Computer Science
CLOCK(8) CLOCK(8)
NAME
clock - query and set the hardware clock (RTC)
SYNOPSIS
hwclock --show
hwclock --set --date=newdate
hwclock --systohc
hwclock --hctosys
hwclock --getepoch
hwclock --setepoch --epoch=year
hwclock --adjust
hwclock --version
other options:
--utc --directisa --test --debug
Minimum unique abbreviations of all options are accept-
able.
Also, equivalent options -r, -w, -s, -a, -v, -u, and -D
are accepted for compatibility with the program "clock".
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.
--set Set the Hardware Clock to the time given by the
--date option.
--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 vari-
able and/or /usr/lib/zoneinfo, as tzset(3) would
interpret them. EXCEPT: always set the Daylight
Savings Time part of the kernel's timezone value to
0 ("not Daylight Savings Time"). If DST is indi-
cated, just add an hour to the base part.
See the discussion of timezones below.
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.
You need the following option if you specify --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"
--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.
For example,
hwclock --setepoch --epoch=1952
The following options apply to most functions.
--utc Indicates that the Hardware Clock is kept in Coor-
dinated Universal Time. It is your choice whether
to keep your clock in UTC or local time, but noth-
ing in the clock tells which you've chosen. So
this option is how you give that information to
hwclock.
In order to avoid any possible misunderstandings:
all conversation with the program hwclock is done
in local time. If you have your clock in UTC (as is
common on a Unix system) then the option --utc
tells hwclock to do the appropriate conversions:
`hwclock --show --utc' will tell you the local
time, assuming the hardware clock is in UTC, and
`hwclock --set --date="XXX" --utc' will set the
hardware clock to the UTC equivalent of the local
time XXX.
--directisa
is meaningful only on an ISA machine. For all
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.
--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.
On an ISA system, this clock is specified as part of the
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
using the --hctosys option.
A complication is that the timezone value actually con-
sists of two parts: 1) how far from the Standard Meridian
the locality is geographically, and 2) whether or not a
Daylight Savings Time (DST) convention is in effect in the
locality at the present time. In practice, the DST part
of the timezone value is almost never used, so if the geo-
graphical part were to be set to its correct value, the
users of the timezone value would actually compute the
wrong local time.
Therefore, hwclock violates the definition of the kernel's
timezone value and always sets the DST part to zero. If
DST is supposed to be in effect, hwclock simply adds an
hour to the geographical part.
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.
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 can only do this if running
with superuser effective userid.
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 with
older Linux kernels for ISA machines.
On an m68k system, hwclock can access the clock via the
console driver, via the device special file /dev/tty1.
On an Alpha, /dev/rtc is the only choice.
There are or were some Alpha Linux systems that don't have
/dev/rtc and there are or were programs that accessed the
clock via almost direct I/O using /dev/port. However,
this is not as good a method as /dev/rtc and such programs
were not widely enough used that hwclock has any need to
be backward compatible with them. So hwclock does not
provide the /dev/port method and consequently will not
work on an Alpha that doesn't have /dev/rtc.
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 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 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 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 ).
Line 2: 1 number: Resulting number of seconds since 1969
UTC of most recent calibration.
You can use an adjtime file that was previously used with
the clock 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.
ENVIRONMENT VARIABLES
TZ
FILES
/etc/adjtime /usr/lib/zoneinfo/
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.
02 March 1998 1
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