time and localization

Time

  • the time is determined by four parts:
    • time value
    • time standard (localtime, UTC)
    • time zone (Asia/Tehran)
    • Daylight Saving Time (DST)
  • Hardware clock - Real Time Clock (RTC) - CMOS clock [N01]
    • Only have the ability to store time: year, month, day, hour, minute, and second
    • No timezone, DST or standard
In [6]:
timedatectl

      Local time: Wed 2017-03-22 16:20:13 IRDT
  Universal time: Wed 2017-03-22 11:50:13 UTC
        RTC time: Wed 2017-03-22 11:50:12
       Time zone: Asia/Tehran (IRDT, +0430)
 Network time on: yes
NTP synchronized: yes
 RTC in local TZ: no

Standards

In [2]:
sudo hwclock --show --localtime # do not adjust to local timezone. (it's already localtime)

It will display what the current time is according to the hardware clock.
If this time matches whatever your watch says, then the hardware clock is set to local time.
If the output from hwclock is not local time, chances are it is set to UTC time.

systemd-timedated reads /etc/adjtime, and depending on the contents of the file, it sets the clock to either UTC or local time.
If /etc/adjtime isn't present at boot, systemd-timedated will assume that hardware clock is set to UTC.

When a os uses UTC it will consider CMOS time as a UTC then makes an adjustment to it.

when using localtime:

In [ ]:
sudo timedatectl set-local-rtc true

cat /etc/adjtime # used by hwclock

In [ ]:
sudo timedatectl set-local-rtc false # do not use localtime -> Set to UTC
In [5]:
cat /etc/adjtime

cat: /etc/adjtime: No such file or directory





















Why UTC
    

  • Multiple operating systems
      

    • All should use same standard
        

      • conflict when turning off
        • 

      

      • 

    • when using localtime more than one operating system may adjust it after a DST
      • 

    • Moving between timezones and using one of the operating systems to reset the system/hardware clock. 
      • 

    

    • 
















NTP (network time protocol)
Network Time Protocol is a networking protocol for clock synchronization between computer systems













In [17]:



    


grep -i ntp /etc/systemd/timesyncd.conf



    















    






#NTP=
#FallbackNTP=ntp.ubuntu.com



















In [ ]:



    


NTP=ir.pool.ntp.org



    













In [23]:



    


systemctl is-enabled systemd-timesyncd.service



    















    






disabled










    
























In [1]:



    


sudo timedatectl set-ntp true



    













In [10]:



    


systemctl is-enabled systemd-timesyncd.service



    















    






enabled



















In [11]:



    


timedatectl status | grep -i ntp



    















    






NTP synchronized: yes





















Timezone
tzdata












In [ ]:



    


sudo dpkg-reconfigure tzdata



    















<img src="img/dpkg-rec-tzdata.png" /img>













In [1]:



    


cat /etc/timezone



    















    






Asia/Tehran



















In [2]:



    


file /etc/localtime



    















    






/etc/localtime: symbolic link to /usr/share/zoneinfo/Asia/Tehran





















We will get back to these files for manual configuration.















timedatectl












In [10]:



    


timedatectl list-timezones | grep -i tehran



    















    






Asia/Tehran



















In [11]:



    


sudo timedatectl set-timezone Asia/Tehran



    













In [13]:



    


timedatectl status | grep zone



    















    






       Time zone: Asia/Tehran (IRST, +0330)



















In [12]:



    


timedatectl status | grep zone # In DST



    















    






       Time zone: Asia/Tehran (IRDT, +0430)





















tzselect
It comes handy when you want to know what time it is in other countries, or if you just wonder what timezones exist.













In [ ]:



    


tzselect



    















<img src="img/tzselect.png" /img>















    

  • You can make this change permanent for yourself by appending the line:
      

    • TZ='Asia/Tehran'; export TZ to the file '.profile' in your home directory; then log out and log in again.  
      • 

    

    • 



Here is that TZ value again, this time on standard output so that you can use the /usr/bin/tzselect command in shell scripts:

Asia/Tehran













In [ ]:



    


tzselect 2> /dev/null



    













In [ ]:



    


4
16
1
Asia/Tehran



    













In [8]:



    


export TZ=Europe/Germany



    













In [9]:



    


date



    















    






Wed Mar 22 12:59:35 Europe 2017



















In [10]:



    


export TZ=Asia/Tehran



    













In [11]:



    


date



    















    






Wed Mar 22 17:29:38 IRDT 2017





















Config files












In [6]:



    


cat /etc/timezone



    















    






Asia/Tehran



















In [11]:



    


ll /etc/localtime



    















