RHEL 7 NTP configuration

• Introduction
• Server hardware clock
• Using NTP for synchronizing time for all managed server
• New utilities and daemon for managing time on RHEL7
• Using date command for checking and converting system time
• Setting hardware clock time via hwclock command
• Using timedatectl to check if you system is configured correctly
• Cheatsheet for configuring NTP in RHEL7
• Checking Cronyd status via systemctl
• Setting proper time zone
• Using Graphical Tools to Manage Time

Introduction

Most client-server applications depends on synchronization of time on all servers with central time services provider. Unsynchronized server is a source of very difficult to troubleshoot errors, so the correctness of NTP configuration should be changed first in any such troubleshooting. So it is very important that this daemon is configured correctly and NTP works as expected. That main difficulties happen if you you are behind firewall or proxy.

RHEL7 introduced a new chrony daemon and new utilities for managing NTP.

RHEL 7 changes default provide of NTP to chrony, which create the need to learn this new daemon. The idea behind chrony is that most sever do not need full fledged NTP daemon as they are consumer of those servers and never the providers.

TIP:

While on RHEL 7, chrony is the default solution to manage network time. NTP can still be implemented through the old ntpd, daemon. If you have services that need ntpd, you are free to use it, but you should realize that in that case you cannot use the timedatectl command.

Server hardware clock

Each server contains a hardware clock. Typically, it is an integrated circuit on the system board that is completely independent of the current state of the operating system and keeps running even when the computer is shut down. From the hardware clock, the system gets its initial time setting during the boot.

The time on the hardware clock on Linux servers is usually set to universal time coordinated (UTC). UTC is a time that is the same everywhere on the planet, and based on UTC, the current local time is calculated. To do this we need to provide offset called the time zone.

System time is a time maintained by the operating system. Once the system has booted, the system clock became independent of the hardware clock. The system also can synchronize hardware c lock with the external source of precise time -- the time server. Time server can be external or internal. For large corporation it is typically internal.

System time is a time that is maintained by the operating system and it is kept in UTC. Applications running on the server are converting system time into local time. Local time is the actual time in the current time zone. Moreover there are twice a year changes (to and from winter time), which are called daylight savings time (DST) which moves the local time one hour and creates a lot of difficulties.

Using NTP for synchronizing time for all managed server

Network Time Protocol (NTP). NTP is a method of maintaining system time that is provided through NTP servers on the Internet. It is an easy solution to provide an accurate time to servers, because most servers are connected to the Internet anyway.

NTP was developed by David Mills and others at the University of Delaware as a means for solving the problem of time synchronization between different servers connected to the Internet. Detailed information about NTP, version of the protocol and corresponding RFCs can be found at www.ntp.org

NTP runs on UDP port 123. The NTP servers advertise every 64 seconds, by means of a multicast address (224.0.1.1), that they are NTP servers. Any NTP client that is not configured with the unicast address of an NTP server multicasts . The NTP client sends request packets to all the NTP servers that it knows using their unicast addresses. Included in the request packet is the client’s local time. The NTP server replies by inserting UTC time into the return packet. The client compares its original request time with its own time when it receives the response from the server. This allows the client to determine how long the packet was in transit on the network.

Despite trivial semantic NTP is a rather complex protocol. Servers are organized into hierarchical levels called stratum that act in a hierarchy.

• Stratum 1 is the highest hierarchical level and those servers are directly attached to highly accurate reference sources such as atomic clocks. This is backbone of NTP and regular servers should not interface with backbone directly, but using NTP servers on lower stratums .
• Stratum 2 servers tend to operate out of academic institutions and large corporations. Those can be used by "regular" Internet-connected servers but any sizable organization usually has its own "local" NTP server(s) that interfaces with Stratum 2 servers.
• Below Stratum 2 you will find local NTP server that most organizations on the Internet now have. In the past most organization used free xntp daemon, but some prefer using appliances. Appliances often provide alternative synchronization case of lost internet connectivity using GPS.

