hwclock - query and set the hardware clock (RTC)
SYNOPSIS
hwclock -r or hwclock --show
hwclock -w or hwclock --systohc
hwclock -s or hwclock --hctosys
hwclock -a or hwclock --adjust
hwclock -v or hwclock --version
hwclock --set --date=newdate
hwclock --getepoch
hwclock --setepoch --epoch=year
other options:
[-u|--utc] --localtime --noadjfile --directisa --test
[-D|--debug]
and arcane options for DEC Alpha:
[-A|--arc] [-J|--jensen] [-S|--srm] [-F|--funky-toy]
Minimum unique abbreviations of all options are accept
able.
Also, -h asks for a help message.
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 to the System Time,
and set the System Time from the Hardware Clock.
You can also run hwclock periodically to insert or remove
time from the Hardware Clock to compensate for systematic
drift (where the clock consistently gains or loses time at
a certain rate if left to run).
OPTIONS
You need exactly one of the following options to tell
hwclock what function to perform:
--show Read the Hardware Clock and print the time on Stan
dard Output. The time shown is always in local
time, even if you keep your Hardware Clock in Coor
dinated Universal Time. See the --utc option.
--set Set the Hardware Clock to the time given by the
--date option.
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 vari
able and/or /usr/lib/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.
--systohc
Set the Hardware Clock to the current System Time.
--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 out standard output the kernel's Hardware
Clock epoch value. This 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 Counter 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.
--version
Print the version of hwclock on Standard Output.
--epoch=year
Specifies the year which is the beginning of the
Hardware Clock's epoch. I.e. 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.
hwclock --setepoch --epoch=1952
You need the following option if you specify the --set
option. Otherwise, it is ignored.
--date=date_string
Specifies the time to which to set the Hardware
Clock. The value of this option is an argument to
the date(1) program. For example,
hwclock --set --date="9/22/96 16:45:05"
The argument is in local time, even if you keep
your Hardware Clock in Coordinated Universal time.
See the --utc option.
The following options apply to most functions.
--utc
--localtime
Indicates that the Hardware Clock is kept in Coor
dinated 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 specified the last time
hwclock was used to set the clock (i.e. hwclock was
successfully run with the --set , --systohc , or
--adjust options), as recorded in the adjtime file.
If the adjtime file doesn't exist, the default is
local 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.
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 read), it
will use the explicit I/O instructions anyway.
The rtc device driver was new in Linux Release 2.
--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 calibrated date in the
adjtime file, by assuming that 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.
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 main
tainer to see if the program can be improved to
detect your system automatically. Output of
`hwclock --debug' and `cat /proc/cpuinfo' may be of
interest.)
--jensen means you are running on a Jensen model.
--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 indepen
dently of any control program running in the CPU and even
when the machine is powered off.
ISA standard. The control program 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 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 because all of the other names are inappro
priate to the point of being misleading.
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 stan
dard). 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 num
ber of seconds since 1969). 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 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 discon
tinuities 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. You can also use the program adjtimex(8) to
smoothly adjust the System Time while the system runs.
A Linux kernel maintains a concept of a local timezone for
the system. But don't be misled -- almost nobody cares
what timezone the kernel thinks it is in. Instead, pro
grams 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/local/timezone 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 ker
nel 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.
hwclock sets the kernel timezone to the value indicated by
TZ and/or /usr/local/timezone when you set the System Time
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_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).)
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 hanging).
On older systems, the method of accessing the Hardware
Clock depends on the system hardware.
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 inter
face 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
provides 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, 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 regis
ters without even trying /dev/rtc by specifying the
--directisa option.
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 make systematic corrections to correct the systematic
drift.
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. 5 days later, the clock has gained 10
seconds, so you issue another hwclock --set command to set
it back 10 seconds. Hwclock updates the adjtime file to
show the current time as the last time the clock was cali
brated, 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 calibra
tion, 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 peri
odically 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.
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 calibra
tion 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 Hard
ware Clock every 11 minutes. This is a good mode to use
when you are using something sophisticated 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 any
thing, including hwclock --hctosys, that sets the System
Time the old fashioned way.
To see if it is on or off, use the command adjtimex
--print and look at the value of "status". If the "64"
bit of this number (expressed in binary) equal to 0, 11
minute mode is on. Otherwise, it is off.
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.
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, con
tact 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.
ENVIRONMENT VARIABLES
TZ
FILES
/etc/adjtime /usr/lib/zoneinfo/ /dev/rtc /dev/port
/dev/tty1 /proc/cpuinfo
SEE ALSO
adjtimex(8), date(1), 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
program by Charles Hedrick, Rob Hooft, and Harald Koenig.
See the source code for complete history and credits.
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