hwclock — read or set the hardware clock (RTC)
hwclock [function] [
hwclock is a tool for accessing the Hardware Clock. 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); compare the System and Hardware Clocks; and predict future Hardware Clock values based on its drift rate.
Since v2.26 important changes were made to the
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
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.
Periodically compare the Hardware Clock to the System Time and output the difference every 10 seconds. This will also print the frequency offset and tick.
These functions are for Alpha machines only.
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 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 Clock epoch value must be 1952.
requires using the
−−epoch option to specify
This epoch value is used whenever hwclock reads or sets the Hardware Clock.
Predict what the Hardware Clock will read in the
future based upon the time given by the
−−date option and the
/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.
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
Showing the Hardware Clock time is the default when no function is specified.
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.
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
function does this based upon the information in the
/etc/adjtime file or the
command line arguments
−−utc. Note: no
daylight saving adjustment is made. See the discussion
below, under LOCAL vs
The kernel also keeps a timezone value, the
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
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
−−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
is not used with
−−systohc at shutdown (or
anywhere else). Another way to inhibit this is by using
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
−−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 the Hardware Clock to the time given by the
and update the timestamps in
/etc/adjtime. With the
(re)calculate the drift factor.
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
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.
Set the Hardware Clock from the System Clock, and
update the timestamps in
/etc/adjtime. When the
is given, then also (re)calculate the drift factor.
Display version information and exit.
Display help text and exit.
Override the default
Indicate that the Hardware Clock is incapable of storing years outside the range 1994-1999. There is a problem in some BIOSes (almost all Award BIOSes made between 4/26/94 and 5/31/95) wherein they are unable to deal with years after 1999. If one attempts to set the year-of-century value to something less than 94 (or 95 in some cases), the value that actually gets set is 94 (or 95). Thus, if you have one of these machines, hwclock cannot set the year after 1999 and cannot use the value of the clock as the true time in the normal way.
To compensate for this (without your getting a BIOS
update, which would definitely be preferable), always
if you have one of these machines. When hwclock knows it's
working with a brain-damaged clock, it ignores the year
part of the Hardware Clock value and instead tries to
guess the year based on the last calibrated date in the
adjtime file, by assuming that date is within the past
year. For this to work, you had better do a
−−set or hwclock
−−systohc at least once a
Though hwclock ignores the year value when it reads the Hardware Clock, it sets the year value when it sets the clock. It sets it to 1995, 1996, 1997, or 1998, whichever one has the same position in the leap year cycle as the true year. That way, the Hardware Clock inserts leap days where they belong. Again, if you let the Hardware Clock run for more than a year without setting it, this scheme could be defeated and you could end up losing a day.
You need this option if you specify the
otherwise it is ignored. It specifies the time to which
to set the Hardware Clock, or the time for which to
predict the Hardware Clock reading. The value of this
option is used as an argument to the date(1) program's
\-\-date option. For example:
hwclock −−set −−date='2011-08-14 16:45:05'
The argument must be in local time, even if you keep your Hardware Clock in UTC. See the
−−localtimeoption. The argument must 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.
Display a lot of information about what hwclock is doing internally. Some of its functions are complex and this output can help you understand how the program works.
This option is meaningful for: ISA compatible
machines including x86, and x86_64; and Alpha (which
has a similar Hardware Clock interface). For other
machines, it has no effect. This option tells
to use explicit I/O instructions to access the Hardware
Clock. Without this option, hwclock will use the
rtc device, which it assumes to be driven by the 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
Override hwclock's default rtc device file name. Otherwise it will use the first one found in this order:
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
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
−−localtime then the one
last given with a set function (
−−adjust), as recorded in
/etc/adjtime, will be
used. If the adjtime file doesn't exist, the default is
Daylight saving time changes may be inconsistent when the Hardware Clock is kept in local time. See the discussion below, under LOCAL vs UTC.
Disable the facilities provided by
/etc/adjtime. hwclock will not read
nor write to that file with this option. Either
must be specified when using this option.
Do not actually change anything on the system, i.e.,
the Clocks or adjtime file. This is useful, especially
in conjunction with
−−debug, in learning about
the internal operations of hwclock.
Update the Hardware Clock's drift factor in
/etc/adjtime. It is used
otherwise it is ignored.
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 behaviour this would automatically (re)calculate the drift factor which caused several problems:
When using ntpd 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.
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/adjtimefile. 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 is equivalent to
−−epoch=1980 and is used to
specify the most common epoch on Alphas with an ARC
console (although Ruffians have an epoch of 1900).
Specifies the year which is the beginning of the
Hardware Clock's epoch, that is the number of years
into AD to which a zero value in the Hardware Clock's
year counter refers. It is used together with the
option to set the kernel's idea of the epoch of the
For example, on a Digital Unix machine:
hwclock −−setepoch −−epoch=1952
These two options specify what kind of Alpha machine
you have. They are invalid if you do not have an Alpha
and are usually unnecessary if you do; hwclock should be
able to determine what it is running on when
/proc is mounted.
option is used for Jensen models;
that the machine requires the UF bit instead of the UIP
bit in the Hardware Clock to detect a time transition.
The "toy" in the option name refers to the Time Of Year
facility of the machine.
This option is equivalent to
−−epoch=1900 and is used to
specify the most common epoch on Alphas with an SRM
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
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.
sets the kernel's timezone to the value indicated by TZ or
/etc/localtime with the
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).
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
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
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 compatible and Alpha systems which do not have a working rtc device driver.
In the case of a Jensen Alpha, there is no way for
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.
On an m68k system, hwclock can access the
clock with the console driver, via the device special file
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
It works like this: hwclock keeps a file,
/etc/adjtime, that keeps some
historical information. This is called the adjtime
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
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
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.
−−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
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 decimal integer.
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.
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 ntpd(1), 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
−−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
−−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'.
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.
This discussion is based on the following conditions:
Nothing is running that alters the date-time clocks, such as ntpd(1) 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 −−tickvalue
During shutdown the following is called:
*Systems without adjtimex may use ntptime.
Whether maintaining precision time with ntpd(1) 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
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,
ppm may be
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 −−logoptions.
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(8) 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:
Ensure that ntpd(1) will not be launched at startup.
The System Clocktime must be correct at shutdown!
Shut down the system.
Let an extended period pass without changing the Hardware Clock.
Start the system.
Immediately use hwclock to set the
correct time, adding the
If step 6 uses
Having hwclock calculate the
drift factor is a good starting point, but for optimal
results it will likely need to be adjusted by directly
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.
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
local time for its output and as the argument to the
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.
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
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:
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.
If this variable is set its value takes precedence over the system configured timezone.
If this variable is set its value takes precedence over the system configured timezone database directory path.
The configuration and state file for hwclock.
The system timezone file.
The system timezone database directory.
Device files hwclock
may try for Hardware Clock access:
Written by Bryan Henderson, September 1996 (firstname.lastname@example.org), 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.
The hwclock command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
hwclock.8.in -- man page for util-linux' hwclock
2015-01-07 J William Piggott
Authored new section: DATE-TIME CONFIGURATION.
Subsections: Keeping Time..., LOCAL vs UTC, POSIX vs 'RIGHT'.