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News NTP -- Network Time Protocol Recommended Links RHEL 7 NTP configuration Linux Networking Configuration Troubleshooting NTP on Red Hat Linux
RHEL handling of DST change Troubleshooting NTP on Solaris ILO 3 NTP configuration Changing timezone in RHEL6 from the command line Humor Etc


NTP is built on the UDP protocol rather than TCP.  UDP raises different issues with firewalls compared with TCP. Most of the time server firewalls should allow NTP to work without special configuration. NTP daemon has its own capabilities to restrict access so duplication is harmful.


  1. Trobleshooting issues are discussed at Troubleshooting NTP on Red Hat Linux
  2. Red Hat documentation can be found at chapter 29 of Red Hat Deployment Guide
  3. 29. Date and Time Configuration
    29.1. Time and Date Properties
    29.2. Network Time Protocol (NTP) Properties
    29.3. Time Zone Configuration

Enabling ntpd daemon

In RHEL 5 ntpd behavior is controlled by the file /etc/ntp/ntpservers but it looks like it is not changed during the installation.  You need to change it manually or via GUI.

If you did not put your ntp servers during the installation you can change  /etc/ntp/ntpservers manually by  adding the lines to the end. For example

restrict mask nomodify notrap noquery
restrict mask nomodify notrap noquery

To ensure that  NTP server start up automatically on reboot, you need enble it via the chkconfig command as follows:

chkconfig ntpd on
chkconfig --list | grep ntpd
ntpd 0:off 1:off 2:off 3:on 4:on 5:on 6:off

To start the server you can use service command

service ntpd start

To see that your NTP server is listening on UDP port 123, use the following command:

netstat -tuna

Typically NTP is configured during installation.


There is an additional problem that can be experienced with HP servers that have ILO 3. ILO 3 allows server clocks be synchronized via NTP too, but few people put those setting and local clock can be significantly off. On reboot if local clock is mentioned in /etc/ntp.conf clocks NTP daemon use then as initial setup. And if the value is set to local clock discrepancy can't be remedies by NTP daemon and it quits. One solution is to remove local clock as a time source from /etc/ntp.conf.

How to check if ntpd is running correctly

To check if ntpd is running correctly you can use the command ntpq -p

ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================    3 u   59   64    1   23.555   -5.558   0.001    3 u   58   64    1    0.455   -1.511   0.001

How to set the date to value of NTP server

To set the date to value of NTP server you can use


See also Troubleshooting NTP on Red Hat Linux

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Old News ;-)

[Feb 09, 2020] How To Install And Configure Chrony As NTP Client

See also chrony – Comparison of NTP implementations
Another installation manual Steps to configure Chrony as NTP Server & Client (CentOS-RHEL 8)
Feb 09, 2020 |

It can synchronize the system clock faster with better time accuracy and it can be very much useful for the systems which are not online all the time.

Chronyd is smaller in size, it uses less system memory and it wakes up the CPU only when necessary, which is better for power saving.

It can perform well even when the network is congested for longer periods of time.

You can use any of the below commands to check Chrony status.

To check chrony tracking status.

# chronyc tracking

Reference ID    : C0A80105 (
Stratum         : 3
Ref time (UTC)  : Thu Mar 28 05:57:27 2019
System time     : 0.000002545 seconds slow of NTP time
Last offset     : +0.001194361 seconds
RMS offset      : 0.001194361 seconds
Frequency       : 1.650 ppm fast
Residual freq   : +184.101 ppm
Skew            : 2.962 ppm
Root delay      : 0.107966967 seconds
Root dispersion : 1.060455322 seconds
Update interval : 2.0 seconds
Leap status     : Normal

Run the sources command to displays information about the current time sources.

# chronyc sources

210 Number of sources = 1
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
^*          2   6    17    62    +36us[+1230us] +/- 1111ms

[Dec 12, 2019] Use timedatectl to Control System Time and Date in Linux

Dec 12, 2019 |

Mastering the Command Line: Use timedatectl to Control System Time and Date in Linux By Himanshu Arora – Posted on Nov 11, 2014 Nov 9, 2014 in Linux

The timedatectl command in Linux allows you to query and change the system clock and its settings. It comes as part of systemd, a replacement for the sysvinit daemon used in the GNU/Linux and Unix systems.