Several additional terms are used when describing NTP-related topics:

• clock A clock that provides current time by accurately following a time standard, such as Universal Time Coordinate (UTC).
• Strata NTP servers are arranged in a hierarchy of levels, called strata. A stratum-1 server is more accurate than a stratum-2 server. There are 15 strata, but only first three are usually used.
• Resolution The smallest increment in time that a clock offers. For example, a wristwatch usually has a resolution of one second.
• Precision The smallest increase in time that a computer program can use.
• Jitter The difference of the differences experienced when repeatedly measuring time.
• Accuracy How close a clock follows an official time reference, such as UTC.
• Reliability The length of time that a clock can remain accurate within a specified range.
• Wander All clocks suffer from frequency variations. This variation is called wander.
• Drift file A file that contains the frequency offset of the local system’s clock oscillator. Drift file contents can be used by protocols, like NTP, to cause a system’s clock to be more accurate.
• Discipline A general term used for various actions carried out by some protocol, which helps keep a local clock better synchronized to an official time source, such as UTC.
• Tick. The Sun system central processing units (CPUs) generate the regular interrupts. By default, 100 interrupts are generated per second. For the system’s counter to increment, the CPUs interrupt must be processed by the kernel. Each interrupt that gets processed is known as a clock tick.

NTP client software is essentially nothing more then an intelligent corrector of the latency errors due to transmission via IP. All modern OSes provide daemons for NTP protocol but the devil in in details. RHEL 6 used different daemon then RHEL 7.

Setting up a server to use NTP time on RHEL 7 means that you need to put into the file /etc/chrony.conf the list of NTP servers that should be used.

You can switch on NTP and provided initial set of NTP servers in Anaconda during the installation and this is preferable way to doing this task. If you made a mistake or the situation changed you need manually switch on NTP, by using timedatectl set-ntp 1 and correct the set proper NTP servers. This is especially important if the server is behind firewall or proxy, because in this case default server do not work -- they are not assessable.

New utilities and daemon for managing time on RHEL7

RHEL 7 introduced a new set of command for managing NTP. Old books describing RHEL6 techniques of managing NTP for the most part are no longer valid

On a Linux system, time is calculated as an offset of epoch time. Epoch time is the number of seconds since January 1, 1970, in UTC. In some logs (such as /var/log/audit/audit.log), you’ll find time stamps in epoch time and not in human time. To convert such an epoch time stamp to human time, you can use the --date option, followed by the epoch string that is starting with an @:

date --date '@1420987251'

The use of epoch time is also creating a potential timing problem on Linux. On a 32-bit system, the number of seconds that can be counted in the field that is reserved for time notation is finished in 2037. But at this point hardware that uses 32 bit presentation will long be obsolete so this is not a problem.

Using date command for checking and converting system time

Date command is a lone survivor from older version on Linux and UNIX. While most sysadmin know about the date command existence, they often do not suspect what additional capabilities it has.

Like most Linux commands it can do more than a typical sysadmin expects. For example, you can also use it to show the current time in different formats. Some useful examples of date are listed here:

 date              # Shows the current system time
 date +%d-%m-%y    # Shows the current system day of month, month, and year
 date -s 16:03     # Sets the current time to 3 minutes past 4 p.m.

Setting hardware clock time via hwclock command

The date command enables you to set and show the current system time. Using the date command will not change the hardware time that is used on your system.

To synchronize hardware clock with system clock , you can use the hwclock command. The hwclock command has many options. Among most useful:

 hwclock -c         # shows the difference between hardware time and system time. The output refreshed every 10 seconds
 hwclock --systohc  # synchronizes current system time to the hardware clock.
 hwclock --hctosys  # synchronizes current hardware time to the system clock.

[root@server1 ~]# hwclock -c
hw-time      system-time         freq-offset-ppm   tick
1428584002   1428584002.011018
1428584012   1428584012.033019              2200     22
1428584022   1428584022.054953              2197     22
1428584032   1428584032.083572              2418     24
1428584042   1428584042.111683              2517     25

Using timedatectl to check if you system is configured correctly

This is new command introduced in RHEL 7. It enables you to manage many aspects of time is When used without any arguments, timedatectl shows detailed information about the current time and date. It also displays the time zone your system is in, in addition to information about the use of NTP network time and information about the use of DST.