    






lrwxrwxrwx 1 root root 31 Mar 19 22:11 /etc/localtime -> /usr/share/zoneinfo/Asia/Tehran



















In [4]:



    


ls /usr/share/zoneinfo/



    















    






Africa      Cuba     GMT0         Japan              Pacific     Turkey
America     EET      GMT-0        Kwajalein          Poland      UCT
Antarctica  Egypt    GMT+0        leap-seconds.list  Portugal    Universal
Arctic      Eire     Greenwich    Libya              posix       US
Asia        EST      Hongkong     localtime          posixrules  UTC
Atlantic    EST5EDT  HST          MET                PRC         WET
Australia   Etc      Iceland      Mexico             PST8PDT     W-SU
Brazil      Europe   Indian       MST                right       zone1970.tab
Canada      Factory  Iran         MST7MDT            ROC         zone.tab
CET         GB       iso3166.tab  Navajo             ROK         Zulu
Chile       GB-Eire  Israel       NZ                 Singapore
CST6CDT     GMT      Jamaica      NZ-CHAT            SystemV



















In [7]:



    


ls -l /usr/share/zoneinfo/Asia/Tehran



    















    






lrwxrwxrwx 1 root root 7 Dec  7 14:29 /usr/share/zoneinfo/Asia/Tehran -> ../Iran



















In [5]:



    


file /usr/share/zoneinfo/Iran



    















    






/usr/share/zoneinfo/Iran: timezone data, version 2, 7 gmt time flags, 7 std time flags, no leap seconds, 101 transition times, 7 abbreviation chars





















Manually












In [ ]:



    


echo Asia/Tehran | sudo tee /etc/timezone



    













In [ ]:



    


sudo ln -l /usr/share/zoneinfo/Asia/Tehran /etc/localtime



    















locale
locale is a set of parameters that defines the user's language, region and any special variant preferences that the user wants to see in their user interface. Usually a locale identifier consists of at least a language identifier and a region identifier.













In [1]:



    


locale # out config



    















    






LANG=en_US.UTF-8
LANGUAGE=en_US:en
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=



















In [2]:



    


timedatectl | head -1



    















    






      Local time: Sun 2017-03-19 22:54:37 IRST



















In [3]:



    


locale -a | grep -i fa # show all available locales



    















    






fa_IR
fa_IR.utf8



















In [4]:



    


export LC_TIME=fa_IR.utf8



    













In [5]:



    


timedatectl | head -1



    















    






      Local time: یکشنبه 2017-03-19 22:54:46 IRST





















Managing locales
lets see what we got













In [9]:



    


grep '^[a-z]' /etc/locale.gen



    















    






en_US.UTF-8 UTF-8
fa_IR UTF-8





















lets see if we got any locale enabled for france:













In [1]:



    


localectl list-locales | grep -i fr # eq to locale -a



    















    
























In [3]:



    


locale -a | grep -i fr



    















    


























Generate a new locale












In [ ]:



    


sudo nano /etc/locale.gen # uncomment # fr_FR.UTF-8 UTF-8



    















lets check the file again













In [3]:



    


grep -v '^#' /etc/locale.gen | grep -v '^$'



    















    






en_US.UTF-8 UTF-8
fa_IR UTF-8
fr_FR.UTF-8 UTF-8



















In [4]:



    


whatis locale-gen



    















    






locale-gen (8)       - generates localisation files from templates



















In [ ]:



    


sudo locale-gen



    













In [4]:



    


localectl list-locales | grep -i fr



    















    






fr_FR.utf8



















In [5]:



    


locale -a | grep -i fr



    















    






fr_FR.utf8



















In [6]:



    


export LANG=fr_FR.utf8



    













In [25]:



    


timedatectl | head -1



    















    






      Local time: lun. 2017-03-20 01:55:59 IRST





















Make changes permanent












In [ ]:



    


localectl set-locale LANG=fr_FR.utf8



    















Also like always we can use .profile or anything like that to export LANG or other related environment variables.















References & further reading
man 5 localtime

man localectl

man timedatectl

man locale

man tzselect

https://en.wikipedia.org/wiki/Locale_(computer_software)

http://linuxfromscratch.org/lfs/view/systemd/chapter07/clock.html

https://wiki.archlinux.org/index.php/Time

https://help.ubuntu.com/community/UbuntuTime

https://www.cl.cam.ac.uk/~mgk25/mswish/ut-rtc.html

https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/System_Administrators_Guide/sect-Configuring_the_Date_and_Time-hwclock.html
































Lecture notes



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Linux Notes by 
Milad As (Ravexina) is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.




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