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HWCLOCK(8) System Administration HWCLOCK(8)
NAME
hwclock - query or set the hardware clock (RTC)
SYNOPSIS
hwclock [function] [option...]
DESCRIPTION
hwclock is a tool for accessing the Hardware Clock. You can display the current time, set
the Hardware Clock to a specified time, set the Hardware Clock from the System Time, or
set the System Time from the Hardware Clock.
You can also run hwclock periodically to add or subtract time from the Hardware Clock to
compensate for systematic drift (where the clock consistently loses or gains time at a
certain rate when left to run).
FUNCTIONS
You need exactly one of the following options to tell hwclock what function to perform:
-r, --show
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 Coordinated Universal
Time. See the --utc option. Showing the Hardware Clock time is the default when
no function is specified.
--set Set the Hardware Clock to the time given by the --date option.
-s, --hctosys
Set the System Time from the Hardware Clock.
Also set the kernel's timezone value to the local timezone as indicated by the TZ
environment variable and/or /usr/share/zoneinfo, as tzset(3) would interpret them.
The obsolete tz_dsttime field of the kernel's timezone value is set to DST_NONE.
(For details on what this field used to mean, see settimeofday(2).)
This is a good option to use in one of the system startup scripts.
-w, --systohc
Set the Hardware Clock to the current System Time.
--systz
Set the kernel's timezone and reset the System Time based on the current timezone.
The system time is only reset on the first call after boot.
The local timezone is taken to be what is indicated by the TZ environment variable
and/or /usr/share/zoneinfo, as tzset(3) would interpret them. The obsolete tz_dst-
time field of the kernel's timezone value is set to DST_NONE. (For details on what
this field used to mean, see settimeofday(2).)
This is an alternate option to --hctosys that does not read the hardware clock, and
may be used in system startup scripts for recent 2.6 kernels where you know the
System Time contains the Hardware Clock time. If the Hardware Clock is already in
UTC, it is not reset.
--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 discussion below.
--getepoch
Print the kernel's Hardware Clock epoch value to standard output. This is the num-
ber 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.
This epoch value is used whenever hwclock reads or sets the Hardware Clock.
--setepoch
Set the kernel's Hardware Clock epoch value to the value specified by the --epoch
option. See the --getepoch option for details.
--predict
Predict what the RTC will read at time given by the --date option based on the adj-
time file. This is useful for example if you need to set an RTC wakeup time to dis-
tant future and want to account for the RTC drift.
-c, --compare
Periodically compare the Hardware Clock to the System Time and output the differ-
ence every 10 seconds. This will also print the frequency offset and tick.
-h, --help
Display a help text and exit.
-V, --version
Display the version of hwclock and exit.
OPTIONS
The first two options apply to just a few specific functions, the others apply to most
functions.
--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 an
argument to the date(1) program. 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 Coordi-
nated Universal time. See the --utc option.
--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, or otherwise to specify the epoch for use
with direct ISA access.
For example, on a Digital Unix machine:
hwclock --setepoch --epoch=1952
-u, --utc
--localtime
Indicates that the Hardware Clock is kept in Coordinated Universal Time or local
time, respectively. It is your choice whether to keep your clock in UTC or local
time, but nothing in the clock tells which you've chosen. So this option is how
you give that information to hwclock.
If you specify the wrong one of these options (or specify neither and take a wrong
default), both setting and querying of the Hardware Clock will be messed up.
If you specify neither --utc nor --localtime, the default is whichever was speci-
fied the last time hwclock was used to set the clock (i.e. hwclock was success-
fully run with the --set, --systohc, or --adjust options), as recorded in the adj-
time file. If the adjtime file doesn't exist, the default is UTC time.
--noadjfile
Disables 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.
--adjfile=filename
Overrides the default /etc/adjtime.
-f, --rtc=filename
Overrides the default /dev file name, which is /dev/rtc on many platforms but may
be /dev/rtc0, /dev/rtc1, and so on.
--directisa
This option is meaningful only on an ISA machine or an Alpha (which implements
enough of ISA to be, roughly speaking, an ISA machine for hwclock's purposes). 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 try
to use the /dev/rtc device (which it assumes to be driven by the RTC device
driver). If it is unable to open the device (for reading), it will use the
explicit I/O instructions anyway.
--badyear
Indicates 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 cal-
ibrated 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, which-
ever 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.
--srm This option is equivalent to --epoch=1900 and is used to specify the most common
epoch on Alphas with SRM console.
--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 epoch 1900).
--jensen
--funky-toy
These two options specify what kind of Alpha machine you have. They are invalid if
you don't have an Alpha and are usually unnecessary if you do, because hwclock
should be able to determine by itself what it's running on, at least when /proc is
mounted. (If you find you need one of these options to make hwclock work, contact
the maintainer to see if the program can be improved to detect your system automat-
ically. Output of `hwclock --debug' and `cat /proc/cpuinfo' may be of interest.)
Option --jensen means you are running on a Jensen model. And --funky-toy means
that on your machine one has to use the UF bit instead of the UIP bit in the Hard-
ware Clock to detect a time transition. "Toy" in the option name refers to the
Time Of Year facility of the machine.
--test Do everything except actually updating the Hardware Clock or anything else. This
is useful, especially in conjunction with --debug, in learning about hwclock.
--debug
Display a lot of information about what hwclock is doing internally. Some of its
function is complex and this output can help you understand how the program works.
NOTES
Clocks in a Linux System
There are two main clocks in a Linux system:
The Hardware Clock: This is a clock that runs independently of any control program running
in the CPU and even when the machine is powered off.
