	/**
 85 	 * This value indicates the mode, with values defined as follows:
 86 	 * 
 87 	 * Mode     Meaning
 88 	 * ----     -------
 89 	 * 0        reserved
 90 	 * 1        symmetric active
 91 	 * 2        symmetric passive
 92 	 * 3        client
 93 	 * 4        server
 94 	 * 5        broadcast
 95 	 * 6        reserved for NTP control message
 96 	 * 7        reserved for private use
 97 	 * 
 98 	 * In unicast and anycast modes, the client sets this field to 3 (client)
 99 	 * in the request and the server sets it to 4 (server) in the reply. In
100 	 * multicast mode, the server sets this field to 5 (broadcast).
101 	 */ 
   public byte mode = 0;
 	
 	
    /**
106     * This value indicates the stratum level of the local clock, with values
107     * defined as follows:
108     * 
109     * Stratum  Meaning
110     * ----------------------------------------------
111     * 0        unspecified or unavailable
112     * 1        primary reference (e.g., radio clock)
113     * 2-15     secondary reference (via NTP or SNTP)
114     * 16-255   reserved
115     */
    public short stratum = 0;
 	
 
    /**
120     * This value indicates the maximum interval between successive messages,
121     * in seconds to the nearest power of two. The values that can appear in
122     * this field presently range from 4 (16 s) to 14 (16284 s); however, most
123     * applications use only the sub-range 6 (64 s) to 10 (1024 s).
124     */
    public byte pollInterval = 0;
 	
 	
    /**
129     * This value indicates the precision of the local clock, in seconds to
130     * the nearest power of two.  The values that normally appear in this field
131     * range from -6 for mains-frequency clocks to -20 for microsecond clocks
132     * found in some workstations.
133     */
    public byte precision = 0;
 	
 	
    /**
138     * This value indicates the total roundtrip delay to the primary reference
139     * source, in seconds.  Note that this variable can take on both positive
140     * and negative values, depending on the relative time and frequency
141     * offsets. The values that normally appear in this field range from
142     * negative values of a few milliseconds to positive values of several
143     * hundred milliseconds.
144     */
    public double rootDelay = 0;
 	
 	
    /**
149     * This value indicates the nominal error relative to the primary reference
150     * source, in seconds.  The values  that normally appear in this field
151     * range from 0 to several hundred milliseconds.
152     */ 
    public double rootDispersion = 0;
 	
 	
    /**
157     * This is a 4-byte array identifying the particular reference source.
158     * In the case of NTP Version 3 or Version 4 stratum-0 (unspecified) or
159     * stratum-1 (primary) servers, this is a four-character ASCII string, left
160     * justified and zero padded to 32 bits. In NTP Version 3 secondary
161     * servers, this is the 32-bit IPv4 address of the reference source. In NTP
162     * Version 4 secondary servers, this is the low order 32 bits of the latest
163     * transmit timestamp of the reference source. NTP primary (stratum 1)
164     * servers should set this field to a code identifying the external
165     * reference source according to the following list. If the external
166     * reference is one of those listed, the associated code should be used.
167     * Codes for sources not listed can be contrived as appropriate.
168     * 
169     * Code     External Reference Source
170     * ----     -------------------------
171     * LOCL     uncalibrated local clock used as a primary reference for
172     *          a subnet without external means of synchronization
173     * PPS      atomic clock or other pulse-per-second source
174     *          individually calibrated to national standards
175     * ACTS     NIST dialup modem service
176     * USNO     USNO modem service
177     * PTB      PTB (Germany) modem service
178     * TDF      Allouis (France) Radio 164 kHz
179     * DCF      Mainflingen (Germany) Radio 77.5 kHz
180     * MSF      Rugby (UK) Radio 60 kHz
181     * WWV      Ft. Collins (US) Radio 2.5, 5, 10, 15, 20 MHz
182     * WWVB     Boulder (US) Radio 60 kHz
183     * WWVH     Kaui Hawaii (US) Radio 2.5, 5, 10, 15 MHz
184     * CHU      Ottawa (Canada) Radio 3330, 7335, 14670 kHz
185     * LORC     LORAN-C radionavigation system
186     * OMEG     OMEGA radionavigation system
187     * GPS      Global Positioning Service
188     * GOES     Geostationary Orbit Environment Satellite
189     */
    public byte[] referenceIdentifier = {0, 0, 0, 0};
 	
