Provided by: util-linux-extra_2.39.3-9ubuntu6.1_amd64 
NAME
hwclock - time clocks utility
SYNOPSIS
hwclock [function] [option...]
DESCRIPTION
hwclock is an administration tool for the time clocks. It can: display the Hardware Clock
time; set the Hardware Clock to a specified time; set the Hardware Clock from the System
Clock; set the System Clock from the Hardware Clock; compensate for Hardware Clock drift;
correct the System Clock timescale; set the kernel’s timezone, NTP timescale, and epoch
(Alpha only); and predict future Hardware Clock values based on its drift rate.
Since v2.26 important changes were made to the --hctosys function and the --directisa
option, and a new option --update-drift was added. See their respective descriptions
below.
FUNCTIONS
The following functions are mutually exclusive, only one can be given at a time. If none
is given, the default is --show.
-a, --adjust
Add or subtract time from the Hardware Clock to account for systematic drift since the
last time the clock was set or adjusted. See the discussion below, under The Adjust
Function.
--getepoch; --setepoch
These functions are for Alpha machines only, and are only available through the Linux
kernel RTC driver.
They are used to read and set the kernel’s Hardware Clock epoch value. Epoch is the
number of years into AD to which a zero year value in the Hardware Clock refers. For
example, if the machine’s BIOS sets the year counter in the Hardware Clock to contain
the number of full years since 1952, then the kernel’s Hardware Clock epoch value must
be 1952.
The --setepoch function requires using the --epoch option to specify the year. For
example:
hwclock --setepoch --epoch=1952
The RTC driver attempts to guess the correct epoch value, so setting it may not be
required.
This epoch value is used whenever hwclock reads or sets the Hardware Clock on an Alpha
machine. For ISA machines the kernel uses the fixed Hardware Clock epoch of 1900.
--param-get=parameter; --param-set=parameter=value
Read and set the RTC’s parameter. This is useful, for example, to retrieve the RTC’s
feature or set the RTC’s Backup Switchover Mode.
parameter is either a numeric RTC parameter value (see the Kernel’s
include/uapi/linux/rtc.h) or an alias. See --help for a list of valid aliases.
parameter and value, if prefixed with 0x, are interpreted as hexadecimal, otherwise
decimal values.
--predict
Predict what the Hardware Clock will read in the future based upon the time given by
the --date option and the information in /etc/adjtime. This is useful, for example, to
account for drift when setting a Hardware Clock wakeup (aka alarm). See rtcwake(8).
Do not use this function if the Hardware Clock is being modified by anything other
than the current operating system’s hwclock command, such as '11 minute mode' or from
dual-booting another OS.
-r, --show; --get
Read the Hardware Clock and print its time to standard output in the ISO 8601 format.
The time shown is always in local time, even if you keep your Hardware Clock in UTC.
See the --localtime option.
Showing the Hardware Clock time is the default when no function is specified.
The --get function also applies drift correction to the time read, based upon the
information in /etc/adjtime. Do not use this function if the Hardware Clock is being
modified by anything other than the current operating system’s hwclock command, such
as '11 minute mode' or from dual-booting another OS.
-s, --hctosys
Set the System Clock from the Hardware Clock. The time read from the Hardware Clock is
compensated to account for systematic drift before using it to set the System Clock.
See the discussion below, under The Adjust Function.
The System Clock must be kept in the UTC timescale for date-time applications to work
correctly in conjunction with the timezone configured for the system. If the Hardware
Clock is kept in local time then the time read from it must be shifted to the UTC
timescale before using it to set the System Clock. The --hctosys function does this
based upon the information in the /etc/adjtime file or the command line arguments
--localtime and --utc. Note: no daylight saving adjustment is made. See the discussion
below, under LOCAL vs UTC.
The kernel also keeps a timezone value, the --hctosys function sets it to the timezone
configured for the system. The system timezone is configured by the TZ environment
variable or the /etc/localtime file, as tzset(3) would interpret them. The obsolete
tz_dsttime field of the kernel’s timezone value is set to zero. (For details on what
this field used to mean, see settimeofday(2).)
When used in a startup script, making the --hctosys function the first caller of
settimeofday(2) from boot, it will set the NTP '11 minute mode' timescale via the
persistent_clock_is_local kernel variable. If the Hardware Clock’s timescale
configuration is changed then a reboot is required to inform the kernel. See the
discussion below, under Automatic Hardware Clock Synchronization by the Kernel.
