Managing Accurate Date and Time

Avi Alkalay


avi at unix.sh
avi at br.ibm.com
Senior IT and Software Architect :: OpenSource/Linux Solutions
IBM Linux Impact Team :: [http://ibm.com/linux] ibm.com/linux

Kent Borg - Suggestion to use ntpq instead of ntpdc

Yura Moron - Good explanations on ntpq and ntpdc info

Copyright  2002 by Avi Alkalay

1.0 :: 2002/04/28
Revision History                                                             
Revision 1.0             28 Apr 2002            Revised by: avi              
Finalized image.                                                             
Revision 0.8             27 Apr 2002            Revised by: avi              
Switched from ntpdc example to ntpq, based on contributions.                 
Revision 0.8.1           20 Apr 2002            Revised by: avi              
Improved graphic. Links to other doc locations.                              
Revision 0.8             14 Apr 2002            Revised by: avi              
Beter tunning of NTP graphic.                                                
Revision 0.76            13 Apr 2002            Revised by: avi              
Inclusion of architecture graphic.                                           
Revision 0.75            10 Apr 2002            Revised by: avi              
Spell checked. Using DocBook XSLT 1.50.                                      
Revision 0.65            31 Mar 2002            Revised by: avi              
Upgraded to XML 4.1.2 DocBook format                                         
Revision 0.6             29 Mar 2002            Revised by: avi              
Finished timezone mechanism on Linux. Created appendix. Now only drawings are
needed.                                                                      
Revision 0.4             24 Mar 2002            Revised by: avi              
All skeleton defined. All command examples written.                          
Revision 0.2             19 Mar 2002            Revised by: avi              
First DocBook version                                                        
-----------------------------------------------------------------------------

Table of Contents
1. Computer Global Date and Time Concept
2. What are Time Zones?
    2.1. Daylight Savings Time
    2.2. Time Zones Examples
    2.3. Time Zone Mechanism on Linux
   
   
3. The Correct Settings for Your Linux Box
    3.1. Setting Time Zone
    3.2. Setting the Hardware Clock
   
   
4. Accurate Global Time Synchronization
    4.1. NTP: The Network Time Protocol
    4.2. Building a Simple Time Synchronization Architecture
    4.3. NTP Configurations
    4.4. Watching Your Box Synchronizing
    4.5. Configure to Automatically Run NTP at Boot
   
   
A. About this Document

1. Computer Global Date and Time Concept

To determine the current time for some planet region, a computer needs
exactly this two informations:

 1. Correct UTC (universal time as in Greenwich, but not GMT) time
   
 2. Region's current Time Zone
   

For computers, there is also the hardware clock, which is used as a base by
the OS to set his time.

OS date and time (we'll use only date or time from now on) is set on boot, by
some script that reads the hardware clock, makes Time Zone calculations
(there is no time zone data stored in BIOS) and sets the OS. After this
synchronization, BIOS and OS time are completely independent. So after a
while they may have some seconds of difference. Which one is correct? If you
don't make special configurations, none.

We'll discuss here how to make them both globally 100% accurate.
-----------------------------------------------------------------------------

2. What are Time Zones?

Time Zones are a geophysical world globe division of 15o each, starting at
Greenwich, in England, created to help people know what time is it now in
another part of the world.

Nowadays it is much more a political division than geophysical, because
sometimes people needs to have the same time as other people in not-so-far
locations. And for energy savings reasons, we have today the Daylight Savings
Time, that are also a Time Zone variation.

Time Zones are usually defined by your country government or some
astronomical institute, and is represented by 3 or 4 letters. See Section 2.2
for examples.

If you want to know what time is it now in a different world region, you can
use the [http://www.timezoneconverter.com/] timezoneconverter.com website.
-----------------------------------------------------------------------------

2.1. Daylight Savings Time

For energy savings reasons, governments created the Daylight Savings Time.
Our clocks are forwarded one hour, and this makes our days look longer. In
fact, what really happens is only a Time Zone change. The primitive time (UTC
) is still, and will allways be, the same.