 	
    /**
194     * This is the time at which the local clock was last set or corrected, in
195     * seconds since 00:00 1-Jan-1900.
196     */
    public double referenceTimestamp = 0;
 	
 	
    /**
201     * This is the time at which the request departed the client for the
202     * server, in seconds since 00:00 1-Jan-1900.
203     */
    public double originateTimestamp = 0;
 	
 	
    /**
208     * This is the time at which the request arrived at the server, in seconds
209     * since 00:00 1-Jan-1900.
210     */
    public double receiveTimestamp = 0;
	
	
    /**
215     * This is the time at which the reply departed the server for the client,
216     * in seconds since 00:00 1-Jan-1900.
217     */
    public double transmitTimestamp = 0;
 	
 	
    /**
222     * Constructs a new NtpMessage from an array of bytes.
223     */
    public NtpMessage(byte[] array) {
       // See the packet format diagram in RFC 2030 for details 
       leapIndicator = (byte) ((array[0] >> 6) & 0x3);
       version = (byte) ((array[0] >> 3) & 0x7);
       mode = (byte) (array[0] & 0x7);
       stratum = unsignedByteToShort(array[1]);
       pollInterval = array[2];
       precision = array[3];
 		
       rootDelay = (array[4] * 256.0) + 
          unsignedByteToShort(array[5]) +
          (unsignedByteToShort(array[6]) / 256.0) +
          (unsignedByteToShort(array[7]) / 65536.0);
 		
       rootDispersion = (unsignedByteToShort(array[8]) * 256.0) + 
          unsignedByteToShort(array[9]) +
          (unsignedByteToShort(array[10]) / 256.0) +
          (unsignedByteToShort(array[11]) / 65536.0);
 		
      referenceIdentifier[0] = array[12];
       referenceIdentifier[1] = array[13];
       referenceIdentifier[2] = array[14];
       referenceIdentifier[3] = array[15];
 		
       referenceTimestamp = decodeTimestamp(array, 16);
       originateTimestamp = decodeTimestamp(array, 24);
       receiveTimestamp = decodeTimestamp(array, 32);
       transmitTimestamp = decodeTimestamp(array, 40);
    }
 	
 
    private static final long DAYS = 25567; // 1 Jan 1900 to 1 Jan 1970
    private static final long SECS = 60 * 60 * 24 * DAYS;
 
    // Translate Java/Unix's epoch (1 Jan 1970) to NTP's epoch
    // (1 Jan 1900) and convert from milliseconds to fractions of seconds
    public static final double now () {
       return System.currentTimeMillis()/1000.0 + SECS;
    }
 
 	
 	
    /**
267     * Constructs a new NtpMessage in client -> server mode, and sets the
268     * transmit timestamp to the current time.
269     */
    public NtpMessage() {
       // Note that all the other member variables are already set with
       // appropriate default values.
       this.mode = 3;
       this.transmitTimestamp = now();
    }
 	
	
 	
    /**
280     * This method constructs the data bytes of a raw NTP packet.
281     */
    public byte[] toByteArray() {
       // All bytes are automatically set to 0
       byte[] p = new byte[48];
 
       p[0] = (byte) (leapIndicator << 6 | version << 3 | mode);
       p[1] = (byte) stratum;
       p[2] = (byte) pollInterval;
       p[3] = (byte) precision;
 		
       // root delay is a signed 16.16-bit FP, in Java an int is 32-bits
       int l = (int) (rootDelay * 65536.0);
       p[4] = (byte) ((l >> 24) & 0xFF);
       p[5] = (byte) ((l >> 16) & 0xFF);
       p[6] = (byte) ((l >> 8) & 0xFF);
       p[7] = (byte) (l & 0xFF);
 		