This is a good function to use in one of the system startup scripts before the file
systems are mounted read/write.
This function should never be used on a running system. Jumping system time will cause
problems, such as corrupted filesystem timestamps. Also, if something has changed the
Hardware Clock, like NTP’s '11 minute mode', then --hctosys will set the time
incorrectly by including drift compensation.
Drift compensation can be inhibited by setting the drift factor in /etc/adjtime to
zero. This setting will be persistent as long as the --update-drift option is not used
with --systohc at shutdown (or anywhere else). Another way to inhibit this is by using
the --noadjfile option when calling the --hctosys function. A third method is to
delete the /etc/adjtime file. Hwclock will then default to using the UTC timescale for
the Hardware Clock. If the Hardware Clock is ticking local time it will need to be
defined in the file. This can be done by calling hwclock --localtime --adjust; when
the file is not present this command will not actually adjust the Clock, but it will
create the file with local time configured, and a drift factor of zero.
A condition under which inhibiting hwclock's drift correction may be desired is when
dual-booting multiple operating systems. If while this instance of Linux is stopped,
another OS changes the Hardware Clock’s value, then when this instance is started
again the drift correction applied will be incorrect.
For hwclock's drift correction to work properly it is imperative that nothing changes
the Hardware Clock while its Linux instance is not running.
--set
Set the Hardware Clock to the time given by the --date option, and update the
timestamps in /etc/adjtime. With the --update-drift option also (re)calculate the
drift factor. Try it without the option if --set fails. See --update-drift below.
--systz
This is an alternate to the --hctosys function that does not read the Hardware Clock
nor set the System Clock; consequently there is not any drift correction. It is
intended to be used in a startup script on systems with kernels above version 2.6
where you know the System Clock has been set from the Hardware Clock by the kernel
during boot.
It does the following things that are detailed above in the --hctosys function:
• Corrects the System Clock timescale to UTC as needed. Only instead of
accomplishing this by setting the System Clock, hwclock simply informs the kernel
and it handles the change.
• Sets the kernel’s NTP '11 minute mode' timescale.
• Sets the kernel’s timezone.
The first two are only available on the first call of settimeofday(2) after boot.
Consequently this option only makes sense when used in a startup script. If the Hardware
Clocks timescale configuration is changed then a reboot would be required to inform the
kernel.
-w, --systohc
Set the Hardware Clock from the System Clock, and update the timestamps in
/etc/adjtime. With the --update-drift option also (re)calculate the drift factor. Try
it without the option if --systohc fails. See --update-drift below.
-h, --help
Display help text and exit.
-V, --version
Print version and exit.
OPTIONS
--adjfile=filename
Override the default /etc/adjtime file path.
--date=date_string
This option must be used with the --set or --predict functions, otherwise it is
ignored.
hwclock --set --date='16:45'
hwclock --predict --date='2525-08-14 07:11:05'
The argument must be in local time, even if you keep your Hardware Clock in UTC. See
the --localtime option. Therefore, the argument should not include any timezone
information. It also should not be a relative time like "+5 minutes", because
hwclock's precision depends upon correlation between the argument’s value and when the
enter key is pressed. Fractional seconds are silently dropped. This option is capable
of understanding many time and date formats, but the previous parameters should be
observed.
--delay=seconds
This option can be used to overwrite the internally used delay when setting the clock
time. The default is 0.5 (500ms) for rtc_cmos, for another RTC types the delay is 0.
If RTC type is impossible to determine (from sysfs) then it defaults also to 0.5 to be
backwardly compatible.
The 500ms default is based on commonly used MC146818A-compatible (x86) hardware clock.
This Hardware Clock can only be set to any integer time plus one half second. The
integer time is required because there is no interface to set or get a fractional
second. The additional half second delay is because the Hardware Clock updates to the
following second precisely 500 ms after setting the new time. Unfortunately, this
behavior is hardware specific and in some cases another delay is required.
-D, --debug
Use --verbose. The --debug option has been deprecated and may be repurposed or removed
in a future release.
--directisa
This option is meaningful for ISA compatible machines in the x86 and x86_64 family.
