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NAME | SYNOPSIS | DESCRIPTION | FUNCTIONS | OPTIONS | NOTES | DATE-TIME CONFIGURATION | EXIT STATUS | ENVIRONMENT | FILES | SEE ALSO | AUTHORS | REPORTING BUGS | AVAILABILITY |
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HWCLOCK(8) System Administration HWCLOCK(8)
hwclock - time clocks utility
hwclock [function] [option...]
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.
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.
--vl-read, --vl-clear
Some RTC devices are able to monitor the voltage of the
backup battery and thus provide a way for the user to know
that the battery should be replaced. The --vl-read function
retrieves the Voltage Low information and decodes the result
into human-readable form. The --vl-clear function resets the
Voltage Low information, which is necessary for some RTC
devices after a battery replacement.
See the Kernel’s include/uapi/linux/rtc.h for details on
which pieces of information may be returned. Note that not
all RTC devices have this monitoring capability, nor do all
drivers necessarily support reading the information.
-h, --help
Display help text and exit.
-V, --version
Print version and exit.
--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 some 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.
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.
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.
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.
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.
/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
date(1), adjtime_config(5), adjtimex(8), gettimeofday(2),
settimeofday(2), crontab(1p), tzset(3)
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.
For bug reports, use the issue tracker at
https://github.com/util-linux/util-linux/issues.
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/>. This page
is part of the util-linux (a random collection of Linux
utilities) project. Information about the project can be found at
⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have
a bug report for this manual page, send it to
util-linux@vger.kernel.org. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-06-10.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in
this COLOPHON (which is not part of the original manual page),
send a mail to man-pages@man7.org
util-linux 2.39.594-1e0ad 2023-07-19 HWCLOCK(8)
Pages that refer to this page: timedatectl(1), adjtimex(2), clock_getres(2), gettimeofday(2), rtc(4), adjtime_config(5), time(7), rtcwake(8), systemd-timedated.service(8), systemd-timesyncd.service(8)
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