       // root dispersion is an unsigned 16.16-bit FP, in Java there are no
       // unsigned primitive types, so we use a long which is 64-bits 
       long ul = (long) (rootDispersion * 65536.0);
       p[8] = (byte) ((ul >> 24) & 0xFF);
       p[9] = (byte) ((ul >> 16) & 0xFF);
       p[10] = (byte) ((ul >> 8) & 0xFF);
       p[11] = (byte) (ul & 0xFF);
 		
       p[12] = referenceIdentifier[0];
       p[13] = referenceIdentifier[1];
       p[14] = referenceIdentifier[2];
       p[15] = referenceIdentifier[3];
 		
       encodeTimestamp(p, 16, referenceTimestamp);
       encodeTimestamp(p, 24, originateTimestamp);
       encodeTimestamp(p, 32, receiveTimestamp);
       encodeTimestamp(p, 40, transmitTimestamp);
 		
       return p; 
    }
 	
 	
    /**
     * Returns a string representation of a NtpMessage
     */
    public String toString() {
       String precisionStr =
          new DecimalFormat("0.#E0").format(Math.pow(2, precision));
 			
       return "Leap indicator: " + leapIndicator + "\n" +
          "Version: " + version + "\n" +
          "Mode: " + mode + "\n" +
          "Stratum: " + stratum + "\n" +
          "Poll: " + pollInterval + "\n" +
          "Precision: " + precision + " (" + precisionStr + " seconds)\n" + 
          "Root delay: " + new DecimalFormat("0.00").format(rootDelay*1000) + " ms\n" +
          "Root dispersion: " + new DecimalFormat("0.00").format(rootDispersion*1000) + " ms\n" + 
          "Reference identifier: " + referenceIdentifierToString(referenceIdentifier, stratum, version) + "\n" +
          "Reference timestamp: " + timestampToString(referenceTimestamp) + "\n" +
          "Originate timestamp: " + timestampToString(originateTimestamp) + "\n" +
          "Receive timestamp:   " + timestampToString(receiveTimestamp) + "\n" +
          "Transmit timestamp:  " + timestampToString(transmitTimestamp);
    }
 	
 	
 	
    /**
     * Converts an unsigned byte to a short.  By default, Java assumes that
     * a byte is signed.
     */
    public static short unsignedByteToShort(byte b)
    {
       if((b & 0x80)==0x80) return (short) (128 + (b & 0x7f));
       else return (short) b;
    }
 	
 	
 	
    /**
     * Will read 8 bytes of a message beginning at <code>pointer</code>
     * and return it as a double, according to the NTP 64-bit timestamp
     * format.
     */
    public static double decodeTimestamp(byte[] array, int pointer)
    {
       double r = 0.0;
 		
       for(int i=0; i<8; i++)
          {
             r += unsignedByteToShort(array[pointer+i]) * Math.pow(2, (3-i)*8);
          }
 		
       return r;
    }
 	
 	
 	
    /**
    * Encodes a timestamp in the specified position in the message
     */
    public static void encodeTimestamp (final byte[] array, final int pointer, double timestamp)	{
       // Converts a double into a 64-bit fixed point
       for(int i=0; i<8; i++) {
          // 2^24, 2^16, 2^8, .. 2^-32
          final double base = Math.pow(2, (3-i)*8);
          // Capture byte value
          array[pointer+i] = (byte) (timestamp / base);
 
          // Subtract captured value from remaining total
          timestamp = timestamp - (double) (unsignedByteToShort(array[pointer+i]) * base);
       }
 		
       // From RFC 2030: It is advisable to fill the non-significant
       // low order bits of the timestamp with a random, unbiased
       // bitstring, both to avoid systematic roundoff errors and as
       // a means of loop detection and replay detection.
       array[7] = (byte) (Math.random()*255.0);
    }
 	
 	
 	