For other machines, it has no effect. This option tells hwclock to use explicit I/O
instructions to access the Hardware Clock. Without this option, hwclock will use the
rtc device file, which it assumes to be driven by the Linux RTC device driver. As of
v2.26 it will no longer automatically use directisa when the rtc driver is
unavailable; this was causing an unsafe condition that could allow two processes to
access the Hardware Clock at the same time. Direct hardware access from userspace
should only be used for testing, troubleshooting, and as a last resort when all other
methods fail. See the --rtc option.
--epoch=year
This option is required when using the --setepoch function. The minimum year value is
1900. The maximum is system dependent (ULONG_MAX - 1).
-f, --rtc=filename
Override hwclock's default rtc device file name. Otherwise it will use the first one
found in this order: /dev/rtc0, /dev/rtc, /dev/misc/rtc. For IA-64: /dev/efirtc
/dev/misc/efirtc
-l, --localtime; -u, --utc
Indicate which timescale the Hardware Clock is set to.
The Hardware Clock may be configured to use either the UTC or the local timescale, but
nothing in the clock itself says which alternative is being used. The --localtime or
--utc options give this information to the hwclock command. If you specify the wrong
one (or specify neither and take a wrong default), both setting and reading the
Hardware Clock will be incorrect.
If you specify neither --utc nor --localtime then the one last given with a set
function (--set, --systohc, or --adjust), as recorded in /etc/adjtime, will be used.
If the adjtime file doesn’t exist, the default is UTC.
Note: daylight saving time changes may be inconsistent when the Hardware Clock is kept
in local time. See the discussion below, under LOCAL vs UTC.
--noadjfile
Disable the facilities provided by /etc/adjtime. hwclock will not read nor write to
that file with this option. Either --utc or --localtime must be specified when using
this option.
--test
Do not actually change anything on the system, that is, the Clocks or /etc/adjtime
(--verbose is implicit with this option).
--update-drift
Update the Hardware Clock’s drift factor in /etc/adjtime. It can only be used with
--set or --systohc.
A minimum four hour period between settings is required. This is to avoid invalid
calculations. The longer the period, the more precise the resulting drift factor will
be.
This option was added in v2.26, because it is typical for systems to call hwclock
--systohc at shutdown; with the old behavior this would automatically (re)calculate
the drift factor which caused several problems:
• When using NTP with an '11 minute mode' kernel the drift factor would be clobbered
to near zero.
• It would not allow the use of 'cold' drift correction. With most configurations
using 'cold' drift will yield favorable results. Cold, means when the machine is
turned off which can have a significant impact on the drift factor.
• (Re)calculating drift factor on every shutdown delivers suboptimal results. For
example, if ephemeral conditions cause the machine to be abnormally hot the drift
factor calculation would be out of range.
• Significantly increased system shutdown times (as of v2.31 when not using
--update-drift the RTC is not read).
Having hwclock calculate the drift factor is a good starting point, but for optimal
results it will likely need to be adjusted by directly editing the /etc/adjtime file. For
most configurations once a machine’s optimal drift factor is crafted it should not need to
be changed. Therefore, the old behavior to automatically (re)calculate drift was changed
and now requires this option to be used. See the discussion below, under The Adjust
Function.
This option requires reading the Hardware Clock before setting it. If it cannot be read,
then this option will cause the set functions to fail. This can happen, for example, if
the Hardware Clock is corrupted by a power failure. In that case, the clock must first be
set without this option. Despite it not working, the resulting drift correction factor
would be invalid anyway.
-v, --verbose
Display more details about what hwclock is doing internally.
NOTES
Clocks in a Linux System
There are two types of date-time clocks:
The Hardware Clock: This clock is an independent hardware device, with its own power
domain (battery, capacitor, etc), that operates when the machine is powered off, or even
unplugged.
On an ISA compatible system, this clock is specified as part of the ISA standard. A
control program can read or set this clock only to a whole second, but it can also detect
the edges of the 1 second clock ticks, so the clock actually has virtually infinite
precision.
This clock is commonly called the hardware clock, the real time clock, the RTC, the BIOS
clock, and the CMOS clock. Hardware Clock, in its capitalized form, was coined for use by
hwclock. The Linux kernel also refers to it as the persistent clock.
