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HWCLOCK(8)		     System Administration		    HWCLOCK(8)

NAME
       hwclock - read or set the hardware clock (RTC)

SYNOPSIS
       hwclock [function] [option...]

DESCRIPTION
       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); 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.

       --adjust
	      Add or subtract time from the Hardware Clock to account for sys‐
	      tematic 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 avail‐
	      able 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.

       --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 any‐
	      thing 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 discus‐
	      sion 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 configura‐
	      tion is changed then a reboot is required to inform the  kernel.
	      See  the	discussion  below, under Automatic Hardware Clock Syn‐
	      chronization 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 func‐
	      tion.  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 call‐
	      ing 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 (re)calculate the drift factor.

       --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.  When the --update-drift  option  is
	      given, then also (re)calculate the drift factor.

       -V, --version
	      Display version information and exit.

       -h, --help
	      Display help text 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	 Hard‐
	      ware  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	 argu‐
	      ment's value and when the enter key is pressed.  Fractional sec‐
	      onds are silently dropped.  This option  is  capable  of	under‐
	      standing many time and date formats, but the previous parameters
	      should be observed.

       -D, --debug
	      Display a lot of information about what hwclock is doing	inter‐
	      nally.   Some  of	 its functions are complex and this output can
	      help you understand how the program works.

       --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.

       -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

       --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, i.e., the	Clocks
	      or adjtime file.	This is useful, especially in conjunction with
	      --debug, in learning about the internal operations of hwclock.

       --update-drift
	      Update the Hardware Clock's drift factor in /etc/adjtime.	 It is
	      used with --set or --systohc, 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 sys‐
	      tems to call hwclock --systohc at shutdown; with the old	behav‐
	      iour  this  would	 automatically	(re)calculate the drift factor
	      which caused several problems:

	      · When using ntpd with an '11 minute mode' kernel the drift fac‐
		tor 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 subop‐
		timal 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/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.

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 virtu‐
       ally 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 Hard‐
       ware 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 vari‐
       able 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 local‐
       ity 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 mem‐
       ory"  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  rea‐
       sons  that  userspace  programs are generally not supposed to do direct
       I/O and disable interrupts.  hwclock provides  it  for  testing,	 trou‐
       bleshooting,  and   because  it may be the only method available on ISA
       systems which do not have a working rtc device driver.

       On an m68k system, hwclock  can	access	the  clock  with  the  console
       driver, via the device special file /dev/tty1.

   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 com‐
       mand  to set the Hardware Clock to the true current time.  hwclock cre‐
       ates 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 cur‐
       rent time as the last time the clock was calibrated, and records 2 sec‐
       onds  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	 time‐
       stamp  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 sec‐
       ond.  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 invoca‐
       tion 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 num‐
       ber 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 con‐
       tain 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 syn‐
       chronized 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	 capa‐
       bility.	 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
       --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 capa‐
       bility 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  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 --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 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 fac‐
       tors) 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 automa‐
       tion may yield an improvement  over  no	configuration,	but  expecting
       optimum results would be in error.  A better choice for manual configu‐
       ration 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(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:

       1 Ensure that ntpd(1) 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 cor‐
       rectly (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	 oper‐
       ate 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 ver‐
       sions of MS Windows. From Windows 7 on,	the  RealTimeIsUniversal  reg‐
       istry 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  administra‐
       tors  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.

ENVIRONMENT
       TZ     If this variable is set its value takes precedence over the sys‐
	      tem configured timezone.

       TZDIR  If this variable is set its value takes precedence over the sys‐
	      tem configured timezone database directory path.

FILES
       /etc/adjtime
	      The configuration and state file for hwclock.

       /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
       /dev/port
       /dev/tty1

SEE ALSO
       date(1),	 adjtimex(8),  gettimeofday(2),	 settimeofday(2),  crontab(1),
       tzset(3)

AUTHORS
       Written	by  Bryan Henderson, September 1996 (bryanh@giraffe-data.com),
       based on work done on the clock(8)  program  by	Charles	 Hedrick,  Rob
       Hooft, and Harald Koenig.  See the source code for complete history and
       credits.

AVAILABILITY
       The hwclock command is part of the util-linux package and is  available
       from https://www.kernel.org/pub/linux/utils/util-linux/.

util-linux			  April 2015			    HWCLOCK(8)
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