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Managing Accurate Date and Time

Avi Alkalay

Brad Knowles - Suggestion to use and NTP's stratum 2 public time

Kent Borg - Suggestion to use ntpq instead of ntpdc

Yura Moron - Good explanations on ntpq and ntpdc info

Takeo Nakano -


Mihaly Gyulai -

Philippe Wautelet -

Kemal Ökmen -

Copyright � 2002,2003,2004,2005 Avi Alkalay

Revision History                                                             
Revision 2005-11-18      18 Nov 2005      Revised by: Avi                    
Included link to turkish translation                                         
Revision 2005-10-12      12 Oct 2005      Revised by: Avi                    
The Brazil zic file is now external and updated                              
Revision 2005-05-03      03 May 2005      Revised by: Avi                    
Included link to french translation                                          
Revision 1.0.8           2004-10-29       Revised by: Mihaly Gyulai          
Hungarian translation, the chorny section and the very first section         
Revision 1.0.7           9 Jul 2004       Revised by: avi                    
Disclaimer near to license, to let people know they can translate without    
asking permission.Small fixes in some phrases.                               
Revision 1.0.6           25 Feb 2004      Revised by: avi                    
Included credits and link to russian translation.                            
Revision 1.0.5           05 Jul 2003      Revised by: avi                    
Changed public time servers list URL. Added                    
Revision 1.0.4           21 Dec 2002      Revised by: avi                    
Added japanese translation link and credits.                                 
Revision 1.0.3           24 Aug 2002      Revised by: avi                    
Fixed wrong screen and programlisting widths.                                
Revision 1.0.2           04 Aug 2002      Revised by: avi                    
Link to public time servers page.                                            
Revision 1.0.1           07 May 2002      Revised by: avi                    
Final XML conversion. Files reorganization.                                  
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
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. Why do We Need a Precise Clock?
    1.1. On What Factors Does the Hardware-Clock Depend?
2. Computer Global Date and Time Concept
3. What are Time Zones?
    3.1. Daylight Savings Time
    3.2. Time Zones Examples
    3.3. Time Zone Mechanism on Linux
4. The Correct Settings for Your Linux Box
    4.1. Setting Time Zone
    4.2. Setting the Hardware Clock
5. Accurate Global Time Synchronization
    5.1. NTP: The Network Time Protocol
    5.2. Building a Simple Time Synchronization Architecture
    5.3. NTP Configurations
    5.4. Watching Your Box Synchronizing
    5.5. Configure to Automatically Run NTP at Boot
6. Precise Time with the chrony Program
    6.1. How chrony Differs from the ntp Suite?
    6.2. How to Use chrony?
A. About this Document

1. Why do We Need a Precise Clock?

If our computer never connects to other computers (or other devices that use
a clock), the precision of the clock is not critical itself, it depends on
the need of the user. However, programs that some way use the net are
dependent on a precise date and time. Some examples, when you may need
precise clock:

��*�Softwares that deal with transactions
��*�Commercial applications (e.g. eBay)
��*�Mail and messaging-related client and servers
��*�Websites that use cookies
��*�Distributed web applications
��*�Web services
��*�Distributed component-based applications as J2EE, .NET, etc
��*�Advanced modern and paralel filesystems, as AFS, DFS, GFS, GPFS, etc

And of course, to use the computer to adjust our wristwatch clock.

1.1. On What Factors Does the Hardware-Clock Depend?

Here we talk a little about the hardware-clock precision.

In PCs we find quartz-oscillators maintaining the hardware clock. The
frequency of the oscillator is divided, and at the end we get a counter
stepping once in one second (in reality it is more complicated, but now it's
enough for us). The clock-oscillator runs even if the computer is switched
off, so after starting the computer (and starting Linux) the hardware clock
can give the values of the actual time. The stability of this clock is mostly
dependent on the temperature of its surroundings, but it is also dependent on
the air-pressure and the stability of the power supply voltage. The hardware
clock is inaccurate in short term, however in the long term it shows a
certain difference from the exact time. As we continously can compare the
frequency of our hardware clock and an exact clock, we can calculate the
frequency of the hardware clock and so to create the exact time. If this
exact clock is inside your LAN (local area network), the accuracy of your
Linux machine clock is within 0.01 sec. If you use the internet for this
purpose, the accuracy of your clock will be within 0.2 sec regarding to the
exact time.

