System time

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.

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

# hwclock --show

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) § The Adjtime File for more information on this file as well as the #Time skew section.

# hwclock --systohc

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):

$ timedatectl

Set system clock

To set the local time of the system clock directly:

# timedatectl set-time "yyyy-MM-dd hh:mm:ss"

For example:

# timedatectl set-time "2014-05-26 11:13:54"

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 set it to UTC to avoid conflicts across systems. 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:

$ timedatectl | grep local

RTC in local TZ: no

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

# timedatectl set-local-rtc 1

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

# timedatectl set-local-rtc 0

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 (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.

Note:

The use of timedatectl requires an active D-Bus. Therefore, it may not be possible to use this command under a chroot (such as during installation). In these cases, you can revert back to the hwclock command, or use systemd-nspawn instead of chroot.
If /etc/adjtime is not present, systemd assumes the hardware clock is set to 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 .)

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

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation\RealTimeIsUniversal

You can do this from an Administrator Command Prompt running:

reg add "HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TimeZoneInformation" /v RealTimeIsUniversal /d 1 /t REG_DWORD /f

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

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation]
"RealTimeIsUniversal"=dword:00000001

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 and then setting your time zone again:

# timedatectl set-timezone America/Los_Angeles

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 value with hexadecimal value of 1 instead of a value. This bug has been fixed in newer builds and now only 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 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:

$ timedatectl status

To list available zones:

$ timedatectl list-timezones

To set your time zone:

# timedatectl set-timezone Zone/SubZone

Example:

# timedatectl set-timezone Canada/Eastern

This will create an symlink that points to a zoneinfo file under . 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 :

# ln -sf /usr/share/zoneinfo/Zone/SubZone /etc/localtime

See timedatectl(1) and for details.

Setting based on geolocation

Note: Some desktop environments have support for automatic time zone selection (e.g. see GNOME#Date & time).

To set the timezone automatically based on the IP address location, one can use a geolocation API to retrieve the timezone, for example , and pass the output to 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

Create a NetworkManager dispatcher script:

Alternatively, the tool 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 , 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 is run; this also occurs on shutdown but only if the 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:

See #Time synchronization.

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

NTPsec — A fork of NTPd, focused on security.

https://ntpsec.org/ || ntpsec^AUR

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 utility (from ).

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.

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.

Install , start/enable the service .

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 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.

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