Later we'll see how to enable and disable DST automatically in Linux.
-----------------------------------------------------------------------------

2.2. Time Zones Examples

There is nothing better than examples:


Table 1. Brazilian Time Zones. Shifts relative to UTC
+----------+------------+---------------------------------------------------+
|Name and  |DST Name and|Locations                                          |
|Shift     |Shift       |                                                   |
+----------+------------+---------------------------------------------------+
|BREST -2: |BREDT -1:00 |Fernando de Noronha                                |
|00        |            |                                                   |
+----------+------------+---------------------------------------------------+
|BRST -3:00|BRDT -2:00  |So Paulo, Rio, Brasilia, Minas Gerais, North East |
|          |            |Region, South Region,etc                           |
+----------+------------+---------------------------------------------------+
|BRWST -4: |BRWDT -3:00 |West Region                                        |
|00        |            |                                                   |
+----------+------------+---------------------------------------------------+
|BRAST -5: |BRADT -4:00 |Acre                                               |
|00        |            |                                                   |
+----------+------------+---------------------------------------------------+

Please send me contributions like this table for US Time Zone.
-----------------------------------------------------------------------------

2.3. Time Zone Mechanism on Linux

Linux systems uses the GLIBC dynamic Time Zones, based on /etc/localtime.
This file is a link to (or a copy of) a zone information file, usually
located under /usr/share/zoneinfo directory.

From a geophysical perspective, there is only 360o/15o=24 Time Zones in the
world. But to make things easy to people, and to accommodate all the
political variations (like Daylight Savings Time), you'll find hundreds of
zoneinfo files in /usr/share/zoneinfo, each for every world city, country,
etc, and its not complete (it can never be).

Some countries, like Brazil, don't have a fixed day to start Daylight Savings
Time. It is defined every year, a couple of months before summer, and you may
end up in a situation you'll have to change your zoneinfo file, which was
compiled by zic from a text file like this.


Example 1. Brazilian Zone Info text file
# Brazil Time Zones                                                           
#                                                                             
# Brazilian Time Zones are:                                                   
# BREST: East of Brasilia. Fernando de Noronha.                               
# BRST:  Brasilia, So Paulo, Rio, Northeast, South etc                       
# BRWST: West of Brasilia. Mato Grosso, Manaus                                
# BRAST: Acre.                                                                
#                                                                             
# In daylight saving time, letter 'S' changes to 'D'.                         
#                                                                             
# To install, make:                                                           
#                                                                             
# # zic Brazil.txt                                                            
#                                                                             
# Zone files will be installed in /usr/share/zoneinfo (depends on your        
# distribution). Then, make a symbolic link from your zone to /etc/localtime: 
#                                                                             
# # ln -sf /usr/share/zoneinfo/Brazil/Brasilia /etc/localtime                 
#                                                                             
#                                                                             
# If you have updates and new standards to this file please send to           
#                                                                             
# Avi Alkalay <avi @ unix.sh>                                                 
#                                                                             
# Last update: 18 Nov 2000                                                    
# This file is available at http://avi.alkalay.net/linux/zoneinfo/            
#                                                                             
                                                                              