2. 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 its 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 independent from each other. So after a
while they may have some seconds of difference. Which one is correct? If you
don't make special configurations, none of them.

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

3. What are Time Zones?

Time Zones are a geographical 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 geographical, 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 3.2
for examples.

Use the [] to know
what time is it now at any part of the globe.

3.1. 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.

3.2. 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  |S�o 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 Zones.

3.3. 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,

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, São Paulo, Rio, Northeast, South etc                       
# BRWST: West of Brasilia. Mato Grosso, Manaus                                 
# BRAST: Acre.                                                                 
# In daylight saving time, letter 'S' changes to 'D'.                          
# All the brazilian daylight changes can be found here:                        
# 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:  
# # cd /etc; ln -sf /usr/share/zoneinfo/Brazil/Brasilia localtime              
# If you have updates and new standards to this file please send to            
# Avi Alkalay (avi @                                                  
# Fred Neves (fneves @                                            
# Last update: 12 Oct 2005                                                     
# This file is available at                                                    
# Rule  NAME    FROM    TO      TYPE    IN    ON      AT      SAVE    LETTER/S 
Rule    Brazil  1931    1932    -       Oct    3      00:00   1:00    D        
Rule    Brazil  1932    1933    -       Mar   31      00:00   0       S        
Rule    Brazil  1949    only    -       Dec    1      00:00   1:00    D        
Rule    Brazil  1950    only    -       Apr   30      00:00   0       S        
Rule    Brazil  1950    1952    -       Dec    1      00:00   1:00    D        
Rule    Brazil  1951    only    -       Apr   16      00:00   0       S        
Rule    Brazil  1952    only    -       Mar   31      00:00   0       S        
Rule    Brazil  1953    only    -       Feb   28      00:00   0       S        
Rule    Brazil  1963    only    -       Oct   23      00:00   1:00    D        
Rule    Brazil  1964    only    -       Mar    1      00:00   0       S        
Rule    Brazil  1965    only    -       Jan   31      00:00   1:00    D        
Rule    Brazil  1965    only    -       Mar   31      00:00   0       S        
Rule    Brazil  1965    only    -       Dec    1      00:00   1:00    D        
Rule    Brazil  1966    1968    -       Mar    1      00:00   0       S        
Rule    Brazil  1966    1967    -       Nov    1      00:00   1:00    D        
Rule    Brazil  1984    only    -       Nov    2      00:00   1:00    D        
Rule    Brazil  1985    only    -       Mar   15      00:00   0       S        
Rule    Brazil  1985    only    -       Nov    2      00:00   1:00    D        
Rule    Brazil  1986    only    -       Mar   15      00:00   0       S        
Rule    Brazil  1986    only    -       Oct   25      00:00   1:00    D        
