HWCLOCK(8) System Administration HWCLOCK(8)
NAME
hwclock - time clocks utility
SYNOPSIS
hwclock [function] [option...]
DESCRIPTION
hwclock is an administration tool for the time clocks. 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); 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.
-a, --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.
--getepoch; --setepoch
These functions are for Alpha machines only, and are only available
through the Linux kernel RTC driver.
They are used to 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 the machine’s
BIOS sets the year counter in the Hardware Clock to contain 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. For example:
hwclock --setepoch --epoch=1952
The RTC driver attempts to guess the correct epoch value, so
setting it may not be required.
This epoch value is used whenever hwclock reads or sets the
Hardware Clock on an Alpha machine. For ISA machines the kernel
uses the fixed Hardware Clock epoch of 1900.
--param-get=parameter; --param-set=parameter=value
Read and set the RTC’s parameter. This is useful, for example, to
retrieve the RTC’s feature or set the RTC’s Backup Switchover Mode.
parameter is either a numeric RTC parameter value (see the Kernel’s
include/uapi/linux/rtc.h) or an alias. See --help for a list of
valid aliases. parameter and value, if prefixed with 0x, are
interpreted as hexadecimal, otherwise decimal values.
--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 its time to standard output in
the ISO 8601 format. 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 also (re)calculate the drift factor. Try it without the
option if --set fails. See --update-drift below.
--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. With the --update-drift option also
(re)calculate the drift factor. Try it without the option if
--systohc fails. See --update-drift below.
-h, --help
Display help text and exit.
-V, --version
Print version and exit.
OPTIONS
--adjfile=filename
Override the default /etc/adjtime file path.
--date=date_string
This option must be used with the --set or --predict functions,
otherwise it is ignored.
hwclock --set --date='16:45'
hwclock --predict --date='2525-08-14 07:11:05'
The argument must be in local time, even if you keep your Hardware
Clock in UTC. See the --localtime option. Therefore, the argument
should not include any timezone information. It also should 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. Fractional seconds are silently dropped. This
option is capable of understanding many time and date formats, but
the previous parameters should be observed.
--delay=seconds
This option can be used to overwrite the internally used delay when
setting the clock time. The default is 0.5 (500ms) for rtc_cmos,
for another RTC types the delay is 0. If RTC type is impossible to
determine (from sysfs) then it defaults also to 0.5 to be
backwardly compatible.
The 500ms default is based on commonly used MC146818A-compatible
(x86) hardware clock. This Hardware Clock can only be set to any
integer time plus one half second. The integer time is required
because there is no interface to set or get a fractional second.
The additional half second delay is because the Hardware Clock
updates to the following second precisely 500 ms after setting the
new time. Unfortunately, this behavior is hardware specific and in
same cases another delay is required.
-D, --debug
Use --verbose. The --debug option has been deprecated and may be
repurposed or removed in a future release.
--directisa
This option is meaningful for ISA compatible machines in the x86
and x86_64 family. 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 file, which it assumes to be driven by the Linux 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.
--epoch=year
This option is required when using the --setepoch function. The
minimum year value is 1900. The maximum is system dependent
(ULONG_MAX - 1).
-f, --rtc=filename
Override hwclock's default rtc device file name. Otherwise it will
use the first one found in this order: /dev/rtc0, /dev/rtc,
/dev/misc/rtc. For IA-64: /dev/efirtc /dev/misc/efirtc
-l, --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, that is, the Clocks
or /etc/adjtime (--verbose is implicit with this option).
--update-drift
Update the Hardware Clock’s drift factor in /etc/adjtime. It can
only be used with --set or --systohc.
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 behavior this
would automatically (re)calculate the drift factor which caused
several problems:
• When using NTP 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.
• Significantly increased system shutdown times (as of v2.31 when
not using --update-drift the RTC is not read).
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.
This option requires reading the Hardware Clock before setting it. If
it cannot be read, then this option will cause the set functions to
fail. This can happen, for example, if the Hardware Clock is corrupted
by a power failure. In that case, the clock must first be set without
this option. Despite it not working, the resulting drift correction
factor would be invalid anyway.
-v, --verbose
Display more details about what hwclock is doing internally.
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
systems which do not have a working rtc device driver.
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 floating point decimal.
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 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 NTP
daemon 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 NTP daemon 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(2) 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 NTP daemon 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.
EXIT STATUS
One of the following exit values will be returned:
EXIT_SUCCESS ('0' on POSIX systems)
Successful program execution.
EXIT_FAILURE ('1' on POSIX systems)
The operation failed or the command syntax was not valid.
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. See also
adjtime_config(5).
/etc/localtime
The system timezone file.
/usr/share/zoneinfo/
The system timezone database directory.
Device files hwclock may try for Hardware Clock access: /dev/rtc0
/dev/rtc /dev/misc/rtc /dev/efirtc /dev/misc/efirtc
SEE ALSO
date(1), adjtime_config(5), adjtimex(8), gettimeofday(2),
settimeofday(2), crontab(1p), tzset(3)
AUTHORS
Written by Bryan Henderson <bryanh@giraffe-data.com>, September 1996,
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.
REPORTING BUGS
For bug reports, use the issue tracker at
https://github.com/util-linux/util-linux/issues.
AVAILABILITY
The hwclock command is part of the util-linux package which can be
downloaded from Linux Kernel Archive
<https://www.kernel.org/pub/linux/utils/util-linux/>.
util-linux 2.38.1 2022-05-11 HWCLOCK(8)
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