# Rule  NAME    FROM  TO    TYPE  IN    ON   AT    SAVE   LETTER/S            
Rule    Brazil  1931  1932  -     Oct   3    0:00  1:00   D                   
Rule    Brazil  1932  1933  -     Mar   31   0:00  0      S                   
Rule    Brazil  1949  only  -     Dec   1    0:00  1:00   D                   
Rule    Brazil  1950  only  -     Apr   30   0:00  0      S                   
Rule    Brazil  1950  1952  -     Dec   1    0:00  1:00   D                   
Rule    Brazil  1951  only  -     Apr   16   0:00  0      S                   
Rule    Brazil  1952  only  -     Mar   31   0:00  0      S                   
Rule    Brazil  1953  only  -     Feb   28   0:00  0      S                   
Rule    Brazil  1963  only  -     Oct   23   0:00  1:00   D                   
Rule    Brazil  1964  only  -     Mar   1    0:00  0      S                   
Rule    Brazil  1965  only  -     Jan   31   0:00  1:00   D                   
Rule    Brazil  1965  only  -     Mar   31   0:00  0      S                   
Rule    Brazil  1965  only  -     Dec   1    0:00  1:00   D                   
Rule    Brazil  1966  1968  -     Mar   1    0:00  0      S                   
Rule    Brazil  1966  1967  -     Nov   1    0:00  1:00   D                   
Rule    Brazil  1984  only  -     Nov   2    0:00  1:00   D                   
Rule    Brazil  1985  only  -     Mar   15   0:00  0      S                   
Rule    Brazil  1985  only  -     Nov   2    0:00  1:00   D                   
Rule    Brazil  1986  only  -     Mar   15   0:00  0      S                   
Rule    Brazil  1986  only  -     Oct   25   0:00  1:00   D                   
Rule    Brazil  1987  only  -     Feb   14   0:00  0      S                   
Rule    Brazil  1987  only  -     Oct   25   0:00  1:00   D                   
Rule    Brazil  1988  only  -     Feb   7    0:00  0      S                   
Rule    Brazil  1988  only  -     Oct   16   0:00  1:00   D                   
Rule    Brazil  1989  only  -     Jan   29   0:00  0      S                   
Rule    Brazil  1989  only  -     Oct   15   0:00  1:00   D                   
Rule    Brazil  1990  only  -     Feb   11   0:00  0      S                   
Rule    Brazil  1990  only  -     Oct   21   0:00  1:00   D                   
Rule    Brazil  1991  only  -     Feb   17   0:00  0      S                   
Rule    Brazil  1991  only  -     Oct   20   0:00  1:00   D                   
Rule    Brazil  1992  only  -     Feb   9    0:00  0      S                   
Rule    Brazil  1992  only  -     Oct   25   0:00  1:00   D                   
Rule    Brazil  1993  only  -     Jan   31   0:00  0      S                   
Rule    Brazil  1993  only  -     Oct   17   0:00  1:00   D                   
Rule    Brazil  1994  only  -     Feb   20   0:00  0      S                   
Rule    Brazil  1994  only  -     Oct   16   0:00  1:00   D                   
Rule    Brazil  1995  only  -     Feb   19   0:00  0      S                   
Rule    Brazil  1995  only  -     Oct   15   0:00  1:00   D                   
Rule    Brazil  1996  only  -     Feb   11   0:00  0      S                   
Rule    Brazil  1996  only  -     Oct   06   0:00  1:00   D                   
Rule    Brazil  1997  only  -     Feb   16   0:00  0      S                   
Rule    Brazil  1997  only  -     Oct   06   0:00  1:00   D                   
Rule    Brazil  1998  only  -     Mar   01   0:00  0      S                   
Rule    Brazil  1998  only  -     Oct   11   0:00  1:00   D                   
Rule    Brazil  1999  only  -     Feb   21   0:00  0      S                   
Rule    Brazil  1999  only  -     Oct   03   0:00  1:00   D                   
Rule    Brazil  2000  only  -     Feb   27   0:00  0      S                   
Rule    Brazil  2000  only  -     Oct   8    0:00  1:00   D                   
Rule    Brazil  2001  only  -     Feb   18   0:00  0      S                   
                                                                              
                                                                              
# Zone  NAME                    GMTOFF  RULES/SAVE      FORMAT  [UNTIL]       
Zone    Brazil/DeNoronha        -2:00   Brazil          BRE%sT                
Zone    posix/Brazil/DeNoronha  -2:00   Brazil          BRE%sT                
Zone    right/Brazil/DeNoronha  -2:00   Brazil          BRE%sT                
Zone    Brazil/East             -2:00   Brazil          BRE%sT                
Zone    posix/Brazil/East       -2:00   Brazil          BRE%sT                
Zone    right/Brazil/East       -2:00   Brazil          BRE%sT                
                                                                              