Some non-ISA systems have a few real time clocks with only one of them having its own
power domain. A very low power external I2C or SPI clock chip might be used with a backup
battery as the hardware clock to initialize a more functional integrated real-time clock
which is used for most other purposes.
The System Clock: This clock is part of the Linux kernel and is driven by a timer
interrupt. (On an ISA machine, the timer interrupt is part of the ISA standard.) It has
meaning only while Linux is running on the machine. The System Time is the number of
seconds since 00:00:00 January 1, 1970 UTC (or more succinctly, the number of seconds
since 1969 UTC). The System Time is not an integer, though. It has virtually infinite
precision.
The System Time is the time that matters. The Hardware Clock’s basic purpose is to keep
time when Linux is not running so that the System Clock can be initialized from it at
boot. Note that in DOS, for which ISA was designed, the Hardware Clock is the only real
time clock.
It is important that the System Time not have any discontinuities such as would happen if
you used the date(1) program to set it while the system is running. You can, however, do
whatever you want to the Hardware Clock while the system is running, and the next time
Linux starts up, it will do so with the adjusted time from the Hardware Clock. Note:
currently this is not possible on most systems because hwclock --systohc is called at
shutdown.
The Linux kernel’s timezone is set by hwclock. But don’t be misled — almost nobody cares
what timezone the kernel thinks it is in. Instead, programs that care about the timezone
(perhaps because they want to display a local time for you) almost always use a more
traditional method of determining the timezone: They use the TZ environment variable or
the /etc/localtime file, as explained in the man page for tzset(3). However, some programs
and fringe parts of the Linux kernel such as filesystems use the kernel’s timezone value.
An example is the vfat filesystem. If the kernel timezone value is wrong, the vfat
filesystem will report and set the wrong timestamps on files. Another example is the
kernel’s NTP '11 minute mode'. If the kernel’s timezone value and/or the
persistent_clock_is_local variable are wrong, then the Hardware Clock will be set
incorrectly by '11 minute mode'. See the discussion below, under Automatic Hardware Clock
Synchronization by the Kernel.
hwclock sets the kernel’s timezone to the value indicated by TZ or /etc/localtime with the
--hctosys or --systz functions.
The kernel’s timezone value actually consists of two parts: 1) a field tz_minuteswest
indicating how many minutes local time (not adjusted for DST) lags behind UTC, and 2) a
field tz_dsttime indicating the type of Daylight Savings Time (DST) convention that is in
effect in the locality at the present time. This second field is not used under Linux and
is always zero. See also settimeofday(2).
Hardware Clock Access Methods
hwclock uses many different ways to get and set Hardware Clock values. The most normal way
is to do I/O to the rtc device special file, which is presumed to be driven by the rtc
device driver. Also, Linux systems using the rtc framework with udev, are capable of
supporting multiple Hardware Clocks. This may bring about the need to override the default
rtc device by specifying one with the --rtc option.
However, this method is not always available as older systems do not have an rtc driver.
On these systems, the method of accessing the Hardware Clock depends on the system
hardware.
On an ISA compatible system, hwclock can directly access the "CMOS memory" registers that
constitute the clock, by doing I/O to Ports 0x70 and 0x71. It does this with actual I/O
instructions and consequently can only do it if running with superuser effective userid.
This method may be used by specifying the --directisa option.
This is a really poor method of accessing the clock, for all the reasons that userspace
programs are generally not supposed to do direct I/O and disable interrupts. hwclock
provides it for testing, troubleshooting, and because it may be the only method available
on ISA systems which do not have a working rtc device driver.
The Adjust Function
The Hardware Clock is usually not very accurate. However, much of its inaccuracy is
completely predictable - it gains or loses the same amount of time every day. This is
called systematic drift. hwclock's --adjust function lets you apply systematic drift
corrections to the Hardware Clock.
It works like this: hwclock keeps a file, /etc/adjtime, that keeps some historical
information. This is called the adjtime file.