Click here to view code image

[root@localhost ~]# timedatectl
      Local time: Sun 2015-01-11 10:02:41 EST
  Universal time: Sun 2015-01-11 15:02:41 UTC
        RTC time: Sun 2015-01-11 15:02:51
        Timezone: America/New_York (EST, -0500)
     NTP enabled: n/a
NTP synchronized: no
 RTC in local TZ: no
      DST active: no
 Last DST change: DST ended at
                  Sun 2014-11-02 01:59:59 EDT
                  Sun 2014-11-02 01:00:00 EST
 Next DST change: DST begins (the clock jumps one hour forward) at
                  Sun 2015-03-08 01:59:59 EST
                  Sun 2015-03-08 03:00:00 EDT

The timedatectl command works with commands to perform time operations.

The timedatectl command was developed as a generic solution to manage time on RHEL 7. It has some functions that are offered through other commands, but the purpose of the command is that eventually it will replace other commands used for managing time and date settings.

When timedatectl is used to switch on NTP time, it talks to the chronyd process.

Cheatsheet for configuring NTP in RHEL7

1. Open a root shell and type date.
2. Now type hwclock and see whether both commands are showing more or less the same time.
3. Type hwclock -c. Notice that this shows differences between system time and hardware time in much more detail. Use Ctrl+C to interrupt.
4. You have just seen that the current time displayed by hwclock -c is in epoch time.
5. Use date -d ‘@12345678’ (in which you replace 12345678 with the epoch time hwclock -c has just shown) to translate the current epoch time to human-readable time.
6. Type timedatectl status to show current time settings.
7. Use timedatectl list-timezones to show a list of all time zone definitions.
8. Use timedatectl set-timezone Europe/Amsterdam to set the current time zone to Amsterdam.
9. Type timedatectl show and notice the differences with the previous output.
10. Type timedatectl set-ntp 1 to switch on NTP use. You might see the error “failed to issue method call.” If you get this message, type yum -y install chrony and try again.
11. Open the configuration file /etc/chrony.conf and look up the server lines. These are used to specify the servers that should be used for NTP time synchronization.
12. Type systemctl status chronyd and verify that the chrony service is started and enabled. If this is not the case, use systemctl start chronyd; systemctl enable chronyd to make sure that it is operational.
13. Type systemctl status -l chronyd and read the status information.

Checking Cronyd Status via systemctl

Click here to view code image

[root@localhost system]# systemctl status -l chronyd
chronyd.service - NTP client/server
   Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled)
   Active: active (running) since Sun 2015-01-11 10:20:15 EST; 2min 14s
ago
  Process: 13938 ExecStartPost=/usr/libexec/chrony-helper add-dhclient-
servers (code=exited, status=0/SUCCESS)
  Process: 13935 ExecStart=/usr/sbin/chronyd -u chrony $OPTIONS
(code=exited, status=0/SUCCESS)
 Main PID: 13937 (chronyd)
   CGroup: /system.slice/chronyd.service
           13937 /usr/sbin/chronyd -u chrony

Jan 11 10:20:15 localhost.localdomain chronyd[13937]: chronyd version
1.29.1 starting
Jan 11 10:20:15 localhost.localdomain chronyd[13937]: Linux kernel
major=3 minor=10 patch=0
Jan 11 10:20:15 localhost.localdomain chronyd[13937]: hz=100 shift_hz=7
freq_scale=1.00000000 nominal_tick=10000 slew_delta_tick=833 max_tick_
bias=1000 shift_pll=2
Jan 11 10:20:15 localhost.localdomain chronyd[13937]: Generated key 1
Jan 11 10:20:15 localhost.localdomain systemd[1]: Started NTP client/
server.
Jan 11 10:20:20 localhost.localdomain chronyd[13937]: Selected source
178.21.23.127
Jan 11 10:20:20 localhost.localdomain chronyd[13937]: System clock
wrong by 11.256802 seconds, adjustment started
Jan 11 10:20:31 localhost.localdomain chronyd[13937]: System clock was
stepped by 11.257 seconds
    Jan 11 10:20:33 localhost.localdomain chronyd[13937]: Selected
    source 87.195.109.207

Setting proper time zone

Between Linux servers, time is normally communicated in UTC. This allows servers across different time zones to all use the same time settings, which makes managing time in large organizations a lot easier. To make it easier for end users, though, the local time must also be set. To do this, the appropriate time zone needs to be selected.

