Linux set time command line

Устанавливаем в системе Linux время, дату и часовой пояс из командной строки или из Gnome | Используем ntp

В системе Linux очень важно иметь правильное время и дату, поскольку от этого зависит многое. Причем неважно, используете ли вы систему Linux на своем персональном компьютере или у вас Linux-сервер. Серверные и системные часы должны указывать правильное время.

Аппаратные часы — это те часы, которые работают на вашем компьютере даже тогда, как питание компьютера отключено. Это возможно благодаря наличию в современных компьютерах литиевой батареи или батареи другого типа в более старых компьютерах.

Мы можем увидеть различие между аппаратными и системными часами

Вы увидите что-то вроде следующего:

Теперь проверьте системные часы

Вы увидите что-то вроде следующего:

Давайте установим аппаратные часы по местному времени:

Если вы хотите установить их на использование времени по Гринвичу (UTC):

Установка часового пояса

Чтобы в ваших системных часах установить часовой пояс, выполните следующее:

Правильно укажите ваш часовой пояс.

Автоматическая регулировка часов

Для того, чтобы ваша система могла автоматический регулировать время, вам нужно установить программу ntp . Получите ее из репозитария. После того, как она будет установлена, вы можете настроить ее следующим образом:

Отредактируйте файл /etc/ntpd.conf . Он будет выглядеть следующим образом:

Удостоверьтесь, что вы запустили демон, и сделайте так, чтобы он автоматически запускался при загрузке системы.

Для Arch Linux это: /etc/rc.d/ntpd start ; для Debian и производных систем: /etc/init.d/ntpd start

Обновление из командной строки времени по значению, получаемому с сервера времени

Вы можете обновлять время вручную без использования демона ntpdate

Вы получите что-то вроде следующего:

Бонус: Установка времени и даты в Gnome

Если вы используете Gnome, щелкните правой кнопкой мыши по изображению часов и выберите пункт настройки adjust, или в меню выберите пункт System > Administration > Time and Date (Система > Администрирование > Время и дата). Вам, возможно, будет предложено ввести пароль.

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Set Time, Date Timezone in Linux from Command Line or Gnome | Use ntp

Written by Guillermo Garron
Date: 2012-04-19 15:55:00 00:00

To have the correct time and date in Linux is very important, a lot of things depends on it. It does not matter if you are using Linux to power your personal computer or you have a Linux server. The server and system clock needs to be on time.

Set date from the command line

Set time from the command line

Set time and date from the command line

Linux check date from command line

Will show you something like this:

Set hardware clock

The hardware clock is the clock that runs in you PC hardware even if you disconnect it from the main power supply. This is because it has a lithium battery in the modern computers and another type of battery in the old ones.

We’ll see differences between hardware clock and system clock

Will output something like this:

Now check the system clock

Will output something like this:

Let’s set the hardware clock to local time:

If you want to set it to UTC time use:

Set the timezone

To set the timezone of your system clock do the following:

Choose the right timezone for you.

Automatically adjust your computer clock

To have your system to automatically adjust time we need to install ntp . Get it from your repository. Once installed you can configure it this way:

Edit the file /etc/ntpd.conf . It will look like this:

Be sure to start the daemon, and to make it start automatically when the system boots.

On Arch Linux is: /etc/rc.d/ntpd start on Debian and derivatives /etc/init.d/ntpd start

Update from the command line against a time server

You can update the clock manually, without the need of the daemon with ntpdate

You will get something like this:

Bonus: Set the time and Date on Gnome

If you are using Gnome right click on the clock and select adjust, or go to: System > Administration > Time and Date (You may be asked for root password)

You will see a window similar to this one:

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Set the time and date in Linux systems

Written by Guillermo Garron
Date: 2007-08-18 10:36:30 00:00

How important is time for Linux

Lots of the task your Linux machine are controlled by the time, things like cron jobs, emails’ Date, file’s dates, so it is really important to have your computer’s clock on time.

It is the responsability of the sysadmin to keep their Linux servers with clock running as accurately as possible.

Utilities for the command Line

From the command line we have lots of utilities to set up the clock, set the date of the system, set the date of the hardware or have time automatically updated from Internet time servers.

Date displays the date and time of the system, used by root it can also be used to change the date and time of the system.

