RTC(Real Time Clock)

Real Time Clock

RTC stands for real time clock, also known as hardware clock. This clock is powered by CMOS battery of your computer’s motherboard. This clock runs all the time even if you shutdown your computer. Sometimes RTC time is incorrect.

To find out RTC time on your computer, run this command:

timedatectl

Output:

Local time: Wed 2016-03-02 17:47:19 CST  Universal time: Wed 2016-03-02 09:47:19 UTC  RTC time: Wed 2016-03-02 09:47:19  Time zone: Asia/Shanghai (CST, +0800)  Network time on: yes  NTP synchronized: yes  RTC in local TZ: no

As you can see, my local time is now 17:49:19, but RTC time is 09:47:19. RTC time is not in my local time zone and sometimes this can causes problems if the RTC time is inaccurate. RTC time is unreliable.

To let your system read RTC time in UTC standard, execute the following command.

timedatectl set-local-rtc 0

To let your system read RTC time in local time zone, run this command

timedatectl set-local-rtc 1

It’s recommended your system read RTC time in UTC standard to prevent unexpected behavior.

Synchronize System Clock to Your Time Zone

Sometimes, your system clock can still be incorrect even if you set the correct time zone. To make sure your system clock is synchronized to your local time zone, you need NTP.

NTP’s job is to provide accurate time on your network and there are many service that rely on accurate time to function correctly. NTP synchronize clock across your network so that time is as accurate as possible. NTP is an application layer protocol and it use UDP as the transport layer protocol. Network time servers get their time from atomic clocks.

On systemd you can enable NTP synchronization using timedatectl.

sudo timedatectl set-ntp true

The above command will start and enable the systemd-timesyncd service. Now check status

timedatectl status

output:

Local time: Wed 2016-03-02 18:30:35 CST Universal time: Wed 2016-03-02 10:30:35 UTC RTC time: Wed 2016-03-02 10:30:35 Time zone: Asia/Shanghai (CST, +0800) Network time on: yes NTP synchronized: yes RTC in local TZ: no

You can see that network time is on and NTP synchronization is enabled. To check the status of timesyncd:

sudo systemctl status systemd-timesyncd

Output:

● systemd-timesyncd.service - Network Time Synchronization   Loaded: loaded (/usr/lib/systemd/system/systemd-timesyncd.service; enabled; vendor preset: enabled)   Acti; ve: active (running) since Wed 2016-03-02 18:30:29 CST; 1h 37min ago     Docs: man:systemd-timesyncd.service(8) Main PID: 2243 (systemd-timesyn)   Status: "Synchronized to time server 202.112.29.82:123 (0.arch.pool.ntp.org)."    Tasks: 2 (limit: 512)   CGroup: /system.slice/systemd-timesyncd.service   └─2243 /usr/lib/systemd/systemd-timesyncd

source : https://www.linuxbabe.com/desktop-linux/set-time-zone-and-synchronize-system-clock-to-your-time-zone-in-linux

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Introduction

Accurate timekeeping has become a critical component of modern software deployments. Whether it's making sure logs are recorded in the right order or database updates are applied correctly, out-of-sync time can cause errors, data corruption, and other hard to debug issues.

Ubuntu 16.04 has time synchronization built in and activated by default using systemd's timesyncd service. In this article we will look at some basic time-related commands, verify that timesyncd is active, and learn how to install an alternate network time service.

Prerequisites

Before starting this tutorial, you will need an Ubuntu 16.04 server with a non-root, sudo-enabled user, as described in this Ubuntu 16.04 server setup tutorial.

Navigating Basic Time Commands

The most basic command for finding out the time on your server is date. Any user can type this command to print out the date and time:

date

Output

Wed Apr 26 17:44:38 UTC 2017

Most often your server will default to the UTC time zone, as highlighted in the above output. UTC is Coordinated Universal Time, the time at zero degrees longitude. Consistently using Universal Time reduces confusion when your infrastructure spans multiple time zones.

If you have different requirements and need to change the time zone, you can use the timedatectl command to do so.

First, list the available time zones:

timedatectl list-timezones

A list of time zones will print to your screen. You can press SPACE to page down, and b to page up. Once you find the correct time zone, make note of it then type q to exit the list.

