跟老韩学Ubuntu Server 2204-hwclock指令帮助手册

hwclock是设置Linux时间的，将系统时间同步到BIOS中的CMOS硬件中，是每个从事Linux必知必会的指令之一。

HWCLOCK(8)                                       System Administration                                      HWCLOCK(8)

NAME
       hwclock - time clocks utility

SYNOPSIS
       hwclock [function] [option...]

DESCRIPTION
       hwclock is an administration tool for the time clocks. It can: display the Hardware Clock time; set the
       Hardware Clock to a specified time; set the Hardware Clock from the System Clock; set the System Clock from the
       Hardware Clock; compensate for Hardware Clock drift; correct the System Clock timescale; set the kernel’s
       timezone, NTP timescale, and epoch (Alpha only); and predict future Hardware Clock values based on its drift
       rate.

       Since v2.26 important changes were made to the --hctosys function and the --directisa option, and a new option
       --update-drift was added. See their respective descriptions below.

FUNCTIONS
       The following functions are mutually exclusive, only one can be given at a time. If none is given, the default
       is --show.

       -a, --adjust
           Add or subtract time from the Hardware Clock to account for systematic drift since the last time the clock
           was set or adjusted. See the discussion below, under The Adjust Function.

       --getepoch; --setepoch
           These functions are for Alpha machines only, and are only available through the Linux kernel RTC driver.

           They are used to read and set the kernel’s Hardware Clock epoch value. Epoch is the number of years into AD
           to which a zero year value in the Hardware Clock refers. For example, if the machine’s BIOS sets the year
           counter in the Hardware Clock to contain the number of full years since 1952, then the kernel’s Hardware
           Clock epoch value must be 1952.

           The --setepoch function requires using the --epoch option to specify the year. For example:

           hwclock --setepoch --epoch=1952

           The RTC driver attempts to guess the correct epoch value, so setting it may not be required.

           This epoch value is used whenever hwclock reads or sets the Hardware Clock on an Alpha machine. For ISA
           machines the kernel uses the fixed Hardware Clock epoch of 1900.

       --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.

       -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 Hardware Clock in UTC. See the --localtime
           option. Therefore, the argument should not include any timezone information. It also should not be a
           relative time like "+5 minutes", because hwclock's precision depends upon correlation between the
           argument’s value and when the enter key is pressed. Fractional seconds are silently dropped. This option is
           capable of understanding many time and date formats, but the previous parameters should be observed.

       --delay=seconds
           This option can be used to overwrite the internally used delay when setting the clock time. The default is
           0.5 (500ms) for rtc_cmos, for another RTC types the delay is 0. If RTC type is impossible to determine
           (from sysfs) then it defaults also to 0.5 to be backwardly compatible.

           The 500ms default is based on commonly used MC146818A-compatible (x86) hardware clock. This Hardware Clock
           can only be set to any integer time plus one half second. The integer time is required because there is no
           interface to set or get a fractional second. The additional half second delay is because the Hardware Clock
           updates to the following second precisely 500 ms after setting the new time. Unfortunately, this behavior
           is hardware specific and in same cases another delay is required.

       -D, --debug
           Use --verbose. The --debug option has been deprecated and may be repurposed or removed in a future release.

       --directisa
           This option is meaningful for ISA compatible machines in the x86 and x86_64 family. For other machines, it
           has no effect. This option tells hwclock to use explicit I/O instructions to access the Hardware Clock.
           Without this option, hwclock will use the rtc device file, which it assumes to be driven by the Linux RTC
           device driver. As of v2.26 it will no longer automatically use directisa when the rtc driver is
           unavailable; this was causing an unsafe condition that could allow two processes to access the Hardware
           Clock at the same time. Direct hardware access from userspace should only be used for testing,
           troubleshooting, and as a last resort when all other methods fail. See the --rtc option.

       --epoch=year
           This option is required when using the --setepoch function. The minimum year value is 1900. The maximum is
           system dependent (ULONG_MAX - 1).

       -f, --rtc=filename
           Override hwclock's default rtc device file name. Otherwise it will use the first one found in this order:
           /dev/rtc0, /dev/rtc, /dev/misc/rtc. For IA-64: /dev/efirtc /dev/misc/efirtc

       -l, --localtime; -u, --utc
           Indicate which timescale the Hardware Clock is set to.

           The Hardware Clock may be configured to use either the UTC or the local timescale, but nothing in the clock
           itself says which alternative is being used. The --localtime or --utc options give this information to the
           hwclock command. If you specify the wrong one (or specify neither and take a wrong default), both setting
           and reading the Hardware Clock will be incorrect.