In this article, we will discuss this command and the features it provides using relevant examples.

Timedatectl examples

Note – All examples described in this article are tested on GNU bash, version 4.3.11(1).

Display system date/time information

Simply run the command without any command line options or flags, and it gives you information on the system's current date and time, as well as time-related settings. For example, here is the output when I executed the command on my system:

$ timedatectl
      Local time: Sat 2014-11-08 05:46:40 IST
  Universal time: Sat 2014-11-08 00:16:40 UTC
        Timezone: Asia/Kolkata (IST, +0530)
     NTP enabled: yes
NTP synchronized: yes
 RTC in local TZ: no
      DST active: n/a

So you can see that the output contains information on LTC, UTC, and time zone, as well as settings related to NTP, RTC and DST for the localhost.

Update the system date or time using the set-time option

To set the system clock to a specified date or time, use the set-time option followed by a string containing the new date/time information. For example, to change the system time to 6:40 am, I used the following command:

$ sudo timedatectl set-time "2014-11-08 06:40:00"

and here is the output:

$ timedatectl
      Local time: Sat 2014-11-08 06:40:02 IST
  Universal time: Sat 2014-11-08 01:10:02 UTC
        Timezone: Asia/Kolkata (IST, +0530)
     NTP enabled: yes
NTP synchronized: no
 RTC in local TZ: no
      DST active: n/a

Observe that the Local time field now shows the updated time. Similarly, you can update the system date, too.

Update the system time zone using the set-timezone option

To set the system time zone to the specified value, you can use the set-timezone option followed by the time zone value. To help you with the task, the timedatectl command also provides another useful option. list-timezones provides you with a list of available time zones to choose from.

For example, here is the scrollable list of time zones the timedatectl command produced on my system:


To change the system's current time zone from Asia/Kolkata to Asia/Kathmandu, here is the command I used:

$ timedatectl set-timezone Asia/Kathmandu

and to verify the change, here is the output of the timedatectl command:

$ timedatectl
      Local time: Sat 2014-11-08 07:11:23 NPT
  Universal time: Sat 2014-11-08 01:26:23 UTC
        Timezone: Asia/Kathmandu (NPT, +0545)
     NTP enabled: yes
NTP synchronized: no
 RTC in local TZ: no
      DST active: n/a

You can see that the time zone was changed to the new value.

Configure RTC

You can also use the timedatectl command to configure RTC (real-time clock). For those who are unaware, RTC is a battery-powered computer clock that keeps track of the time even when the system is turned off. The timedatectl command offers a set-local-rtc option which can be used to maintain the RTC in either local time or universal time.

This option requires a boolean argument. If 0 is supplied, the system is configured to maintain the RTC in universal time:

$ timedatectl set-local-rtc 0

but in case 1 is supplied, it will maintain the RTC in local time instead.

$ timedatectl set-local-rtc 1

A word of caution : Maintaining the RTC in the local time zone is not fully supported and will create various problems with time zone changes and daylight saving adjustments. If at all possible, use RTC in UTC.

Another point worth noting is that if set-local-rtc is invoked and the --adjust-system-clock option is passed, the system clock is synchronized from the RTC again, taking the new setting into account. Otherwise the RTC is synchronized from the system clock.

Configure NTP-based network time synchronization

NTP, or Network Time Protocol, is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. It is intended to synchronize all participating computers to within a few milliseconds of UTC.

The timedatectl command provides a set-ntp option that controls whether NTP based network time synchronization is enabled. This option expects a boolean argument. To enable NTP-based time synchronization, run the following command:

$ timedatectl set-ntp true

To disable, run:

$ timedatectl set-ntp false

As evident from the examples described above, the timedatectl command is a handy tool for system administrators who can use it to to adjust various system clocks and RTC configurations as well as poll remote servers for time information. To learn more about the command, head over to its man page .