    /**
     * Returns a timestamp (number of seconds since 00:00 1-Jan-1900) as a
     * formatted date/time string. 
     */
    private static String dtf = "%1$ta, %1$td %1$tb %1$tY, %1$tI:%1$tm:%1$tS.%1$tL %1$tp %1$tZ";
 
    public static String timestampToString (final double timestamp) {
       if(timestamp==0) return "0";
       // timestamp is relative to 1900, utc is used by Java and is relative to 1970 
       return millisToDate (Math.round (1000.0*(timestamp-SECS)));
    }
 
    public static String millisToDate (final long ms) {
    	String abc = "";
    	abc = new SimpleDateFormat("dd/MM/yyyy HH:mm:ss:SSS").format(new java.util.Date(ms));
//    	System.out.println("millisToDate = "+new SimpleDateFormat("dd/MM/yyyy HH:mm:ss:SSS").format(new java.util.Date(ms)) );
//       return String.format (dtf, ms);
    	return abc;
    }
	
	
    /**
     * Returns a string representation of a reference identifier according
     * to the rules set out in RFC 2030.
     */
    public static String referenceIdentifierToString(byte[] ref, short stratum, byte version) {
       // From the RFC 2030:
       // In the case of NTP Version 3 or Version 4 stratum-0 (unspecified)
       // or stratum-1 (primary) servers, this is a four-character ASCII
       // string, left justified and zero padded to 32 bits.
       if(stratum==0 || stratum==1) {
          return new String(ref);
       }
 		
       // In NTP Version 3 secondary servers, this is the 32-bit IPv4
       // address of the reference source.
       else if(version==3) {
             return unsignedByteToShort(ref[0]) + "." +
                unsignedByteToShort(ref[1]) + "." +
                unsignedByteToShort(ref[2]) + "." +
                unsignedByteToShort(ref[3]);
          }
 		
       // In NTP Version 4 secondary servers, this is the low order 32 bits
       // of the latest transmit timestamp of the reference source.
       else if(version==4) {
             return "" + ((unsignedByteToShort(ref[0]) / 256.0) + 
                (unsignedByteToShort(ref[1]) / 65536.0) +
                (unsignedByteToShort(ref[2]) / 16777216.0) +
                (unsignedByteToShort(ref[3]) / 4294967296.0));
       }
 		
       return "";
    }
 }

[Tim Holloway]

Sorry. Long code samples tend to go unread here. It's the downside of free support.

The master timekeeper for a computer is the clock chip that's part of the core motherboard hardware. This is what gets queried when you do an OS function call for time. The clock runs as an independent processor, so it doesn't get affected by, nor does it affect CPU loads.

NTP is NOT directly queried for time. First of all, because the CPU and network overhead would be prohibitive, secondly because network response times can vary wildly, so getting an accurate query is more of a statistical function than an absolute point.

As I said before, the NTP server usually gets time from multiple sources and forms a consensus time which it then uses to periodically correct the hardware clock. Because the hardware clock is usually not very good and so it drifts off of the "true" time unless a more accurate source keeps it corrected.

Here's an analogy. Suppose you have 2 clocks. One is a battery-powered digital crystal (non-atomic) clock. The other is one of the old-time LED clocks that you have to set yourself but it runs off the power line. Which do you think is more accurate?

Trick answer! The A/C power line is noisy, sloppy and slow (typically 50/60 cycles per second). But its overall accuracy is more reliable, since all of the power companies on the grid MUST keep their powerline cycles in sync. Otherwise one could be pushing a positive voltage over the wire at the same time another was in the negative phase - effectively a high-power short circuit. The crystal clock will be more accurate in the short term, but any drift will not be corrected, so eventually it could end up off by a very long amount. And a crystal clock is what your computer uses.

So it isn't necessary to have NTP running all the time, just enough to keep the CPU clock on track.

I don't recall CentOS restarting NTP if one resets the clock manually, but it is definite that NTP shouldn't be expected to do really coarse-grained time setting. NTP attempts to make fine adjustments, so if your clock is off by more than an hour, you need to manually reset it to within a minute or so of the correct time.

You should be able to see diagnostic information in your syslog that tells how well NTP is tracking the absolute time. If you don't, check the NTP config file options for logging settings.

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