           If you specify neither --utc nor --localtime then the one last given with a set function (--set, --systohc,
           or --adjust), as recorded in /etc/adjtime, will be used. If the adjtime file doesn’t exist, the default is
           UTC.

           Note: daylight saving time changes may be inconsistent when the Hardware Clock is kept in local time. See
           the discussion below, under LOCAL vs UTC.

       --noadjfile
           Disable the facilities provided by /etc/adjtime. hwclock will not read nor write to that file with this
           option. Either --utc or --localtime must be specified when using this option.

       --test
           Do not actually change anything on the system, that is, the Clocks or /etc/adjtime (--verbose is implicit
           with this option).

       --update-drift
           Update the Hardware Clock’s drift factor in /etc/adjtime. It can only be used with --set or --systohc.

           A minimum four hour period between settings is required. This is to avoid invalid calculations. The longer
           the period, the more precise the resulting drift factor will be.

           This option was added in v2.26, because it is typical for systems to call hwclock --systohc at shutdown;
           with the old behavior this would automatically (re)calculate the drift factor which caused several
           problems:

           ?   When using NTP with an '11 minute mode' kernel the drift factor would be clobbered to near zero.

           ?   It would not allow the use of 'cold' drift correction. With most configurations using 'cold' drift will
               yield favorable results. Cold, means when the machine is turned off which can have a significant impact
               on the drift factor.

           ?   (Re)calculating drift factor on every shutdown delivers suboptimal results. For example, if ephemeral
               conditions cause the machine to be abnormally hot the drift factor calculation would be out of range.

           ?   Significantly increased system shutdown times (as of v2.31 when not using --update-drift the RTC is not
               read).

       Having hwclock calculate the drift factor is a good starting point, but for optimal results it will likely need
       to be adjusted by directly editing the /etc/adjtime file. For most configurations once a machine’s optimal
       drift factor is crafted it should not need to be changed. Therefore, the old behavior to automatically
       (re)calculate drift was changed and now requires this option to be used. See the discussion below, under The
       Adjust Function.

       This option requires reading the Hardware Clock before setting it. If it cannot be read, then this option will
       cause the set functions to fail. This can happen, for example, if the Hardware Clock is corrupted by a power
       failure. In that case, the clock must first be set without this option. Despite it not working, the resulting
       drift correction factor would be invalid anyway.

       -v, --verbose
           Display more details about what hwclock is doing internally.

NOTES
   Clocks in a Linux System
       There are two types of date-time clocks:

       The Hardware Clock: This clock is an independent hardware device, with its own power domain (battery,
       capacitor, etc), that operates when the machine is powered off, or even unplugged.

       On an ISA compatible system, this clock is specified as part of the ISA standard. A control program can read or
       set this clock only to a whole second, but it can also detect the edges of the 1 second clock ticks, so the
       clock actually has virtually infinite precision.

       This clock is commonly called the hardware clock, the real time clock, the RTC, the BIOS clock, and the CMOS
       clock. Hardware Clock, in its capitalized form, was coined for use by hwclock. The Linux kernel also refers to
       it as the persistent clock.

       Some non-ISA systems have a few real time clocks with only one of them having its own power domain. A very low
       power external I2C or SPI clock chip might be used with a backup battery as the hardware clock to initialize a
       more functional integrated real-time clock which is used for most other purposes.

       The System Clock: This clock is part of the Linux kernel and is driven by a timer interrupt. (On an ISA
       machine, the timer interrupt is part of the ISA standard.) It has meaning only while Linux is running on the
       machine. The System Time is the number of seconds since 00:00:00 January 1, 1970 UTC (or more succinctly, the
       number of seconds since 1969 UTC). The System Time is not an integer, though. It has virtually infinite
       precision.

       The System Time is the time that matters. The Hardware Clock’s basic purpose is to keep time when Linux is not
       running so that the System Clock can be initialized from it at boot. Note that in DOS, for which ISA was
       designed, the Hardware Clock is the only real time clock.

       It is important that the System Time not have any discontinuities such as would happen if you used the date(1)
       program to set it while the system is running. You can, however, do whatever you want to the Hardware Clock
       while the system is running, and the next time Linux starts up, it will do so with the adjusted time from the
       Hardware Clock. Note: currently this is not possible on most systems because hwclock --systohc is called at
       shutdown.