[Dec 12, 2019] Set Time-Date-Timezone using Command Line in Linux

Dec 12, 2019 |

Set Time/Date/Timezone in Ubuntu Linux February 5, 2019 Updated September 27, 2019 By Pungki Arianto LINUX COMMANDS , LINUX HOWTO How to set time and time zone in ubuntu linux

Time is an important aspect in Linux systems especially in critical services such as cron jobs. Having the correct time on the server ensures that the server operates in a healthy environment that consists of distributed systems and maintains accuracy in the workplace.

In this tutorial, we will focus on how to set time/date/time zone and to synchronize the server clock with your Ubuntu Linux machine.

Check Current Time

You can verify the current time and date using the date and the timedatectl commands. These linux commands can be executed straight from the terminal as a regular user or as a superuser. The commands are handy usefulness of the two commands is seen when you want to correct a wrong time from the command line.

Using the date command

Log in as a root user and use the command as follows

$ date


check date using date command

You can also use the same command to check a date 2 days ago

$ date --date="2 days ago"


check date 2 days ago

Using timedatectl command

Checking on the status of the time on your system as well as the present time settings, use the command timedatectl as shown

# timedatectl


# timedatectl  status

how to set time

Changing Time

We use the timedatectl to change system time using the format HH:MM: SS. HH stands for the hour in 24-hour format, MM stands for minutes and SS for seconds.

Setting the time to 09:08:07 use the command as follows (using the timedatectl)

# timedatectl set-time 09:08:07
using date command

Changing time means all the system processes are running on the same clock putting the desktop and server at the same time. From the command line, use date command as follows

# date +%T -s "10:13:13"

• 10: Hour (hh)
• 13: Minute (mm)
• 13: Second (ss)

To change the locale to either AM or PM use the %p in the following format.

# date +%T%p -s "6:10:30AM"
# date +%T%p -s "12:10:30PM"
Change Date

Generally, you want your system date and time is set automatically. If for some reason you have to change it manually using date command, we can use this command :

# date --set="20140125 09:17:00"

It will set your current date and time of your system into 'January 25, 2014' and '09:17:00 AM'. Please note, that you must have root privilege to do this.

You can use timedatectl to set the time and the date respectively. The accepted format is YYYY-MM-DD, YYYY represents the year, MM the month in two digits and DD for the day in two digits. Changing the date to 15 January 2019, you should use the following command

# timedatectl set-time 20190115
Create custom date format

To create custom date format, use a plus sign (+)

$ date +"Day : %d Month : %m Year : %Y"
Day: 05 Month: 12 Year: 2013

$ date +%D

%D format follows Year/Month/Day format .

You can also put the day name if you want. Here are some examples :

$ date +"%a %b %d %y"
Fri 06 Dec 2013

$ date +"%A %B %d %Y"
Friday December 06 2013

$ date +"%A %B %d %Y %T"
Friday December 06 2013 00:30:37

$ date +"%A %B-%d-%Y %c"
Friday December-06-2013 12:30:37 AM WIB

List/Change time zone

Changing the time zone is crucial when you want to ensure that everything synchronizes with the Network Time Protocol. The first thing to do is to list all the region's time zones using the list-time zones option or grep to make the command easy to understand

# timedatectl list-timezones

The above command will present a scrollable format.

list time zones

Recommended timezone for servers is UTC as it doesn't have daylight savings. If you know, the specific time zones set it using the name using the following command

# timedatectl set-timezone America/Los_Angeles

To display timezone execute

# timedatectl | grep "Time"

check timezone

Set the Local-rtc

The Real-time clock (RTC) which is also referred to as the hardware clock is independent of the operating system and continues to run even when the server is shut down.

Use the following command

# timedatectl set-local-rtc 0

In addition, the following command for the local time

# timedatectl set-local-rtc 1
Check/Change CMOS Time

The computer CMOS battery will automatically synchronize time with system clock as long as the CMOS is working correctly.

Use the hwclock command to check the CMOS date as follows

# hwclock

check time using hwclock

To synchronize the CMOS date with system date use the following format

# hwclock –systohc

To have the correct time for your Linux environment is critical because many operations depend on it. Such operations include logging events and corn jobs as well. we hope you found this article useful.

Read Also:

Recommended Links

chrony – Comparison of NTP implementations

Steps to configure Chrony as NTP Server & Client (CentOS-RHEL 8)

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