On an ISA system, this clock is specified as part of the ISA standard. The control pro-
gram can read or set this clock to a whole second, but the control program can also detect
the edges of the 1 second clock ticks, so the clock actually has virtually infinite preci-
sion.
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 because all of the other names are inappropriate to the point of being misleading.
So for example, 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 inte-
grated real-time clock which is used for most other purposes.
The System Time: This is the time kept by a clock inside the Linux kernel and 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). The System Time is not an integer, though. It has virtually infinite preci-
sion.
The System Time is the time that matters. The Hardware Clock's basic purpose in a Linux
system is to keep time when Linux is not running. You initialize the System Time to the
time from the Hardware Clock when Linux starts up, and then never use the Hardware Clock
again. 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(1L) 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.
A Linux kernel maintains a concept of a local timezone for the system. But don't be mis-
led -- 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 and/or the /usr/share/zoneinfo directory, 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 timezone value. An example is the vfat filesystem. If the
kernel timezone value is wrong, the vfat filesystem will report and set the wrong time-
stamps on files.
hwclock sets the kernel timezone to the value indicated by TZ and/or /usr/share/zoneinfo
when you set the System Time using the --hctosys option.
The 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_dst-
time 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).)
Users access and setuid
Sometimes, you need to install hwclock setuid root. If you want users other than the supe-
ruser to be able to display the clock value using the direct ISA I/O method, install it
setuid root. If you have the /dev/rtc interface on your system or are on a non-ISA system,
there's probably no need for users to use the direct ISA I/O method, so don't bother.
In any case, hwclock will not allow you to set anything unless you have the superuser real
uid. (This is restriction is not necessary if you haven't installed setuid root, but it's
there for now).
How hwclock Accesses the Hardware Clock
hwclock uses many different ways to get and set Hardware Clock values. The most normal
way is to do I/O to the device special file /dev/rtc, which is presumed to be driven by
the rtc device driver. However, this method is not always available. For one thing, the
rtc driver is a relatively recent addition to Linux. Older systems don't have it. Also,
though there are versions of the rtc driver that work on DEC Alphas, there appear to be
plenty of Alphas on which the rtc driver does not work (a common symptom is hwclock hang-
ing). Moreover, recent Linux systems have more generic support for RTCs, even systems
that have more than one, so you might need to override the default by specifying /dev/rtc0
or /dev/rtc1 instead.
On older systems, the method of accessing the Hardware Clock depends on the system hard-
ware.
On an ISA 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. (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).
This is a really poor method of accessing the clock, for all the reasons that user space
programs are generally not supposed to do direct I/O and disable interrupts. Hwclock pro-
vides it because it is the only method available on ISA and Alpha systems which don't have
working rtc device drivers available.
On an m68k system, hwclock can access the clock via the console driver, via the device
special file /dev/tty1.
hwclock tries to use /dev/rtc. If it is compiled for a kernel that doesn't have that
function or it is unable to open /dev/rtc (or the alternative special file you've defined
on the command line) hwclock will fall back to another method, if available. On an ISA or
Alpha machine, you can force hwclock to use the direct manipulation of the CMOS registers
without even trying /dev/rtc by specifying the --directisa option.
The Adjust Function
The Hardware Clock is usually not very accurate. However, much of its inaccuracy is com-
pletely predictable - it gains or loses the same amount of time every day. This is called
systematic drift. hwclock's "adjust" function lets you make systematic corrections to
correct the systematic drift.
It works like this: hwclock keeps a file, /etc/adjtime, that keeps some historical infor-
mation. 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. 5 days later, the clock
has gained 10 seconds, so you issue another hwclock --set command to set it back 10 sec-
onds. 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
goes 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.
Every time you calibrate (set) the clock (using --set or --systohc), hwclock recalculates
the systematic drift rate based on how long it has been since the last calibration, how
long it has been since the last adjustment, what drift rate was assumed in any intervening
adjustments, and the amount by which the clock is presently off.
A small amount of error creeps in any time hwclock sets the clock, so it refrains from
making an adjustment that would be less than 1 second. Later on, when you request an
adjustment again, the accumulated drift will be more than a second and hwclock will do the
adjustment then.
It is good to do a hwclock --adjust just before the hwclock --hctosys at system startup
time, and maybe periodically while the system is running via cron.
The adjtime file, while named for its historical purpose of controlling adjustments only,
actually contains other information for use by hwclock in remembering information from one
invocation to the next.
The format of the adjtime file is, in ASCII:
Line 1: 3 numbers, separated by blanks: 1) systematic drift rate in seconds per day,
floating point decimal; 2) 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: 1 number: 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 Univer-
sal 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 is a good mode to use when you are using something sophisti-
cated like ntp to keep your System Time 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).
This mode (we'll call it "11 minute mode") is off until something turns it on. The ntp
daemon xntpd is one thing that turns it on. You can turn it off by running anything,
including hwclock --hctosys, that sets the System Time the old fashioned way.
If your system runs with 11 minute mode on, don't use hwclock --adjust or hwclock
--hctosys. You'll just make a mess. It is acceptable to use a hwclock --hctosys at
startup time to get a reasonable System Time until your system is able to set the System
Time from the external source and start 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.
ENVIRONMENT VARIABLES
TZ
FILES
/etc/adjtime /usr/share/zoneinfo/ (/usr/lib/zoneinfo on old systems) /dev/rtc /dev/rtc0
/dev/port /dev/tty1 /proc/cpuinfo
SEE ALSO
date(1), gettimeofday(2), settimeofday(2), crontab(1), tzset(3)
AUTHORS
Written by Bryan Henderson, September 1996 (bryanh AT giraffe-data.com), based on work done
on the clock 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.ker-
nel.org/pub/linux/utils/util-linux/.
util-linux August 2011 HWCLOCK(8)
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