       The Linux kernel’s timezone is set by hwclock. But don’t be misled — almost nobody cares what timezone the
       kernel thinks it is in. Instead, programs that care about the timezone (perhaps because they want to display a
       local time for you) almost always use a more traditional method of determining the timezone: They use the TZ
       environment variable or the /etc/localtime file, as explained in the man page for tzset(3). However, some
       programs and fringe parts of the Linux kernel such as filesystems use the kernel’s timezone value. An example
       is the vfat filesystem. If the kernel timezone value is wrong, the vfat filesystem will report and set the
       wrong timestamps on files. Another example is the kernel’s NTP '11 minute mode'. If the kernel’s timezone value
       and/or the persistent_clock_is_local variable are wrong, then the Hardware Clock will be set incorrectly by '11
       minute mode'. See the discussion below, under Automatic Hardware Clock Synchronization by the Kernel.

       hwclock sets the kernel’s timezone to the value indicated by TZ or /etc/localtime with the --hctosys or --systz
       functions.

       The kernel’s timezone value actually consists of two parts: 1) a field tz_minuteswest indicating how many
       minutes local time (not adjusted for DST) lags behind UTC, and 2) a field tz_dsttime indicating the type of
       Daylight Savings Time (DST) convention that is in effect in the locality at the present time. This second field
       is not used under Linux and is always zero. See also settimeofday(2).

   Hardware Clock Access Methods
       hwclock uses many different ways to get and set Hardware Clock values. The most normal way is to do I/O to the
       rtc device special file, which is presumed to be driven by the rtc device driver. Also, Linux systems using the
       rtc framework with udev, are capable of supporting multiple Hardware Clocks. This may bring about the need to
       override the default rtc device by specifying one with the --rtc option.

       However, this method is not always available as older systems do not have an rtc driver. On these systems, the
       method of accessing the Hardware Clock depends on the system hardware.

       On an ISA compatible system, hwclock can directly access the "CMOS memory" registers that constitute the clock,
       by doing I/O to Ports 0x70 and 0x71. It does this with actual I/O instructions and consequently can only do it
       if running with superuser effective userid. This method may be used by specifying the --directisa option.

       This is a really poor method of accessing the clock, for all the reasons that userspace programs are generally
       not supposed to do direct I/O and disable interrupts. hwclock provides it for testing, troubleshooting, and
       because it may be the only method available on ISA systems which do not have a working rtc device driver.

   The Adjust Function
       The Hardware Clock is usually not very accurate. However, much of its inaccuracy is completely predictable - it
       gains or loses the same amount of time every day. This is called systematic drift. hwclock's --adjust function
       lets you apply systematic drift corrections to the Hardware Clock.

       It works like this: hwclock keeps a file, /etc/adjtime, that keeps some historical information. This is called
       the adjtime file.

       Suppose you start with no adjtime file. You issue a hwclock --set command to set the Hardware Clock to the true
       current time. hwclock creates the adjtime file and records in it the current time as the last time the clock
       was calibrated. Five days later, the clock has gained 10 seconds, so you issue a hwclock --set --update-drift
       command to set it back 10 seconds. hwclock updates the adjtime file to show the current time as the last time
       the clock was calibrated, and records 2 seconds per day as the systematic drift rate. 24 hours go by, and then
       you issue a hwclock --adjust command. hwclock consults the adjtime file and sees that the clock gains 2 seconds
       per day when left alone and that it has been left alone for exactly one day. So it subtracts 2 seconds from the
       Hardware Clock. It then records the current time as the last time the clock was adjusted. Another 24 hours go
       by and you issue another hwclock --adjust. hwclock does the same thing: subtracts 2 seconds and updates the
       adjtime file with the current time as the last time the clock was adjusted.

       When you use the --update-drift option with --set or --systohc, the systematic drift rate is (re)calculated by
       comparing the fully drift corrected current Hardware Clock time with the new set time, from that it derives the
       24 hour drift rate based on the last calibrated timestamp from the adjtime file. This updated drift factor is
       then saved in /etc/adjtime.

       A small amount of error creeps in when the Hardware Clock is set, so --adjust refrains from making any
       adjustment that is less than 1 second. Later on, when you request an adjustment again, the accumulated drift
       will be more than 1 second and --adjust will make the adjustment including any fractional amount.

       hwclock --hctosys also uses the adjtime file data to compensate the value read from the Hardware Clock before
       using it to set the System Clock. It does not share the 1 second limitation of --adjust, and will correct
       sub-second drift values immediately. It does not change the Hardware Clock time nor the adjtime file. This may
       eliminate the need to use --adjust, unless something else on the system needs the Hardware Clock to be
       compensated.