Zone    America/Sao_Paulo       -3:00   Brazil          BR%sT                 
Zone    America/Rio_de_Janeiro  -3:00   Brazil          BR%sT                 
Zone    America/Brasilia        -3:00   Brazil          BR%sT                 
Zone    posix/America/Sao_Paulo -3:00   Brazil          BR%sT                 
Zone    posix/America/Rio_de_Janeiro -3:00   Brazil     BR%sT                 
Zone    posix/America/Salvador  -3:00   Brazil          BR%sT                 
Zone    posix/America/Brasilia  -3:00   Brazil          BR%sT                 
Zone    posix/Brazil/Central    -3:00   Brazil          BR%sT                 
Zone    posix/Brazil/Brasilia   -3:00   Brazil          BR%sT                 
Zone    posix/Brazil/Sao_Paulo  -3:00   Brazil          BR%sT                 
Zone    posix/Brazil/Salvador   -3:00   Brazil          BR%sT                 
Zone    posix/Brazil/Rio_de_Janeiro  -3:00   Brazil     BR%sT                 
Zone    right/America/Sao_Paulo -3:00   Brazil          BR%sT                 
Zone    right/America/Rio_de_Janeiro -3:00   Brazil     BR%sT                 
Zone    right/America/Salvador  -3:00   Brazil          BR%sT                 
Zone    right/America/Brasilia  -3:00   Brazil          BR%sT                 
Zone    right/Brazil/Central    -3:00   Brazil          BR%sT                 
Zone    right/Brazil/Brasilia   -3:00   Brazil          BR%sT                 
Zone    right/Brazil/Sao_Paulo  -3:00   Brazil          BR%sT                 
Zone    right/Brazil/Salvador   -3:00   Brazil          BR%sT                 
Zone    right/Brazil/Rio_de_Janeiro   -3:00   Brazil    BR%sT                 
Zone    Brazil/Central          -3:00   Brazil          BR%sT                 
Zone    Brazil/Brasilia         -3:00   Brazil          BR%sT                 
Zone    Brazil/Sao_Paulo        -3:00   Brazil          BR%sT                 
Zone    Brazil/Rio_de_Janeiro   -3:00   Brazil          BR%sT                 
Zone    Brazil/Salvador         -3:00   Brazil          BR%sT                 
                                                                              
Zone    Brazil/West             -4:00   Brazil          BRW%sT                
Zone    Brazil/Manaus           -4:00   Brazil          BRW%sT                
Zone    Brazil/Rondonia         -4:00   Brazil          BRW%sT                
Zone    Brazil/Roraima          -4:00   Brazil          BRW%sT                
Zone    Brazil/Mato_Grosso      -4:00   Brazil          BRW%sT                
Zone    posix/Brazil/Manaus     -4:00   Brazil          BRW%sT                
Zone    posix/Brazil/Mato_Grosso -4:00  Brazil          BRW%sT                
Zone    right/Brazil/Manaus     -4:00   Brazil          BRW%sT                
Zone    right/Brazil/Mato_Grosso -4:00  Brazil          BRW%sT                
Zone    posix/America/Manaus    -4:00   Brazil          BRW%sT                
Zone    right/America/Manaus    -4:00   Brazil          BRW%sT                
                                                                              
Zone    Brazil/Acre             -5:00   Brazil          BRA%sT                
                                                                              

The Rule block defines the date and time we change the Time Zone, while in
the Zone block we reference the Rule will manage it. Note that the Zone name
is actually the file name under /usr/share/zoneinfo directory, and here we
defined several different names for the same Time Zone, just to be easyer for
people to find their zone.

This file's comments explains how to install these time zones, using the zic
zoneinfo compiler (which already installs them also). To make it effective,
you only have to link (or copy) the zoneinfo file to /etc/localtime. In some
distributions, there is a higher level (and preferred) way to set the Time
Zone, described in Section 3.1.

After making /etc/localtime pointing to the correct zoneinfo file, you are
already under that zone rules and DST changes are automatic -- you don't have
to change time manually.

The following commands sequence shows Linux Time Zone mechanics dynamism.
Note they were all issued in less than one minute:
bash$ ls -al /etc/localtime                                                                   
lrwxrwxrwx  1 root root 35 May 22  2001 /etc/localtime -> /usr/share/zoneinfo/Brazil/Brasilia 
bash$ date                                                                                    
Fri Mar 29 20:13:38 BRST 2002                                                                 
bash# ln -sf /usr/share/zoneinfo/GMT /etc/localtime                                           
bash$ date                                                                                    
Fri Mar 29 23:13:47 GMT 2002                                                                  
bash# ln -sf /usr/share/zoneinfo/Brazil/Brasilia /etc/localtime                               
bash$ date                                                                                    
Fri Mar 29 20:14:03 BRST 2002                                                                 

At 20:13, I was in my default brazilian Time Zone (BRST), then I switched to
GMT and my system time changed to 23:13! When your Time Zone enters DST,
you'll see a similar effect, but the rules are all inside your Time Zone (/
etc/localtime link doesn't change like this example).