If you just enter:

You will get the date of the system, like this:

You can set the date by issuing (as root):

• nn: month of the year (01 to 12)
• dd: day of the month (01 to 31)
• hh: hour of the day (00 to 23)
• mm: minute of the hour (00 to 59)
• cc: The first to digits of the year
• yy: The last two digits of the year
• .ss: The seconds

hwclock

With this command you are able to set or get the time of you hardware clock, the hardware clock is the clock that runs in you PC hardware even if you disconnect it from the main power supply, this is because it has a lithium battery in the modern computers and another type of battery in the old ones.

Usage

hwclock [function] [options. ]

• –help: show this help
• –show: read hardware clock and print result
• –set: set the rtc to the time given with –date
• –hctosys: set the system time from the hardware clock
• –systohc: set the hardware clock to the current system time
• –adjust: adjust the rtc to account for systematic drift since the clock was last set or adjusted
• –getepoch: print out the kernel’s hardware clock epoch value –setepoch set the kernel’s hardware clock epoch value to the value given with –epoch
• –version: print out the version of hwclock to stdout

• –utc: the hardware clock is kept in coordinated universal time
• –localtime: the hardware clock is kept in local time
• –directisa: access the ISA bus directly instead of /dev/rtc
• –badyear: ignore rtc’s year because the bios is broken
• –date: specifies the time to which to set the hardware clock
• –epoch=year: specifies the year which is the beginning of the hardware clock’s epoch value
• –noadjfile: do not access /etc/adjtime. Requires the use of either –utc or –localtime

If you want to set your hardware clock to your local time you can use this command:

That will set the clock you August 8th, 2007 at 21:08 and will tell your clock that is the local time, you can use –utc instead of –localtime to set your clock to the Universal Time (Greenwich Time).

If you want to read it issue this command:

You should get an output like this:

Keeping it on time automatically

If you want to keep your system time accurate and on time automatically, you can install ntp

To do this just run on Ubuntu or Debian:

Once installed you have to configure it, its configuration file is:

It looks like this by default on Debian or Ubuntu systems:

Commands to update manually from the server

Here we have two commands:

rdate

rdate [-psau] host [port]

Where the options are:

• -p: Do not set, just print the remote time
• -s: Do not print the time.
• -a: Use the adjtimex(2) call to gradually skew the local time to the remote time rather than just hopping.
• -u: Use UDP instead of TCP as the transport.

So to update your clock with this command issue this command:

ntpdate

ntpdate [-bBdoqsuv] [-a key] [-e authdelay] [-k keyfile] [-o version] [-p samples] [-t timeout] server [. ]

As ntpdate will decline to set the date if an NTP server daemon (e.g., ntpd) is running on the same host so you should decide if use the daemon way or use ntpdate in the cron.

To check if your ntp daemon is running enter:

To use ntpdate if you ntp daemon is off enter:

To start the daemon enter

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System time

This article or section needs expansion.

In an operating system, the time (clock) is determined by three parts: time value, whether it is local time or UTC or something else, time zone, and Daylight Saving Time (DST) if applicable. This article explains what they are and how to read/set them. Two clocks are present on systems: a hardware clock and a system clock which are also detailed in this article.

Standard behavior of most operating systems is:

• Set the system clock from the hardware clock on boot.
• Keep accurate time of the system clock, see #Time synchronization.
• Set the hardware clock from the system clock on shutdown.

Contents

Hardware clock

The hardware clock (a.k.a. the Real Time Clock (RTC) or CMOS clock) stores the values of: Year, Month, Day, Hour, Minute, and Seconds. Only 2016, or later, UEFI firmware has the ability to store the timezone, and whether DST is used.

Read hardware clock

Set hardware clock from system clock

The following sets the hardware clock from the system clock. Additionally it updates /etc/adjtime or creates it if not present. See hwclock(8) section «The Adjtime File» for more information on this file as well as the #Time skew section.

System clock

The system clock (a.k.a. the software clock) keeps track of: time, time zone, and DST if applicable. It is calculated by the Linux kernel as the number of seconds since midnight January 1st 1970, UTC. The initial value of the system clock is calculated from the hardware clock, dependent on the contents of /etc/adjtime . After boot-up has completed, the system clock runs independently of the hardware clock. The Linux kernel keeps track of the system clock by counting timer interrupts.

Read clock

To check the current system clock time (presented both in local time and UTC) as well as the RTC (hardware clock):

Set system clock

To set the local time of the system clock directly:

sets the time to May 26th, year 2014, 11:13 and 54 seconds.

