Provided by: util-linux_2.27.1-6ubuntu3.10_amd64 
NAME
hwclock - read or set the hardware clock (RTC)
SYNOPSIS
hwclock [function] [option...]
DESCRIPTION
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.
FUNCTIONS
The following functions are mutually exclusive, only one can be given at a time. If none is given, the
default is --show.
--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.
-c, --compare
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.
--getepoch
--setepoch
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 --setepoch function requires using the --epoch option to specify the year.
This epoch value is used whenever hwclock reads or sets the Hardware Clock.
--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 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 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 (re)calculate the drift factor.
--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. When the
--update-drift option is given, then also (re)calculate the drift factor.
-V, --version
Display version information and exit.
-h, --help
Display help text and exit.
OPTIONS
--adjfile=filename
Override the default /etc/adjtime file path.
--badyear
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 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 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.
--date=date_string
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. For example:
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.
-D, --debug
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.
--directisa
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.
-f, --rtc=filename
Override hwclock's default rtc device file name. Otherwise it will use the first one found in
this order:
/dev/rtc
/dev/rtc0
/dev/misc/rtc
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 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, i.e., the Clocks or adjtime file. This is useful,
especially in conjunction with --debug, in learning about the internal operations of hwclock.
--update-drift
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.
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).
--epoch=year
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. The "toy" in the
option name refers to the Time Of Year facility of the machine.
--srm This option is equivalent to --epoch=1900 and is used to specify the most common epoch on Alphas
with an SRM console.
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 compatible and Alpha systems
which do not have a working rtc device driver.
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
I/O subsystem.
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 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 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 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 ntpd(1) 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 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
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(8) 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 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 --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.
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.
/etc/localtime
The system timezone file.
/usr/share/zoneinfo/
The system timezone database directory.
Device files hwclock may try for Hardware Clock access:
/dev/rtc
/dev/rtc0
/dev/misc/rtc
/dev/efirtc
/dev/misc/efirtc
/dev/port
/dev/tty1
SEE ALSO
date(1), adjtimex(8), gettimeofday(2), settimeofday(2), crontab(1), tzset(3)
AUTHORS
Written by Bryan Henderson, September 1996 (bryanh@giraffe-data.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
credits.
AVAILABILITY
The hwclock command is part of the util-linux package and is available from
ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
util-linux April 2015 HWCLOCK(8)