On Red Hat Enterprise Linux 7, you have four approaches to setting the correct local time zone:

Use the system-config-date utility as discussed in the next section of this chapter.

Go to the directory /usr/share/zoneinfo. In this directory, you’ll find different subdirectories containing files for each of the time zones that has been defined. To set the local time zone on a server, you can create a symbolic link with the name /etc/localtime to the time zone file that is involved. If you want to set local time to Los Angeles time, for instance, use ln -sf /usr/share/zoneinfo/America/Los_Angeles /etc/localtime.

Use the tzselect utility. This tool starts the interface , from which the appropriate region and locale can be selected.

Use timedatectl to set the time zone information.

Using Graphical Tools to Manage Time

If your server is configured with a graphical interface, you can use the graphical tool to manage time. To do so, follow these steps:

1. On the graphical display, click the current time that is shown in the upper-right corner.
2. On the screen that opens, click Date & Time Settings.
3. A new screen opens. To get access to the options that allow you to change time, click Unlock.
4. From the screen you see now, you can switch network time on and off and change the current time zone setting as well as the current time.

Alternatively, you can start the graphical utility to manage time by using the system-config-date command. The interface of this utility is similar to what you see in Anaconda during the initial installation of Red Hat.

[root@server1 ~]# tzselect
Please identify a location so that time zone rules can be set
correctly.
Please select a continent or ocean.
 1) Africa
 2) Americas
 3) Antarctica
 4) Arctic Ocean
 5) Asia
 6) Atlantic Ocean
 7) Australia
 8) Europe
 9) Indian Ocean
10) Pacific Ocean
11) none - I want to specify the time zone using the Posix TZ
format.
#? 2
Please select a country.
 1) Anguilla                 28) Haiti
 2) Antigua & Barbuda        29) Honduras
 3) Argentina                30) Jamaica
 4) Aruba                    31) Martinique
 5) Bahamas                  32) Mexico
 6) Barbados                 33) Montserrat
 7) Belize                   34) Nicaragua
 8) Bolivia                  35) Panama
 9) Brazil                   36) Paraguay
10) Canada                   37) Peru
11) Caribbean Netherlands    38) Puerto Rico
12) Cayman Islands           39) St Barthelemy
13) Chile                    40) St Kitts & Nevis
14) Colombia                 41) St Lucia
15) Costa Rica               42) St Maarten (Dutch part)
16) Cuba                     43) St Martin (French part)
17) Curacao                  44) St Pierre & Miquelon
18) Dominica                 45) St Vincent
19) Dominican Republic       46) Suriname
20) Ecuador                  47) Trinidad & Tobago
21) El Salvador              48) Turks & Caicos Is
22) French Guiana            49) United States
23) Greenland                50) Uruguay
24) Grenada                  51) Venezuela
25) Guadeloupe               52) Virgin Islands (UK)
26) Guatemala                53) Virgin Islands (US)
27) Guyana
#?

NEWS CONTENTS

• 20210528 : How to synchronize time with NTP using systemd-timesyncd daemon ( May 16, 2021 , linuxiac.com )
• 20200209 : How To Install And Configure Chrony As NTP Client ( Feb 09, 2020 , www.2daygeek.com )
• 20191212 : Use timedatectl to Control System Time and Date in Linux ( Dec 12, 2019 , www.maketecheasier.com )
• 20191212 : Set Time-Date-Timezone using Command Line in Linux ( Dec 12, 2019 , linoxide.com )

Old News ;-)

[May 28, 2021] How to synchronize time with NTP using systemd-timesyncd daemon

May 16, 2021 | linuxiac.com

The majority of Linux distributions have adopted systemd, and with it comes the systemd-timesyncd daemon. That means you have an NTP client already preinstalled, and there is no need to run the full-fledged ntpd daemon anymore. The built-in systemd-timesyncd can do the basic time synchronization job just fine.