Rule    Brazil  1987    only    -       Feb   14      00:00   0       S        
Rule    Brazil  1987    only    -       Oct   25      00:00   1:00    D        
Rule    Brazil  1988    only    -       Feb    7      00:00   0       S        
Rule    Brazil  1988    only    -       Oct   16      00:00   1:00    D        
Rule    Brazil  1989    only    -       Jan   29      00:00   0       S        
Rule    Brazil  1989    only    -       Oct   15      00:00   1:00    D        
Rule    Brazil  1990    only    -       Feb   11      00:00   0       S        
Rule    Brazil  1990    only    -       Oct   21      00:00   1:00    D        
Rule    Brazil  1991    only    -       Feb   17      00:00   0       S        
Rule    Brazil  1991    only    -       Oct   20      00:00   1:00    D        
Rule    Brazil  1992    only    -       Feb    9      00:00   0       S        
Rule    Brazil  1992    only    -       Oct   25      00:00   1:00    D        
Rule    Brazil  1993    only    -       Jan   31      00:00   0       S        
Rule    Brazil  1993    only    -       Oct   17      00:00   1:00    D        
Rule    Brazil  1994    only    -       Feb   20      00:00   0       S        
Rule    Brazil  1994    only    -       Oct   16      00:00   1:00    D        
Rule    Brazil  1995    only    -       Feb   19      00:00   0       S        
Rule    Brazil  1995    only    -       Oct   15      00:00   1:00    D        
Rule    Brazil  1996    only    -       Feb   11      00:00   0       S        
Rule    Brazil  1996    only    -       Oct   06      00:00   1:00    D        
Rule    Brazil  1997    only    -       Feb   16      00:00   0       S        
Rule    Brazil  1997    only    -       Oct   06      00:00   1:00    D        
Rule    Brazil  1998    only    -       Mar   01      00:00   0       S        
Rule    Brazil  1998    only    -       Oct   11      00:00   1:00    D        
Rule    Brazil  1999    only    -       Feb   21      00:00   0       S        
Rule    Brazil  1999    only    -       Oct    3      00:00   1:00    D        
Rule    Brazil  2000    only    -       Feb   27      00:00   0       S        
Rule    Brazil  2000    only    -       Oct    8      00:00   1:00    D        
Rule    Brazil  2001    only    -       Feb   18      00:00   0       S        
Rule    Brazil  2001    only    -       Oct   14      00:00   1:00    D        
Rule    Brazil  2002    only    -       Feb   17      00:00   0       S        
Rule    Brazil  2002    only    -       Nov    3      00:00   1:00    D        
Rule    Brazil  2003    only    -       Feb   16      00:00   0       S        
Rule    Brazil  2003    only    -       Oct   19      00:00   1       D        
Rule    Brazil  2004    only    -       Feb   15      00:00   0       S        
Rule    Brazil  2004    only    -       Nov    2      00:00   1       D        
Rule    Brazil  2005    only    -       Feb   20      00:00   0       S        
Rule    Brazil  2005    only    -       Oct   16      00:00   1       D        
Rule    Brazil  2006    only    -       Feb   19      00:00   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    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/East                    -3:00   Brazil        BR%sT             
Zone    posix/Brazil/East              -3:00   Brazil        BR%sT             
Zone    right/Brazil/East              -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 4.1.