Suppose you start with no adjtime file. You issue a hwclock --set command to set the
Hardware Clock to the true current time. hwclock creates the adjtime file and records in
it the current time as the last time the clock was calibrated. Five days later, the clock
has gained 10 seconds, so you issue a hwclock --set --update-drift command to set it back
10 seconds. hwclock updates the adjtime file to show the current time as the last time the
clock was calibrated, and records 2 seconds per day as the systematic drift rate. 24 hours
go by, and then you issue a hwclock --adjust command. hwclock consults the adjtime file
and sees that the clock gains 2 seconds per day when left alone and that it has been left
alone for exactly one day. So it subtracts 2 seconds from the Hardware Clock. It then
records the current time as the last time the clock was adjusted. Another 24 hours go by
and you issue another hwclock --adjust. hwclock does the same thing: subtracts 2 seconds
and updates the adjtime file with the current time as the last time the clock was
adjusted.
When you use the --update-drift option with --set or --systohc, the systematic drift rate
is (re)calculated by comparing the fully drift corrected current Hardware Clock time with
the new set time, from that it derives the 24 hour drift rate based on the last calibrated
timestamp from the adjtime file. This updated drift factor is then saved in /etc/adjtime.
A small amount of error creeps in when the Hardware Clock is set, so --adjust refrains
from making any adjustment that is less than 1 second. Later on, when you request an
adjustment again, the accumulated drift will be more than 1 second and --adjust will make
the adjustment including any fractional amount.
hwclock --hctosys also uses the adjtime file data to compensate the value read from the
Hardware Clock before using it to set the System Clock. It does not share the 1 second
limitation of --adjust, and will correct sub-second drift values immediately. It does not
change the Hardware Clock time nor the adjtime file. This may eliminate the need to use
--adjust, unless something else on the system needs the Hardware Clock to be compensated.
The Adjtime File
While named for its historical purpose of controlling adjustments only, it actually
contains other information used by hwclock from one invocation to the next.
The format of the adjtime file is, in ASCII:
Line 1: Three numbers, separated by blanks: 1) the systematic drift rate in seconds per
day, floating point decimal; 2) the resulting number of seconds since 1969 UTC of most
recent adjustment or calibration, decimal integer; 3) zero (for compatibility with
clock(8)) as a floating point decimal.
Line 2: One number: the resulting number of seconds since 1969 UTC of most recent
calibration. Zero if there has been no calibration yet or it is known that any previous
calibration is moot (for example, because the Hardware Clock has been found, since that
calibration, not to contain a valid time). This is a decimal integer.
Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set to Coordinated Universal
Time or local time. You can always override this value with options on the hwclock command
line.
You can use an adjtime file that was previously used with the clock(8) program with
hwclock.
Automatic Hardware Clock Synchronization by the Kernel
You should be aware of another way that the Hardware Clock is kept synchronized in some
systems. The Linux kernel has a mode wherein it copies the System Time to the Hardware
Clock every 11 minutes. This mode is a compile time option, so not all kernels will have
this capability. This is a good mode to use when you are using something sophisticated
like NTP to keep your System Clock synchronized. (NTP is a way to keep your System Time
synchronized either to a time server somewhere on the network or to a radio clock hooked
up to your system. See RFC 1305.)
If the kernel is compiled with the '11 minute mode' option it will be active when the
kernel’s clock discipline is in a synchronized state. When in this state, bit 6 (the bit
that is set in the mask 0x0040) of the kernel’s time_status variable is unset. This value
is output as the 'status' line of the adjtimex --print or ntptime commands.
It takes an outside influence, like the NTP daemon to put the kernel’s clock discipline
into a synchronized state, and therefore turn on '11 minute mode'. It can be turned off by
running anything that sets the System Clock the old fashioned way, including hwclock
--hctosys. However, if the NTP daemon is still running, it will turn '11 minute mode' back
on again the next time it synchronizes the System Clock.
If your system runs with '11 minute mode' on, it may need to use either --hctosys or
--systz in a startup script, especially if the Hardware Clock is configured to use the
local timescale. Unless the kernel is informed of what timescale the Hardware Clock is
using, it may clobber it with the wrong one. The kernel uses UTC by default.
The first userspace command to set the System Clock informs the kernel what timescale the
Hardware Clock is using. This happens via the persistent_clock_is_local kernel variable.
If --hctosys or --systz is the first, it will set this variable according to the adjtime
file or the appropriate command-line argument. Note that when using this capability and
the Hardware Clock timescale configuration is changed, then a reboot is required to notify
the kernel.
hwclock --adjust should not be used with NTP '11 minute mode'.
