hwclock.8

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.B 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 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.
.PP
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 
.B hwclock 
because all of the other names are inappropriate to the point of being
misleading.
.PP
.B 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 precision.
.PP
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.
.PP
It is important that the System Time not have any discontinuities such as
would happen if you used the 
.BR 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 
.BR adjtimex (8)
to smoothly adjust the System Time while the system runs.
.PP
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, 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/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 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 timestamps on files.
.PP
.B hwclock
sets the kernel timezone to the value indicated by TZ and/or
/usr/local/timezone when you set the System Time using the 
.B \-\-hctosys
option.
.PP
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
.BR settimeofday (2).)

.SH How hwclock Accesses the Hardware Clock
.PP
.B 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).
.PP
On older systems, the method of accessing the Hardware Clock depends on
the system hardware. 
.PP
On an ISA system, 
.B 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
.B 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 provides it because it is
the only method available on ISA and Alpha systems which don't have
working rtc device drivers available.

.PP
On an m68k system,
.B hwclock
can access the clock via the console driver, via the device special
file /dev/tty1.
.PP
.B 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, 
.B hwclock 
will fall back to another method, if available.  On an ISA or Alpha
machine, you can force
.B hwclock
to use the direct manipulation of the CMOS registers without even trying
.I /dev/rtc
by specifying the \-\-directisa option.


.SH The Adjust Function
.PP
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.
.BR hwclock 's 
"adjust" function lets you make systematic corrections to correct the
systematic drift.
.PP
It works like this:  
.B hwclock 
keeps a file,
.I /etc/adjtime,
that keeps some historical information.  This is called the adjtime file.
.PP
Suppose you start with no adjtime file.  You issue a 
.I hwclock \-\-set
command to set the Hardware Clock to the true current time.  
.B 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
.I hwclock \-\-set
command to set it back 10 seconds.  
.B 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
.I hwclock \-\-adjust
command.  
.B 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
.I hwclock \-\-adjust.
.B 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.
.PP
Every time you calibrate (set) the clock (using 
.I \-\-set
or
.I \-\-systohc
),
.B 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.
.PP
A small amount of error creeps in any time 
.B 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
.B hwclock 
will do the adjustment then.
.PP
It is good to do a 
.I hwclock \-\-adjust
just before the 
.I hwclock \-\-hctosys
at system startup time, and maybe periodically while the system is
running via cron.
.PP
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.
.PP
The format of the adjtime file is, in ASCII:
.PP
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
.BR clock (8))
as a decimal integer.
.PP
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.
.PP
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 
.B hwclock
command line.
.PP
You can use an adjtime file that was previously used with the 
.BR clock (8)
program with 
.B hwclock.


.SH "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 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 anything, including
.IR "hwclock \-\-hctosys" ,
that sets the System Time the old fashioned way.

To see if it is on or
off, use the command 
.I 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 
.I hwclock \-\-adjust
or
.IR "hwclock \-\-hctosys" .
You'll just make a mess.  It is acceptable to use a
.I 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.


.SH 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.  
.B 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 
.B 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.



.SH "ENVIRONMENT VARIABLES"
.I TZ

.SH FILES
.I /etc/adjtime
.I /usr/share/zoneinfo/
.RI ( /usr/lib/zoneinfo
on old systems)
.I /dev/rtc
.I /dev/port
.I /dev/tty1
.I /proc/cpuinfo

.SH "SEE ALSO"
.BR adjtimex (8),
.BR date (1),
.BR gettimeofday (2),
.BR settimeofday (2),
.BR crontab (1),
.BR tzset (3)

.SH AUTHORS
Written by Bryan Henderson, September 1996 ([e-mail]),
based on work done on the
.I clock
program by Charles Hedrick, Rob Hooft, and Harald Koenig.  
See the source code for complete history and credits.