HWCLOCK(8) System Administration HWCLOCK(8)
hwclock - read or set the hardware clock (RTC)
hwclock [function] [option...]
hwclock is a tool for accessing the Hardware Clock. 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); compare the System and Hardware Clocks; 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.
The following functions are mutually exclusive, only one can be given
at a time. If none is given, the default is --show.
Add or subtract time from the Hardware Clock to account for sys-
tematic drift since the last time the clock was set or adjusted.
See the discussion below, under The Adjust Function.
Periodically compare the Hardware Clock to the System Time and
output the difference every 10 seconds. This will also print
the frequency offset and tick.
These functions are for Alpha machines only.
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 you are using the con-
vention that the year counter in your Hardware Clock contains
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.
This epoch value is used whenever hwclock reads or sets the
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
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
Read the Hardware Clock and print the time on standard output.
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
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 any-
thing other than the current operating system's hwclock command,
such as '11 minute mode' or from dual-booting another OS.
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 discus-
sion 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
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 configura-
tion is changed then a reboot is required to inform the kernel.
See the discussion below, under Automatic Hardware Clock Syn-
chronization 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 func-
tion. 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 call-
ing 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
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 (re)calculate the drift factor.
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
o 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.
o Sets the kernel's NTP '11 minute mode' timescale.
o 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.
Set the Hardware Clock from the System Clock, and update the
timestamps in /etc/adjtime. When the --update-drift option is
given, then also (re)calculate the drift factor.
Display version information and exit.
Display help text and exit.
Override the default /etc/adjtime file path.
Indicate that the Hardware Clock is incapable of storing years
outside the range 1994-1999. There is a problem in some BIOSes
(almost all Award BIOSes made between 4/26/94 and 5/31/95)
wherein they are unable to deal with years after 1999. If one
attempts to set the year-of-century value to something less than
94 (or 95 in some cases), the value that actually gets set is 94
(or 95). Thus, if you have one of these machines, hwclock can-
not set the year after 1999 and cannot use the value of the
clock as the true time in the normal way.
To compensate for this (without your getting a BIOS update,
which would definitely be preferable), always use --badyear if
you have one of these machines. When hwclock knows it's working
with a brain-damaged clock, it ignores the year part of the
Hardware Clock value and instead tries to guess the year based
on the last calibrated date in the adjtime file, by assuming
that date is within the past year. For this to work, you had
better do a hwclock --set or hwclock --systohc at least once a
Though hwclock ignores the year value when it reads the Hardware
Clock, it sets the year value when it sets the clock. It sets
it to 1995, 1996, 1997, or 1998, whichever one has the same
position in the leap year cycle as the true year. That way, the
Hardware Clock inserts leap days where they belong. Again, if
you let the Hardware Clock run for more than a year without set-
ting it, this scheme could be defeated and you could end up los-
ing a day.
You need this option if you specify the --set or --predict func-
tions, otherwise it is ignored. It specifies the time to which
to set the Hardware Clock, or the time for which to predict the
Hardware Clock reading. The value of this option is used as an
argument to the date(1) program's --date option. For example:
hwclock --set --date='2011-08-14 16:45:05'
The argument must be in local time, even if you keep your Hard-
ware Clock in UTC. See the --localtime option. The argument
must 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.
Display a lot of information about what hwclock is doing inter-
nally. Some of its functions are complex and this output can
help you understand how the program works.
This option is meaningful for: ISA compatible machines including
x86, and x86_64; and Alpha (which has a similar Hardware Clock
interface). 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, which it assumes to be driven by the 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 condi-
tion 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.
Override hwclock's default rtc device file name. Otherwise it
will use the first one found in this order:
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.
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, i.e., the Clocks
or adjtime file. This is useful, especially in conjunction with
--debug, in learning about the internal operations of hwclock.
Update the Hardware Clock's drift factor in /etc/adjtime. It is
used with --set or --systohc, otherwise it is ignored.
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 sys-
tems to call hwclock --systohc at shutdown; with the old behav-
iour this would automatically (re)calculate the drift factor
which caused several problems:
o When using ntpd with an '11 minute mode' kernel the drift fac-
tor would be clobbered to near zero.
o 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.
o (Re)calculating drift factor on every shutdown delivers subop-
timal results. For example, if ephemeral conditions cause the
machine to be abnormally hot the drift factor calculation
would be out of range.