ISA Hardware Clock Century value
There is some sort of standard that defines CMOS memory Byte 50 on an ISA machine as an
indicator of what century it is. hwclock does not use or set that byte because there are
some machines that don’t define the byte that way, and it really isn’t necessary anyway,
since the year-of-century does a good job of implying which century it is.
If you have a bona fide use for a CMOS century byte, contact the hwclock maintainer; an
option may be appropriate.
Note that this section is only relevant when you are using the "direct ISA" method of
accessing the Hardware Clock. ACPI provides a standard way to access century values, when
they are supported by the hardware.
DATE-TIME CONFIGURATION
Keeping Time without External Synchronization
This discussion is based on the following conditions:
• Nothing is running that alters the date-time clocks, such as NTP daemon or a cron
job."
• The system timezone is configured for the correct local time. See below, under POSIX
vs 'RIGHT'.
• Early during startup the following are called, in this order: adjtimex --tick value
--frequency value hwclock --hctosys
• During shutdown the following is called: hwclock --systohc
• Systems without adjtimex may use ntptime.
Whether maintaining precision time with NTP daemon or not, it makes sense to configure the
system to keep reasonably good date-time on its own.
The first step in making that happen is having a clear understanding of the big picture.
There are two completely separate hardware devices running at their own speed and drifting
away from the 'correct' time at their own rates. The methods and software for drift
correction are different for each of them. However, most systems are configured to
exchange values between these two clocks at startup and shutdown. Now the individual
device’s time keeping errors are transferred back and forth between each other. Attempt to
configure drift correction for only one of them, and the other’s drift will be overlaid
upon it.
This problem can be avoided when configuring drift correction for the System Clock by
simply not shutting down the machine. This, plus the fact that all of hwclock's precision
(including calculating drift factors) depends upon the System Clock’s rate being correct,
means that configuration of the System Clock should be done first.
The System Clock drift is corrected with the adjtimex(8) command’s --tick and --frequency
options. These two work together: tick is the coarse adjustment and frequency is the fine
adjustment. (For systems that do not have an adjtimex package, ntptime -f ppm may be used
instead.)
Some Linux distributions attempt to automatically calculate the System Clock drift with
adjtimex's compare operation. Trying to correct one drifting clock by using another
drifting clock as a reference is akin to a dog trying to catch its own tail. Success may
happen eventually, but great effort and frustration will likely precede it. This
automation may yield an improvement over no configuration, but expecting optimum results
would be in error. A better choice for manual configuration would be adjtimex's --log
options.
It may be more effective to simply track the System Clock drift with sntp, or date -Ins
and a precision timepiece, and then calculate the correction manually.
After setting the tick and frequency values, continue to test and refine the adjustments
until the System Clock keeps good time. See adjtimex(2) for more information and the
example demonstrating manual drift calculations.
Once the System Clock is ticking smoothly, move on to the Hardware Clock.
As a rule, cold drift will work best for most use cases. This should be true even for 24/7
machines whose normal downtime consists of a reboot. In that case the drift factor value
makes little difference. But on the rare occasion that the machine is shut down for an
extended period, then cold drift should yield better results.
Steps to calculate cold drift:
1
Ensure that NTP daemon will not be launched at startup.
2
The System Clock time must be correct at shutdown!
3
Shut down the system.
4
Let an extended period pass without changing the Hardware Clock.
5
Start the system.
6
Immediately use hwclock to set the correct time, adding the --update-drift option.
Note: if step 6 uses --systohc, then the System Clock must be set correctly (step 6a) just
before doing so.
Having hwclock calculate the drift factor is a good starting point, but for optimal
results it will likely need to be adjusted by directly editing the /etc/adjtime file.
Continue to test and refine the drift factor until the Hardware Clock is corrected
properly at startup. To check this, first make sure that the System Time is correct before
shutdown and then use sntp, or date -Ins and a precision timepiece, immediately after
startup.
LOCAL vs UTC
Keeping the Hardware Clock in a local timescale causes inconsistent daylight saving time
results:
• If Linux is running during a daylight saving time change, the time written to the
Hardware Clock will be adjusted for the change.
• If Linux is NOT running during a daylight saving time change, the time read from the
Hardware Clock will NOT be adjusted for the change.