An application running in this machine (eg. web-server generating access
logs) will feel this change, so it is very important for developers to
remember that the full Time Concept is the current time plus current Time
Zone, as described in Section 1.

In the end, I switched back to my correct Time Zone.
-----------------------------------------------------------------------------

3. The Correct Settings for Your Linux Box

For any OS installation, you must know your Time Zone. This is expressed in
terms of a city, a state or a country. You must also decide how to set BIOS
time, and we may follow two strategies here:

Linux Only Machine
    In this case you should set BIOS time to UTC time. DST changes will be
    dynamically managed by Time Zone configurations.
   
Dual Boot Linux and MS Windows Machine
    Windows handles time in a more primitive way than Linux. For Windows, 
    BIOS time is allways your local time, so DST changes are more aggressive
    because they directly change hardware clock. And since both Linux and
    Windows initially get and set time from the hardware, when they are
    together, Linux must handle it in the same way. So set BIOS time to your
    localtime.
   

-----------------------------------------------------------------------------
3.1. Setting Time Zone

On Red Hat Linux and derived systems, you can set the hardware clock strategy
and Time Zone using the timeconfig command, that shows a user-friendly
dialog. You can also use it non-interactively:


Example 2. Time Configuration Tool
bash# timeconfig "Brasil/East"   # set HC to localtime, and TZ to "Brazil/East" 
bash# timeconfig --utc "Brasil/East"   # set HC to UTC, and TZ to "Brazil/East" 

Anyway, it changes /etc/sysconfig/clock file that is read at boot time. You
can edit it by hand, and that is how it looks:


Example 3. /etc/sysconfig/clock file
ZONE="Brazil/East"                                                           
UTC=true                                                                     
ARC=false                                                                    
-----------------------------------------------------------------------------

3.2. Setting the Hardware Clock

I encourage you to only set your HC after understanding how to get accurate
time, described on Section 4.

The hwclock command reads and sets the HC, based on several options you give
him, documented in its man page. But you don't have to use it if you have a
modern Linux distribution. After defining your HC strategy and Time Zone, you
can use the high level setclock command to correctly set your HC. You don't
need to pass any parameters because setclock intelligently calls hwclock to
set the BIOS based on your OS current date and time. So you should allways
use the setclock command.

But if you are a minimalist and prefer hard work, here are some hwclock
examples:


Example 4. setclock and hwclock usage
bash# setclock                                  # The easy way to set HC                                 
bash# hwclock                                   # reads HC                                               
bash# hwclock --systohc --utc                   # set HC with UTC time based on OS current time          
bash# hwclock --systohc                         # set HC with local time based on OS current time        
bash# hwclock --set --date "22 Mar 2002 13:17"  # set HC with time specified on string                   

Since the OS time is independent from the hardware clock, any BIOS change we
make will take place in the next boot.

Another option to change HC is rebooting and accessing your computer BIOS
screens. On [http://ibm.com/servers/eserver/zseries/os/linux/] IBM e-server
zSeries platforms you'll have to do it on z/VM level, because Linux here runs
on virtual machines created by z/VM.
-----------------------------------------------------------------------------

4. Accurate Global Time Synchronization

To have accurate time in all your systems is as important as having a solid
network security strategy (achieved by much more than simple firewall boxes).
It is one of the primary components of a system administration based on good
practices, which leads to organization and security. Specially when
administering distributed applications, web-services, or even a distributed
security monitoring tool, accurate time is a must.
-----------------------------------------------------------------------------

4.1. NTP: The Network Time Protocol

We'll not discuss here the protocol, but how this wonderfull invention, added
to the pervasiveness of the Internet, can be usefull for us. You can find
more about it at [http://www.ntp.org/] www.ntp.org.

