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,
the --directisa option, and a new option --update-drift was added.
See their respective sections below.
The following functions are mutually exclusive, only one can be given
at a time. If none are given the default is --show.
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 AdjustFunction.
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
convention 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 set function requires using the --epoch option.
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
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 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
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,
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
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
· 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.
Set the Hardware Clock from the System Clock, and update the
timestamps in /etc/adjtime. With the --update-drift option
(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 cannot set the year after 1999 and
cannot use the value of the clock as the true time in the
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 year!
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 setting it, this scheme could be defeated and you
could end up losing a day.
hwclock warns you that you probably need --badyear whenever it
finds your Hardware Clock set to 1994 or 1995.
You need this option if you specify the --set or --predict
functions, 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.
hwclock --set --date='2011-08-14 16:45:05'
The argument must be in local time, even if you keep your
Hardware 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
internally. 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 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.
Override hwclock's default rtc device file name. Otherwise it
will use the first one found in this order:
For IA-64:/dev/efirtc/dev/misc/efirtc--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
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
systems to call hwclock --systohc at shutdown; with the old
behaviour this would automatically (re)calculate the drift
factor which caused several problems:
· When using ntpd 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.
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.
--arc This option is equivalent to --epoch=1980 and is used to
specify the most common epoch on Alphas with ARC console (but
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--funky-toy--jensen
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. "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 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 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
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 AutomaticHardware 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).
User access and setuid
Sometimes, you need to install hwclock setuid root. If you want
users other than the superuser to be able to display the clock value
using the direct ISA I/O method, install it setuid root. If you have
the rtc device interface on your system, or are on a non-ISA
compatible system, there is probably no need for users to have the
direct ISA I/O method, so do not bother. See the --rtc option.
In any case, hwclock will not allow you to set anything unless you
have the superuser real uid. (This restriction is not necessary if
you haven't installed setuid root, but it's there for now.)
Hardware Clock Access Methodshwclock 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
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
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 compatible and Alpha systems which do not have a working rtc
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 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
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 decimal integer.
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 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 capability and the Hardware Clock timescale
configuration is changed, then a reboot is required to notify the
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
Keeping Time without External Synchronization
This discussion is based on the following conditions:
· Nothing is running that alters the date-time clocks, e.g., ntpd(1),
cron jobs, et al.
· The system timezone is configured for the correct local time. See
below POSIX vs 'RIGHT'.
· Early in 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 ntpd(1) or not, it makes
sense to configure the system to keep reasonably good date-time on
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. If the big picture is not kept in
mind, confusion will soon ensue.
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
course adjustment and frequency is the fine adjustment. (For system
that do not have an adjtimex package, ntptime -f <ppm> may be use
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
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 shutdown for an extended
period then cold drift should yield better results.
Steps to calculate cold drift:
1 Confirm that ntpd(1) will not be launched at startup.
2 The System Clock time must be correct at shutdown!
3 Shutdown 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 with the
Note: if step six 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
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
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
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
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
system configured timezone.
TZDIR If this variable is set its value takes precedence over the
system 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:
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
This page is part of the util-linux (a random collection of Linux
utilities) project. Information about the project can be found at
⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have a
bug report for this manual page, send it to
firstname.lastname@example.org. This page was obtained from the
project's upstream Git repository
2015-04-19. If you discover any rendering problems in this HTML ver‐
sion of the page, or you believe there is a better or more up-to-date
source for the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original man‐
ual page), send a mail to email@example.com
util-linux January 2015 HWCLOCK(8)