# Time

Related articles

- Network Time Protocol

In an operating system, the time (clock) is determined by four parts: time value, time standard, time zone, and Daylight Saving Time (DST) if applicable. This article explains what they are and how to read/set them. To maintain accurate system time on a network see Network Time Protocol.

## Hardware clock and system clock

A computer has two clocks that need to be considered: the "Hardware clock" and the "System/software clock".

Hardware clock (a.k.a. the Real Time Clock (RTC) or CMOS clock) stores the values of: Year, Month, Day, Hour, Minute, and the Seconds. It does not have the ability to store the time standard (localtime or UTC), nor whether DST is used.

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):

    $ timedatectl status

Run the same command as root to display also the hardware clock time.

### Set clock

To set 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.

### RTC clock

Standard behavior of most operating systems is:

- Set the system clock from the hardware clock on boot
- Keep accurate time of the system clock with an NTP daemon
- Set the hardware clock from the system clock on shutdown.

## Time standard

Note: Systemd will use UTC for the hardware clock by default.

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 hardware clock (CMOS clock, the time that appears in BIOS) is defined by the operating system. By default, Windows uses localtime, Mac OS uses UTC, and UNIX-like operating systems vary. An OS that uses the UTC standard, generally, will consider CMOS (hardware clock) time a UTC time (GMT, Greenwich time) and make an adjustment to it while setting the System time on boot according to your time zone.

When using Linux it is beneficial to have the hardware clock set to the UTC standard and made known to all operating systems. Defining the hardware clock in Linux as UTC means that Daylight Saving Time will automatically be accounted for. If using the localtime standard the system clock will not be changed for DST occurrences assuming that another operating system will take care of the DST switch (and provided no NTP agent is operating).

You can set the hardware clock time standard through the command line. You can check what you have set your Arch Linux install to use by:

    $ timedatectl status | grep local

The hardware clock can be queried and set with the `timedatectl` command. To change the hardware clock time standard to localtime, use:

    # timedatectl set-local-rtc true

If you want to revert to the hardware clock being in UTC, do:

    # timedatectl set-local-rtc false

Be warned that, if the hardware clock is set to localtime, dealing with daylight saving time is messy. If the DST changes when your computer is off, your clock will be wrong on next boot (there is a lot more to it). Recent kernels set the system time from the RTC directly on boot, assuming that the RTC is in UTC. This means that if the RTC is in local time, then the system time will first be set up wrongly and then corrected shortly afterwards on every boot. This is the root of certain weird bugs (time going backwards is rarely a good thing).

These will generate `/etc/adjtime` automatically; 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 from 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.

Note: The use of `timedatectl` requires an active dbus. 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.

### UTC in Windows

One reason users often set the RTC in localtime to dual-boot with Windows (which uses localtime). However, Windows is able to deal with the RTC being in UTC with a simple registry fix. It is recommended to configure Windows to use UTC, rather than Linux to use localtime. If you make Windows use UTC, also remember to disable the "Internet Time Update" Windows feature, so that Windows does not mess with the hardware clock, trying to sync it with internet time. You should instead use NTP to modify the RTC and sync to internet time.

Using `regedit`, add a `DWORD` value with hexadecimal value `1` to the registry:

    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation\RealTimeIsUniversal

Alternatively, create a `*.reg` 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

Windows XP and Windows Vista SP1 have support for setting the time standard as UTC and can be activated in the same way. However, there is a bug after resuming from the suspend/hibernation state that resets the clock to localtime. For these operating systems, it is recommended to use localtime.

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:

    # timedatectl set-timezone America/Los_Angeles

It makes sense to disable time synchronization in Windows - otherwise it will mess up the hardware clock.

## Time zone

To check the current zone defined for the system:

    $ timedatectl status

To list available zones:

    $ timedatectl list-timezones

To change your time zone:

    # timedatectl set-timezone Zone/SubZone

Example:

    # timedatectl set-timezone Canada/Eastern

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, keep in mind that it must be a symbolic link, not hard, as specified in archlinux(7).

See `man 1 timedatectl`, `man 5 localtime`, and `man 7 archlinux` for more details.

Note: If the pre-systemd configuration file `/etc/timezone` still exists in your system, you can remove it safely, since it is no longer used.

## 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 systems using systemd, this does not happen).

Note: If the hwclock has been set again less than 24 hours after a previous set, the drift is not recalculated as `hwclock` considers the elapsed time period too short to accurately calculate the drift.

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 Mac OS install. The drift value can be removed by removing the file `/etc/adjtime`, then set the correct hardware clock and system clock time, and check if your time standard is correct.

Note: For those using systemd, but wish to make use of the drift value stored in `/etc/adjtime` (i.e. perhaps cannot or do not want to use NTP); they need to call `hwclock --adjust` on a regular basis, perhaps by creating a cron job.

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:

- NTP can synchronize the software clock of a GNU/Linux system with Internet time servers using the Network Time Protocol. NTP can also adjust the interrupt frequency and the number of ticks per second to decrease system clock drift. Running NTP will also cause the hardware clock to be re-synchronised every 11 minutes.
- adjtimex in the AUR can adjust kernel time variables like interrupt frequency to help improve the system clock time drift.

## 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 (from libfaketime) or the datefudge utilities.

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

    $ date && export TZ="/usr/share/zoneinfo/Pacific/Fiji" && date

    Sa 24. Mai 12:38:26 CEST 2014
    Sa 24. Mai 22:38:26 FJT 2014

This is different than just setting the time, as for example it allows to test the behaviour 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 locally and Autostarting#Shells.

## 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 NTP (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.

## See also

- Linux Tips - Linux, Clocks, and Time
- Sources for Time Zone and Daylight Saving Time Data for tzdata
- Time Scales
- Wikipedia:Time

Categories:

- Mainboards and BIOS
- System administration