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 command 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 2.

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

4. 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

4.1. 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

4.2. Setting the Hardware Clock

I encourage you to set your hardware clock only after understanding how to
get accurate time, described on Section 5.

The hwclock command reads and sets the hardware clock, based on several
options you give to it, documented in its man page. But you don't have to use
it if you have a modern Linux distribution. After defining your hardware
clock strategy and Time Zone, you can use the high level setclock command to
correctly set your hardware clock. 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 always use the setclock command.

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

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 "21 Oct 2004 21:17"  # 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 [] 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.

5. 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.

5.1. NTP: The Network Time Protocol

We won't discuss here the protocol, but how this wonderful invention, added
to the pervasivenes of the Internet, can be useful for us. You can find more
about it at []

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

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

5.2. 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. You can use the public server [http://], or choose some from the [http://] stratum 2 public time servers
    on NTP website. If you don't have an Internet access, your WAN
    administrator (must be a clever guy) can provide you some internal
 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://] make a search on [http:/

Here is an example of good architecture:

Figure 1. Local Relay Servers for NTP


If you have several machines to synchronize, do not make them all access the
remote NTP servers you chose. 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 a dedicated machine for it.

Tip It is a good idea to create hostname aliases for your local Relay Servers
    like and, 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 such aliases.                                                     

5.3. NTP Configurations

For Your Relay Servers
    Edit /etc/ntp.conf and add the remote servers you chose:
    Example 5. Relay machines' /etc/ntp.conf
    server     # A stratum 1 server at       
    server        # A stratum 2 server at     
    Again, you can use the public server [], or get a list of [
    clock2a.html] public stratum 2 time servers from NTP website.
For Your Clients
    Edit /etc/ntp.conf and add your Relay Servers with a standard name:
    Example 6. Client machines' /etc/ntp.conf
    server             # My first local relay                   
    server             # 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     (1)                                          
24 Mar 18:16:36 ntpdate[10254]: step time server offset -15.266188 sec 
bash# ntpdate     (2)                                          
24 Mar 18:16:43 ntpdate[10255]: adjust time server 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                                                   

5.4. 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 
==============================================================================   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  2 u  972 1024  377   38.206   19.589  28.028 
-dr-zaius.cs.wis  2 u  502 1024  357   55.098    3.979   0.333 
+taylor.cs.wisc.  2 u  454 1024  347   54.127    3.379   0.047 
-ntp0.cis.strath  3 u  507 1024  377  115.274   -5.025   1.642 
*   .GPS.            1 u  426 1024  377  107.424   -3.018   2.534 
 ntp1.conectiv.c         16 u    - 1024    0    0.000    0.000 4000.00 .GPS.            1 u  984 1024  377   25.126    0.131  30.939    2 u  119 1024  377   24.229   -6.884   0.421 

The meaning of each column

    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.
    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.
    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 accurate remote server plus 1. Never connect
    to a Stratum 1 server, use Stratum 2 servers! Stratum 2 servers are also
    good for our purposes, and this policy is good for reducing the traffic
    to the Stratum 1 servers.
    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.
    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.
    The amount of time (seconds) needed to receive a response for a "what
    time is it" request.
    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.
    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
    Marks a bad host
    Indicates the current favorite

5.5. 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 the machine.

6. Precise Time with the chrony Program

6.1. How chrony Differs from the ntp Suite?

chrony also uses the NTP protocol, and is also designed to make Linux clock
more accurate. It is also suitable for systems that do not have an Internet
connection. Then the source of the exact time can be any accurate clock, from
which we can read the time and type it to the program. In addition, it is
also capable of calculating the inaccuracy of the hardware clock, and based
on that, adjust the hardware clock at boot time.

chrony 1.20 does not support built-in hardware clocks like GPS and DCF
receivers, but the structure of the program makes such development possible.

6.2. How to Use chrony?

chrony consists of two parts: chronyd daemon and a user interface chronyc.

You can find chrony at []

A. About this Document

Copyright 2002, Avi Alkalay.

This document must be distributed under the terms of [
copyleft/fdl.html] GNU Free Documentation License. Please translate, adapt,
improve, redistribute using the original XML DocBook source right bellow. Let
me know if you want me to put a link to your translation/adaptation/
improvement here.

This document is published in the following locations:

��*�[] Official site [[http:/
    XML (DocBook) Source]
��*�[] TLDP, as a HOWTO [[http:/
    TimePrecision-HOWTO.html] single page] [[
    /docs/HOWTO/other-formats/pdf/TimePrecision-HOWTO.pdf] PDF]
��*�Philippe Wautelet <p.wautelet> [
    /HOWTO/lecture/TimePrecision-HOWTO.html] french translation [[http://
    TimePrecision-HOWTO.xml] XML (DocBook) Source]
��*�Takeo Nakano <nakano at> [
    JFdocs/TimePrecision-HOWTO/] japanese translation [[http://] txt version]
��*�SHAKI <sha-ki at> [
    linuxtime.htm] russian translation
��*�Mihaly Gyulai <gyulai kukac fbi pont hu> [
    TimePrecision-HOWTO-hu/index.html] Hungarian translation - magyar
��*�Kemal Ökmen <kemal AT comu edu tr> [
    time-precision-howto.html] Turkish translation - Türkçe çeviri

When translating, feel free to change all brazilian-related configuration
examples to your country's. And please send me an e-mail containing the
translator's name, e-mail address, the language that this doc was translated
to, the phrase "[YOUR LANGUAGE] translation" in your language, all UTF-8
encoded. This way I can put, in the original document, in your language, a
link to your translation. Also, please let the [] TLDP folks
be aware of your translation, so they can add it to the collection of HOWTOs
in your language.

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