Time standard

There are two time standards: localtime and Coordinated Universal Time (UTC). The localtime standard is dependent on the current time zone, while UTC is the global time standard and is independent of time zone values. Though conceptually different, UTC is also known as GMT (Greenwich Mean Time).

The standard used by the hardware clock (CMOS clock, the BIOS time) is set by the operating system. By default, Windows uses localtime, macOS uses UTC, other UNIX and UNIX-like systems vary. An OS that uses the UTC standard will generally consider the hardware clock as UTC and make an adjustment to it to set the OS time at boot according to the time zone.

If multiple operating systems are installed on a machine, they will all derive the current time from the same hardware clock: it is recommended to adopt a unique standard for the hardware clock to avoid conflicts across systems and set it to UTC. Otherwise, if the hardware clock is set to localtime, more than one operating system may adjust it after a DST change for example, thus resulting in an over-correction; problems may also arise when traveling between different time zones and using one of the operating systems to reset the system/hardware clock.

The hardware clock can be queried and set with the timedatectl command. You can see the current hardware clock time standard of the Arch system using:

To change the hardware clock time standard to localtime, use:

To revert to the hardware clock being in UTC, type:

These generate /etc/adjtime automatically and update the RTC accordingly; no further configuration is required.

During kernel startup, at the point when the RTC driver is loaded, the system clock may be set from the hardware clock. Whether this occurs depends on the hardware platform, the version of the kernel and kernel build options. If this does occur, at this point in the boot sequence, the hardware clock time is assumed to be UTC and the value of /sys/class/rtc/rtcN/hctosys (N=0,1,2. ) will be set to 1.

Later, the system clock is set again from the hardware clock by systemd, dependent on values in /etc/adjtime . Hence, having the hardware clock using localtime may cause some unexpected behavior during the boot sequence; e.g system time going backwards, which is always a bad idea (there is a lot more to it). To avoid it systemd will only synchronize back, if the hardware clock is set to UTC and keep the kernel uninformed about the local timezone. As a consequence timestamps on a FAT filesystem touched by the Linux system will be in UTC.

UTC in Microsoft Windows

To dual boot with Windows it is recommended to configure Windows to use UTC, rather than Linux to use localtime. (Windows by default uses localtime [1].)

It can be done by a simple registry fix: Open regedit and add a DWORD value with hexadecimal value 1 to the registry:

You can do this from an Administrator Command Prompt running:

Alternatively, create a *.reg file (on the desktop) with the following content and double-click it to import it into registry:

Should Windows ask to update the clock due to DST changes, let it. It will leave the clock in UTC as expected, only correcting the displayed time.

The #Hardware clock and #System clock time may need to be updated after setting this value.

If you are having issues with the offset of the time, try reinstalling tzdata and then setting your time zone again:

Historical notes

For really old Windows, the above method fails, due to Windows bugs. More precisely,

• For 64-bit versions of Windows 7 and older builds of Windows 10, there was a bug that made it necessary to have a QWORD value with hexadecimal value of 1 instead of a DWORD value. This bug has been fixed in newer builds and now only DWORD works.
• Before Vista SP2, there is a bug that resets the clock to localtime after resuming from the suspend/hibernation state.
• For XP and older, there is a bug related to the daylight saving time. See [2] for details.
• For even older versions of Windows, you might want to read https://www.cl.cam.ac.uk/

mgk25/mswish/ut-rtc.html — the functionality was not even documented nor officially supported then.

For these operating systems, it is recommended to use localtime.

UTC in Ubuntu

Ubuntu and its derivatives have the hardware clock set to be interpreted as in «localtime» if Windows was detected on any disk during Ubuntu installation. This is apparently done deliberately to allow new Linux users to try out Ubuntu on their Windows computers without editing the registry.

For changing this behavior, see above.

Time zone

To check the current zone defined for the system:

To list available zones:

To set your time zone:

This will create an /etc/localtime symlink that points to a zoneinfo file under /usr/share/zoneinfo/ . In case you choose to create the link manually (for example during chroot where timedatectl will not work), keep in mind that it must be a symbolic link, as specified in archlinux(7) [dead link 2021-02-08] :

Setting based on geolocation

To set the timezone automatically based on the IP address location, one can use a geolocation API to retrieve the timezone, for example curl https://ipapi.co/timezone , and pass the output to timedatectl set-timezone for automatic setting. Some geo-IP APIs that provide free or partly free services are listed below:

Update timezone every time NetworkManager connects to a network

Alternatively, the tool tzupdate AUR automatically sets the timezone based on the geolocation of the IP address. This comparison of the most popular IP geolocation apis may be helpful in deciding which API to use in production.