To check the current status of time and time configuration via timedatectl and timesyncd, run the following command.

timedatectl status

               Local time: Thu 2021-05-13 15:44:11 UTC
           Universal time: Thu 2021-05-13 15:44:11 UTC
                 RTC time: Thu 2021-05-13 15:44:10
                Time zone: Etc/UTC (UTC, +0000)
System clock synchronized: yes
              NTP service: active
          RTC in local TZ: no

If you see NTP service: active in the output, then your computer clock is automatically periodically adjusted through NTP.

If you see NTP service: inactive , run the following command to enable NTP time synchronization.

timedatectl set-ntp true

That's all you have to do. Once that's done, everything should be in place and time should be kept correctly.

In addition, timesyncd itself is still a normal service, so you can check its status also more in detail via.

systemctl status systemd-timesyncd

systemd-timesyncd.service - Network Time Synchronization
      Loaded: loaded (/usr/lib/systemd/system/systemd-timesyncd.service; enabled; vendor preset: enabled) 
      Active: active (running) since Thu 2021-05-13 18:55:18 EEST; 3min 23s ago
      ...

If it is disabled, you can start and make systemd-timesyncd service active like this:

systemctl start systemd-timesyncd
systemctl enable systemd-timesyncd

How to change timezone

Before changing your time zone, start by using timedatectl to find out the currently set time zone.

timedatectl

               Local time: Thu 2021-05-13 16:59:32 UTC
           Universal time: Thu 2021-05-13 16:59:32 UTC
                 RTC time: Thu 2021-05-13 16:59:31
                Time zone: Etc/UTC (UTC, +0000)
System clock synchronized: yes
              NTP service: inactive
          RTC in local TZ: no

Now let's list all the available time zones, so you know the exact name of the time zone you'll use on your system.

timedatectl list-timezones

The list of time zones is quite large. You do need to know the official time-zone name for your location. Say you want to change the time zone to New York.

timedatectl set-timezone America/New_York

This command creates a symbolic link for the time zone you choose from /usr/share/zoneinfo/ to /etc/localtime .

In addition, you can skip the command shown above, create this symbolic link manually and achieve the same result.

ln -s /usr/share/zoneinfo/America/New_York /etc/localtime

[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 | www.2daygeek.com

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 (CentOS7.2daygeek.com)
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               
===============================================================================
^* CentOS7.2daygeek.com          2   6    17    62    +36us[+1230us] +/- 1111ms

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

Dec 12, 2019 | www.maketecheasier.com

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

Conclusion

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 | linoxide.com

Set Time/Date/Timezone in Ubuntu Linux February 5, 2019 Updated September 27, 2019 By Pungki Arianto LINUX COMMANDS , LINUX HOWTO

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

Output

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

$ date --date="2 days ago"

Output

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

or

# timedatectl  status

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"

Where,
• 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
12/05/13

%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.

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"

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

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:

• Tuptime - Tool to Display Uptime History of Linux System
• How to Synchronize Time using NTP Server in Ubuntu
• How to Take Linux System Snapshots with Timeshift

Recommended Links

• date command
• How To Verify NTP Setup (Sync) is Working or Not In Linux 2daygeek.com
• How to configure NTP server and client in CentOS - RHEL 7 – The Geek Diary
• 19.17. Configure NTP Red Hat Enterprise Linux 7 Red Hat Customer Portal

chrony – Comparison of NTP implementations

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

General information

• Network Time Protocol Setup - Red Hat Customer Portal
• How do I configure the ntpd service in Red Hat Enterprise Linux to function as an NTP time server for a network of NTP clients
• Red Hat Linux Configure an NTP Client And Server
• ntp.org- Home of the Network Time Protocol
• Network Time Protocol - Wikipedia, the free encyclopedia
• CentOS-5.2 Deployment Guide (Online | PDF)
• Guide to the Secure Configuration of Red Hat Enterprise Linux 5
• ntpd - Wikipedia, the free encyclopedia
• Network Time Protocol - Wikipedia, the free encyclopedia
• Solaris NTP -- Network Time Protocol

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Last modified: February 10, 2020