The Hardware Clock on an ISA compatible system keeps only a date and time, it has no
concept of timezone nor daylight saving. Therefore, when hwclock is told that it is in
local time, it assumes it is in the 'correct' local time and makes no adjustments to the
time read from it.
Linux handles daylight saving time changes transparently only when the Hardware Clock is
kept in the UTC timescale. Doing so is made easy for system administrators as hwclock uses
local time for its output and as the argument to the --date option.
POSIX systems, like Linux, are designed to have the System Clock operate in the UTC
timescale. The Hardware Clock’s purpose is to initialize the System Clock, so also keeping
it in UTC makes sense.
Linux does, however, attempt to accommodate the Hardware Clock being in the local
timescale. This is primarily for dual-booting with older versions of MS Windows. From
Windows 7 on, the RealTimeIsUniversal registry key is supposed to be working properly so
that its Hardware Clock can be kept in UTC.
POSIX vs 'RIGHT'
A discussion on date-time configuration would be incomplete without addressing timezones,
this is mostly well covered by tzset(3). One area that seems to have no documentation is
the 'right' directory of the Time Zone Database, sometimes called tz or zoneinfo.
There are two separate databases in the zoneinfo system, posix and 'right'. 'Right' (now
named zoneinfo-leaps) includes leap seconds and posix does not. To use the 'right'
database the System Clock must be set to (UTC + leap seconds), which is equivalent to (TAI
- 10). This allows calculating the exact number of seconds between two dates that cross a
leap second epoch. The System Clock is then converted to the correct civil time, including
UTC, by using the 'right' timezone files which subtract the leap seconds. Note: this
configuration is considered experimental and is known to have issues.
To configure a system to use a particular database all of the files located in its
directory must be copied to the root of /usr/share/zoneinfo. Files are never used directly
from the posix or 'right' subdirectories, e.g., TZ='right/Europe/Dublin'. This habit was
becoming so common that the upstream zoneinfo project restructured the system’s file tree
by moving the posix and 'right' subdirectories out of the zoneinfo directory and into
sibling directories:
/usr/share/zoneinfo, /usr/share/zoneinfo-posix, /usr/share/zoneinfo-leaps
Unfortunately, some Linux distributions are changing it back to the old tree structure in
their packages. So the problem of system administrators reaching into the 'right'
subdirectory persists. This causes the system timezone to be configured to include leap
seconds while the zoneinfo database is still configured to exclude them. Then when an
application such as a World Clock needs the South_Pole timezone file; or an email MTA, or
hwclock needs the UTC timezone file; they fetch it from the root of /usr/share/zoneinfo ,
because that is what they are supposed to do. Those files exclude leap seconds, but the
System Clock now includes them, causing an incorrect time conversion.
Attempting to mix and match files from these separate databases will not work, because
they each require the System Clock to use a different timescale. The zoneinfo database
must be configured to use either posix or 'right', as described above, or by assigning a
database path to the TZDIR environment variable.
EXIT STATUS
One of the following exit values will be returned:
EXIT_SUCCESS ('0' on POSIX systems)
Successful program execution.
EXIT_FAILURE ('1' on POSIX systems)
The operation failed or the command syntax was not valid.
ENVIRONMENT
TZ
If this variable is set its value takes precedence over the system configured
timezone.
TZDIR
If this variable is set its value takes precedence over the system configured timezone
database directory path.
FILES
/etc/adjtime
The configuration and state file for hwclock. See also adjtime_config(5).
/etc/localtime
The system timezone file.
/usr/share/zoneinfo/
The system timezone database directory.
Device files hwclock may try for Hardware Clock access: /dev/rtc0 /dev/rtc /dev/misc/rtc
/dev/efirtc /dev/misc/efirtc
SEE ALSO
date(1), adjtime_config(5), adjtimex(8), gettimeofday(2), settimeofday(2), crontab(1p),
tzset(3)
AUTHORS
Written by Bryan Henderson <bryanh@giraffe-data.com>, September 1996, based on work done
on the clock(8) program by Charles Hedrick, Rob Hooft, and Harald Koenig. See the source
code for complete history and credits.
REPORTING BUGS
For bug reports, use the issue tracker at https://github.com/util-linux/util-linux/issues.
AVAILABILITY
The hwclock command is part of the util-linux package which can be downloaded from Linux
Kernel Archive <https://www.kernel.org/pub/linux/utils/util-linux/>.