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.
OPTIONS FOR ALPHA MACHINES ONLY
--arc This option is equivalent to --epoch=1980 and is used to specify
the most common epoch on Alphas with an ARC console (although
Ruffians have an epoch of 1900).
Specifies the year which is the beginning of the Hardware
Clock's epoch, that is the number of years into AD to which a
zero value in the Hardware Clock's year counter refers. It is
used together with the --setepoch option to set the kernel's
idea of the epoch of the Hardware Clock.
For example, on a Digital Unix machine:
hwclock --setepoch --epoch=1952
These two options specify what kind of Alpha machine you have.
They are invalid if you do not have an Alpha and are usually
unnecessary if you do; hwclock should be able to determine what
it is running on when /proc is mounted.
The --jensen option is used for Jensen models; --funky-toy means
that the machine requires the UF bit instead of the UIP bit in
the Hardware Clock to detect a time transition. The "toy" in
the option name refers to the Time Of Year facility of the
--srm This option is equivalent to --epoch=1900 and is used to specify
the most common epoch on Alphas with an SRM console.
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 virtu-
ally 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 Hard-
ware 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 vari-
able 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
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 local-
ity 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 mem-
ory" 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 rea-
sons that userspace programs are generally not supposed to do direct
I/O and disable interrupts. hwclock provides it for testing, trou-
bleshooting, and because it may be the only method available on ISA
compatible and Alpha systems which do not have a working rtc device
In the case of a Jensen Alpha, there is no way for hwclock to execute
those I/O instructions, and so it uses instead the /dev/port device
special file, which provides almost as low-level an interface to the
On an m68k system, hwclock can access the clock with the console
driver, via the device special file /dev/tty1.
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
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 com-
mand to set the Hardware Clock to the true current time. hwclock cre-
ates 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 cur-
rent time as the last time the clock was calibrated, and records 2 sec-
onds 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 time-
stamp from the adjtime file. This updated drift factor is then saved
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 sec-
ond. 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 invoca-
tion 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 num-
ber of seconds since 1969 UTC of most recent adjustment or calibration,
decimal integer; 3) zero (for compatibility with clock(8)) as a 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 con-
tain 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 syn-
chronized 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 capa-
bility. 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 ntpd(1), 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 capa-
bility 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.
Keeping Time without External Synchronization
This discussion is based on the following conditions:
o Nothing is running that alters the date-time clocks, such as ntpd(1)
or a cron job.
o The system timezone is configured for the correct local time. See
below, under POSIX vs 'RIGHT'.
o Early during startup the following are called, in this order:
adjtimex --tick value --frequency value
o During shutdown the following is called:
* Systems without adjtimex may use ntptime.
Whether maintaining precision time with ntpd(1) 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 fac-
tors) 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
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 automa-
tion may yield an improvement over no configuration, but expecting
optimum results would be in error. A better choice for manual configu-
ration 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
After setting the tick and frequency values, continue to test and
refine the adjustments until the System Clock keeps good time. See
adjtimex(8) for more information and the example demonstrating manual
Once the System Clock is ticking smoothly, move on to the Hardware
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 ntpd(1) 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
Note: if step 6 uses --systohc, then the System Clock must be set cor-
rectly (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:
o If Linux is running during a daylight saving time change, the time
written to the Hardware Clock will be adjusted for the change.
o If Linux is NOT running during a daylight saving time change, the
time read from the Hardware Clock will NOT be adjusted for the
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 oper-
ate 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 ver-
sions of MS Windows. From Windows 7 on, the RealTimeIsUniversal reg-
istry 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:
Unfortunately, some Linux distributions are changing it back to the old
tree structure in their packages. So the problem of system administra-
tors 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.
TZ If this variable is set its value takes precedence over the sys-
tem configured timezone.
TZDIR If this variable is set its value takes precedence over the sys-
tem configured timezone database directory path.
The configuration and state file for hwclock.
The system timezone file.
The system timezone database directory.
Device files hwclock may try for Hardware Clock access:
date(1), adjtimex(8), gettimeofday(2), settimeofday(2), crontab(1),
Written by Bryan Henderson, September 1996 (email@example.com),
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
The hwclock command is part of the util-linux package and is available
util-linux April 2015 HWCLOCK(8)
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