Now set the time zone with timedatectl set-timezone, making sure to replace the highlighted portion below with the time zone you found in the list. You'll need to use sudo with timedatectl to make this change:

sudo timedatectl set-timezone Asia/Seoul

You can verify your changes by running date again:

date

Output

Wed Apr 26 13:55:45 EDT 2017

The time zone abbreviation should reflect the newly chosen value.

Now that we know how to check the clock and set time zones, let’s make sure our time is being synchronized properly.

Controlling timesyncd with timedatectl

Until recently, most network time synchronization was handled by the Network Time Protocol daemon or ntpd. This server connects to a pool of other NTP servers that provide it with constant and accurate time updates.

Ubuntu's default install now uses timesyncd instead of ntpd. timesyncd connects to the same time servers and works in roughly the same way, but is more lightweight and more integrated with systemd and the low level workings of Ubuntu.

We can query the status of timesyncd by running timedatectl with no arguments. You don't need to use sudo in this case:

timedatectl

Output

Local time: Wed 2017-04-26 17:20:07 UTC   Universal time: Wed 2017-04-26 17:20:07 UTC         RTC time: Wed 2017-04-26 17:20:07        Time zone: Etc/UTC (UTC, +0000)  Network time on: yes NTP synchronized: yes

 RTC in local TZ: no

This prints out the local time, universal time (which may be the same as local time, if you didn't switch from the UTC time zone), and some network time status information. Network time on: yes means that timesyncd is enabled, and NTP synchronized: yes indicates that the time has been successfully synced.

If timesyncd isn't enabled, turn it on with timedatectl:

sudo timedatectl set-ntp on

Run timedatectl again to confirm the network time status. It may take a minute for the actual sync to happen, but eventually both Network time on: and NTP synchronized: should read yes.

Switching to ntpd

Though timesyncd is fine for most purposes, some applications that are very sensitive to even the slightest perturbations in time may be better served by ntpd, as it uses more sophisticated techniques to constantly and gradually keep the system time on track.

Before installing ntpd, we should turn off timesyncd:

sudo timedatectl set-ntp no

Verify that timesyncd is off:

timedatectl

Look for Network time on: no in the output. This means timesyncd has been stopped. We can now install the ntp package with apt-get:

sudo apt-get install ntp

ntpd will be started automatically after install. You can query ntpd for status information to verify that everything is working:

sudo ntpq -p

Output

remote           refid      st t when poll reach   delay   offset  jitter

====================================================

0.ubuntu.pool.n .POOL.          16 p    -   64    0    0.000    0.000   0.000

1.ubuntu.pool.n .POOL.          16 p    -   64    0    0.000    0.000   0.000

2.ubuntu.pool.n .POOL.          16 p    -   64    0    0.000    0.000   0.000

3.ubuntu.pool.n .POOL.          16 p    -   64    0    0.000    0.000   0.000

ntp.ubuntu.com  .POOL.          16 p    -   64    0    0.000    0.000   0.000

-makaki.miuku.ne 210.23.25.77     2 u   45   64    3  248.007   -0.489   1.137

-69.10.161.7     144.111.222.81   3 u   43   64    3   90.551    4.316   0.550

+static-ip-85-25 130.149.17.21    2 u   42   64    3   80.044   -2.829   0.900

+zepto.mcl.gg    192.53.103.108   2 u   40   64    3   83.331   -0.385   0.391

ntpq is a query tool for ntpd. The -p flag asks for information about the NTP servers (or peers) ntpd has connected to. Your output will be slightly different, but should list the default Ubuntu pool servers plus a few others. Bear in mind that it can take a few minutes for ntpd to establish connections.

Conclusion

In this article we’ve shown how to view the system time, change time zones, work with Ubuntu's default timesyncd, and install ntpd. If you have more sophisticated timekeeping needs than what we’ve covered here, you might reference the offical NTP documentation, and also take a look at the NTP Pool Project, a global group of volunteers providing much of the world's NTP infrastructure.

source : https://www.digitalocean.com/community/tutorials/how-to-set-up-time-synchronization-on-ubuntu-16-04