Time skew

Every clock has a value that differs from real time (the best representation of which being International Atomic Time); no clock is perfect. A quartz-based electronic clock keeps imperfect time, but maintains a consistent inaccuracy. This base ‘inaccuracy’ is known as ‘time skew’ or ‘time drift’.

When the hardware clock is set with hwclock , a new drift value is calculated in seconds per day. The drift value is calculated by using the difference between the new value set and the hardware clock value just before the set, taking into account the value of the previous drift value and the last time the hardware clock was set. The new drift value and the time when the clock was set is written to the file /etc/adjtime overwriting the previous values. The hardware clock can therefore be adjusted for drift when the command hwclock —adjust is run; this also occurs on shutdown but only if the hwclock daemon is enabled, hence for Arch systems which use systemd, this does not happen.

If the hardware clock keeps losing or gaining time in large increments, it is possible that an invalid drift has been recorded (but only applicable, if the hwclock daemon is running). This can happen if you have set the hardware clock time incorrectly or your time standard is not synchronized with a Windows or macOS install. The drift value can be removed by first removing the file /etc/adjtime , then setting the correct hardware clock and system clock time. You should then check if your time standard is correct.

The software clock is very accurate but like most clocks is not perfectly accurate and will drift as well. Though rarely, the system clock can lose accuracy if the kernel skips interrupts. There are some tools to improve software clock accuracy:

Time synchronization

The Network Time Protocol (NTP) is a protocol for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. The following are implementations of NTP available for Arch Linux:

• Chrony — A client and server that is roaming friendly and designed specifically for systems that are not online all the time.

https://chrony.tuxfamily.org/ || chrony

• ConnMan — A lightweight network manager with NTP support.

https://01.org/connman (waybackmachine) || connman

• Network Time Protocol daemon — The reference implementation of the protocol, especially recommended to be used on time servers. It can also adjust the interrupt frequency and the number of ticks per second to decrease system clock drift, and will cause the hardware clock to be re-synchronised every 11 minutes.

https://www.ntp.org/ || ntp

• ntpclient — A simple command-line NTP client.

http://doolittle.icarus.com/ntpclient/ || ntpclientAUR

• NTPsec — A fork of NTPd, focused on security.

https://ntpsec.org/ || ntpsecAUR

• OpenNTPD — Part of the OpenBSD project and implements both a client and a server.

https://www.openntpd.org/ || openntpd

• sntp — An SNTP client that comes with NTPd. It supersedes ntpdate and is recommended in non-server environments.

https://www.ntp.org/ || ntp

• systemd-timesyncd — A simple SNTP daemon that only implements a client side, focusing only on querying time from one remote server. It should be more than appropriate for most installations.

https://www.freedesktop.org/wiki/Software/systemd/ || systemd

Per-user/session or temporary settings

For some use cases it may be useful to change the time settings without touching the global system values. For example to test applications relying on the time during development or adjusting the system time zone when logging into a server remotely from another zone.

To make an application «see» a different date/time than the system one, you can use the faketime(1) utility (from libfaketime ).

If instead you want an application to «see» a different time zone than the system one, set the TZ environment variable, for example:

This is different than just setting the time, as for example it allows to test the behavior of a program with positive or negative UTC offset values, or the effects of DST changes when developing on systems in a non-DST time zone.

Another use case is having different time zones set for different users of the same system: this can be accomplished by setting the TZ variable in the shell’s configuration file, see Environment variables#Defining variables.

Troubleshooting

Clock shows a value that is neither UTC nor local time

This might be caused by a number of reasons. For example, if your hardware clock is running on local time, but timedatectl is set to assume it is in UTC, the result would be that your timezone’s offset to UTC effectively gets applied twice, resulting in wrong values for your local time and UTC.

To force your clock to the correct time, and to also write the correct UTC to your hardware clock, follow these steps:

• Setup ntpd (enabling it as a service is not necessary).
• Set your time zone correctly.
• Run ntpd -qg to manually synchronize your clock with the network, ignoring large deviations between local UTC and network UTC.
• Run hwclock —systohc to write the current software UTC time to the hardware clock.

Tips and tricks

fake-hwclock

alarm-fake-hwclock designed especially for system without battery backed up RTC, it includes a systemd service which on shutdown saves the current time and on startup restores the saved time, thus avoiding strange time travel errors.

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