Once your system is setup, NTP will manage to keep its time accurate, making
very small adjustments to not impact the running applications.

People can get exact time using hardware based on atom's electrons frequency
or cyclotrons. There is also a method based on GPS (Global Positioning
System). The first is more accurate, but the second is pretty good also. Both
require very special and expensive equipment, but their owners (usually
universities and research labs) connects them to computers, that run an NTP
daemon, that are connected to the Internet, that finally lets us access it
for free. And this is how we'll synchronize our systems.
-----------------------------------------------------------------------------

4.2. Building a Simple Time Synchronization Architecture

You will need:

 1. A direct or indirect (through a firewall) connection to the Internet.
   
 2. Choose some NTP servers. There is a list of [http://www.eecis.udel.edu/
    ~mills/ntp/servers.htm] public time servers on NTP website. If you don't
    have an Internet access, your WAN administrator (must be a cleaver guy)
    can provide you some internal addresses.
   
 3. Have the NTP package installed in all systems you want to synchronize.
    You can find RPMs in your favorite Linux distribution CD, or [http://
    rpmfind.net/linux/rpm2html/search.php?query=ntp] make a search on [http:/
    /rpmfind.net/] rpmfind.net.
   

Here is an example of good architecture:


Figure 1. Local Relay Servers for NTP

[ntp]

If you have several machines to synchronize, do not make them all access the
remote NTP servers you choosed. Only 2 of your server farm's machines must
access remote NTP servers, and the other machines will sync with these 2. We
will call them the Relay Servers.

Your Relay Servers can be any machine already available in your network. NTP
consumes low memory and CPU. You don't need to have a dedicated machine for
it.

Tip It is a good idea to create hostname aliases for your local Relay Servers
    like ntp1.mydomain.com and ntp2.maydomain.com, and use only these names  
    when configuring the client machines. This way you can move the NTP      
    functionality to a new Relay Server (with a different IP and hostname),  
    without having to reconfigure the clients. Ask your DNS administrator to 
    create this aliases.                                                     
-----------------------------------------------------------------------------

4.3. NTP Configurations

For Your Relay Servers
    Edit /etc/ntp.conf and add the remote servers you choosed:
   
   
    Example 5. Relay machines /etc/ntp.conf
    .                                                                        
    .                                                                        
    server  otherntp.server.org     # A stratum 1 server at server.org       
    server  ntp.research.gov        # A stratum 2 server at research.gov     
    .                                                                        
    .                                                                        
   
For Your Clients
    Edit /etc/ntp.conf and add your Relay Servers with a standard name:
   
   
    Example 6. Client machines /etc/ntp.conf
    .                                                                        
    .                                                                        
    server  ntp1.my.com             # My first local relay                   
    server  ntp2.my.com             # My second local relay                  
    .                                                                        
    .                                                                        
   

If your machine has a UTC time difference bigger than some minutes comparing
to the NTP servers, NTP will not work. So you must do a first full sync, and
I recommend you to do it in a non-production hour. You need to do it only
when you are making the initial NTP setup. Never more:


Example 7. First sync
bash# ntpdate otherntp.research.gov     (1)                                          
24 Mar 18:16:36 ntpdate[10254]: step time server 200.100.20.10 offset -15.266188 sec 
bash# ntpdate otherntp.research.gov     (2)                                          
24 Mar 18:16:43 ntpdate[10255]: adjust time server 200.100.20.10 offset -0.000267 sec

(1) First full sync. We were 15 seconds late.
(2) Second full sync, just to be sure. Now we are virtually 0 seconds late,
    which is good.