   The Adjtime File
       While named for its historical purpose of controlling adjustments only, it actually contains other information
       used by hwclock from one invocation to the next.

       The format of the adjtime file is, in ASCII:

       Line 1: Three numbers, separated by blanks: 1) the systematic drift rate in seconds per day, floating point
       decimal; 2) the resulting number of seconds since 1969 UTC of most recent adjustment or calibration, decimal
       integer; 3) zero (for compatibility with clock(8)) as a floating point decimal.

       Line 2: One number: the resulting number of seconds since 1969 UTC of most recent calibration. Zero if there
       has been no calibration yet or it is known that any previous calibration is moot (for example, because the
       Hardware Clock has been found, since that calibration, not to contain a valid time). This is a decimal integer.

       Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set to Coordinated Universal Time or local time.
       You can always override this value with options on the hwclock command line.

       You can use an adjtime file that was previously used with the clock(8) program with hwclock.

   Automatic Hardware Clock Synchronization by the Kernel
       You should be aware of another way that the Hardware Clock is kept synchronized in some systems. The Linux
       kernel has a mode wherein it copies the System Time to the Hardware Clock every 11 minutes. This mode is a
       compile time option, so not all kernels will have this capability. This is a good mode to use when you are
       using something sophisticated like NTP to keep your System Clock synchronized. (NTP is a way to keep your
       System Time synchronized either to a time server somewhere on the network or to a radio clock hooked up to your
       system. See RFC 1305.)

       If the kernel is compiled with the '11 minute mode' option it will be active when the kernel’s clock discipline
       is in a synchronized state. When in this state, bit 6 (the bit that is set in the mask 0x0040) of the kernel’s
       time_status variable is unset. This value is output as the 'status' line of the adjtimex --print or ntptime
       commands.

       It takes an outside influence, like the NTP daemon to put the kernel’s clock discipline into a synchronized
       state, and therefore turn on '11 minute mode'. It can be turned off by running anything that sets the System
       Clock the old fashioned way, including hwclock --hctosys. However, if the NTP daemon is still running, it will
       turn '11 minute mode' back on again the next time it synchronizes the System Clock.

       If your system runs with '11 minute mode' on, it may need to use either --hctosys or --systz in a startup
       script, especially if the Hardware Clock is configured to use the local timescale. Unless the kernel is
       informed of what timescale the Hardware Clock is using, it may clobber it with the wrong one. The kernel uses
       UTC by default.

       The first userspace command to set the System Clock informs the kernel what timescale the Hardware Clock is
       using. This happens via the persistent_clock_is_local kernel variable. If --hctosys or --systz is the first, it
       will set this variable according to the adjtime file or the appropriate command-line argument. Note that when
       using this capability and the Hardware Clock timescale configuration is changed, then a reboot is required to
       notify the kernel.

       hwclock --adjust should not be used with NTP '11 minute mode'.

   ISA Hardware Clock Century value
       There is some sort of standard that defines CMOS memory Byte 50 on an ISA machine as an indicator of what
       century it is. hwclock does not use or set that byte because there are some machines that don’t define the byte
       that way, and it really isn’t necessary anyway, since the year-of-century does a good job of implying which
       century it is.

       If you have a bona fide use for a CMOS century byte, contact the hwclock maintainer; an option may be
       appropriate.

       Note that this section is only relevant when you are using the "direct ISA" method of accessing the Hardware
       Clock. ACPI provides a standard way to access century values, when they are supported by the hardware.

DATE-TIME CONFIGURATION
   Keeping Time without External Synchronization
       This discussion is based on the following conditions:

       ?   Nothing is running that alters the date-time clocks, such as NTP daemon or a cron job."

       ?   The system timezone is configured for the correct local time. See below, under POSIX vs 'RIGHT'.

       ?   Early during startup the following are called, in this order: adjtimex --tick value --frequency value
           hwclock --hctosys

       ?   During shutdown the following is called: hwclock --systohc

           ?   Systems without adjtimex may use ntptime.

       Whether maintaining precision time with NTP daemon or not, it makes sense to configure the system to keep
       reasonably good date-time on its own.

       The first step in making that happen is having a clear understanding of the big picture. There are two
       completely separate hardware devices running at their own speed and drifting away from the 'correct' time at
       their own rates. The methods and software for drift correction are different for each of them. However, most
       systems are configured to exchange values between these two clocks at startup and shutdown. Now the individual
       device’s time keeping errors are transferred back and forth between each other. Attempt to configure drift
       correction for only one of them, and the other’s drift will be overlaid upon it.

