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.
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.
To set the system clock directly:
# timedatectl set-time "2012-10-30 18:17:16"
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.
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 or not 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.
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.
regedit, add a
DWORD value with hexadecimal value
1 to the registry:
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 reinstallingand 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.
To check the current zone:
$ timedatectl status
To list available zones:
$ timedatectl list-timezones
To change your time zone:
# timedatectl set-timezone Zone/SubZone
# 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 relative link, not absolute, as specified in archlinux(7).
man 1 timedatectl,
man 5 localtime, and
man 7 archlinux for more details.
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).
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.
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.
- AUR can adjust kernel time variables like interrupt frequency to help improve the system clock time drift. AUR in the
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.
ntpd -qgto manually synchronize your clock with the network, ignoring large deviations between local UTC and network UTC.
hwclock --systohcto write the current software UTC time to the hardware clock.