The last step is to start or restart the NTP daemons in each machine:
bash# service ntpd restart                                                   
-----------------------------------------------------------------------------

4.4. Watching Your Box Synchronizing

Now you have everything setup. NTP will softly keep your machine time
synchronized. You can watch this process using the NTP Query (ntpq command:


Example 8. A time synchronization status
bash# ntpq -p                                                                  
     remote           refid      st t when poll reach   delay   offset  jitter 
============================================================================== 
-jj.cs.umb.edu   gandalf.sigmaso  3 u   95 1024  377   31.681  -18.549   1.572 
 milo.mcs.anl.go ntp0.mcs.anl.go  2 u  818 1024  125   41.993  -15.264   1.392 
-mailer1.psc.edu ntp1.usno.navy.  2 u  972 1024  377   38.206   19.589  28.028 
-dr-zaius.cs.wis ben.cs.wisc.edu  2 u  502 1024  357   55.098    3.979   0.333 
+taylor.cs.wisc. ben.cs.wisc.edu  2 u  454 1024  347   54.127    3.379   0.047 
-ntp0.cis.strath harris.cc.strat  3 u  507 1024  377  115.274   -5.025   1.642 
*clock.via.net   .GPS.            1 u  426 1024  377  107.424   -3.018   2.534 
 ntp1.conectiv.c 0.0.0.0         16 u    - 1024    0    0.000    0.000 4000.00 
+bonehed.lcs.mit .GPS.            1 u  984 1024  377   25.126    0.131  30.939 
-world.std.com   204.34.198.40    2 u  119 1024  377   24.229   -6.884   0.421 

The meaning of each column

remote
    Is the name of the remote NTP server. If you use the -n switch, you will
    see the IP addresses of these servers instead of their hostnames.
   
refid
    Indicates where each server is getting its time right now. It can be a
    server hostname or something like .GPS., indicating a Global Positioning
    System source.
   
st
    Stratum is a number from 1 to 16, to indicate the remote server
    precision. 1 is the most accurate, 16 means server unreachable. Your
    Stratum will be equal to the less accurate remote server plus 1.
   
poll
    The polling interval (in seconds) between time requests. The value will
    range between the minimum and maximum allowed polling values. Initially
    the value will be smaller to allow synchronization to occur quickly.
    After the clocks are 'in sync' the polling value will increase to reduce
    network traffic and load on popular time servers.
   
reach
    This is an octal representation of an array of 8 bits, representing the
    last 8 times the local machine tried to reach the server. The bit is set
    if the remote server was reached.
   
delay
    The amount of time (seconds) needed to receive a response for a "what
    time is it" request.
   
offset
    The most important value. The difference of time between the local and
    remote server. In the course of synchronization, the offset time lowers
    down, indicating that the local machine time is getting more accurate.
   
jitter
    Dispersion, also called Jitter, is a measure of the statistical variance
    of the offset across several successive request/response pairs. Lower
    dispersion values are preferred over higher dispersion values. Lower
    dispersions allow more accurate time synchronization.
   

The meaning of the signs before server hostname

-
    Means the local NTP service doesn't like this server very much
   
+
    Means the local NTP service likes this server
   
x
    Marks a bad host
   
*
    Indicates the current favorite
   

-----------------------------------------------------------------------------
4.5. Configure to Automatically Run NTP at Boot

You may want to have NTP running all the time even if you reboot your
machine. On each machine, do the following:
bash# chkconfig --level 2345 ntpd on                                         

This will ensure autostart.

If your machine is up and running for a long time (months, years) without
rebooting, you'll find a big discrepancy between the inaccurate hardware
clock and the (now very accurate) system time. Modern Linux distributions
copy OS time to the HC everytime the system is shutdown, using a mechanism
similar to the setclock command. This way, in the next OS boot, you'll get
date and time almost as accurate as it was when you shutdown.
-----------------------------------------------------------------------------

A. About this Document

Copyright 2002, Avi Alkalay.

This document must be distributed under the terms of [http://www.gnu.org/
copyleft/fdl.html] GNU Free Documentation License.

This document is published in the following locations:

*[http://avi.alkalay.net/linux/docs/TimePrecision/] Main distribution
    [[http://avi.alkalay.net/linux/docs/TimePrecision/timeprecision.tar.gz]
    XML Source]
   
*[http://en.tldp.org/HOWTO/TimePrecision-HOWTO/] LinuxDoc, as a HOWTO
    [[http://www.ibiblio.org/pub/Linux/docs/HOWTO/other-formats/html_single/
    TimePrecision-HOWTO.html] single page] [[http://www.ibiblio.org/pub/Linux
    /docs/HOWTO/other-formats/pdf/TimePrecision-HOWTO.pdf] PDF]
   