       This problem can be avoided when configuring drift correction for the System Clock by simply not shutting down
       the machine. This, plus the fact that all of hwclock's precision (including calculating drift factors) depends
       upon the System Clock’s rate being correct, means that configuration of the System Clock should be done first.

       The System Clock drift is corrected with the adjtimex(8) command’s --tick and --frequency options. These two
       work together: tick is the coarse adjustment and frequency is the fine adjustment. (For systems that do not
       have an adjtimex package, ntptime -f ppm may be used instead.)

       Some Linux distributions attempt to automatically calculate the System Clock drift with adjtimex's compare
       operation. Trying to correct one drifting clock by using another drifting clock as a reference is akin to a dog
       trying to catch its own tail. Success may happen eventually, but great effort and frustration will likely
       precede it. This automation may yield an improvement over no configuration, but expecting optimum results would
       be in error. A better choice for manual configuration would be adjtimex's --log options.

       It may be more effective to simply track the System Clock drift with sntp, or date -Ins and a precision
       timepiece, and then calculate the correction manually.

       After setting the tick and frequency values, continue to test and refine the adjustments until the System Clock
       keeps good time. See adjtimex(2) for more information and the example demonstrating manual drift calculations.

       Once the System Clock is ticking smoothly, move on to the Hardware Clock.

       As a rule, cold drift will work best for most use cases. This should be true even for 24/7 machines whose
       normal downtime consists of a reboot. In that case the drift factor value makes little difference. But on the
       rare occasion that the machine is shut down for an extended period, then cold drift should yield better
       results.

       Steps to calculate cold drift:

       1
           Ensure that NTP daemon will not be launched at startup.

       2
           The System Clock time must be correct at shutdown!

       3
           Shut down the system.

       4
           Let an extended period pass without changing the Hardware Clock.

       5
           Start the system.

       6
           Immediately use hwclock to set the correct time, adding the --update-drift option.

       Note: if step 6 uses --systohc, then the System Clock must be set correctly (step 6a) just before doing so.

       Having hwclock calculate the drift factor is a good starting point, but for optimal results it will likely need
       to be adjusted by directly editing the /etc/adjtime file. Continue to test and refine the drift factor until
       the Hardware Clock is corrected properly at startup. To check this, first make sure that the System Time is
       correct before shutdown and then use sntp, or date -Ins and a precision timepiece, immediately after startup.

   LOCAL vs UTC
       Keeping the Hardware Clock in a local timescale causes inconsistent daylight saving time results:

       ?   If Linux is running during a daylight saving time change, the time written to the Hardware Clock will be
           adjusted for the change.

       ?   If Linux is NOT running during a daylight saving time change, the time read from the Hardware Clock will
           NOT be adjusted for the change.

       The Hardware Clock on an ISA compatible system keeps only a date and time, it has no concept of timezone nor
       daylight saving. Therefore, when hwclock is told that it is in local time, it assumes it is in the 'correct'
       local time and makes no adjustments to the time read from it.

       Linux handles daylight saving time changes transparently only when the Hardware Clock is kept in the UTC
       timescale. Doing so is made easy for system administrators as hwclock uses local time for its output and as the
       argument to the --date option.

       POSIX systems, like Linux, are designed to have the System Clock operate in the UTC timescale. The Hardware
       Clock’s purpose is to initialize the System Clock, so also keeping it in UTC makes sense.

       Linux does, however, attempt to accommodate the Hardware Clock being in the local timescale. This is primarily
       for dual-booting with older versions of MS Windows. From Windows 7 on, the RealTimeIsUniversal registry key is
       supposed to be working properly so that its Hardware Clock can be kept in UTC.

   POSIX vs 'RIGHT'
       A discussion on date-time configuration would be incomplete without addressing timezones, this is mostly well
       covered by tzset(3). One area that seems to have no documentation is the 'right' directory of the Time Zone
       Database, sometimes called tz or zoneinfo.

       There are two separate databases in the zoneinfo system, posix and 'right'. 'Right' (now named zoneinfo-leaps)
       includes leap seconds and posix does not. To use the 'right' database the System Clock must be set to (UTC +
       leap seconds), which is equivalent to (TAI - 10). This allows calculating the exact number of seconds between
       two dates that cross a leap second epoch. The System Clock is then converted to the correct civil time,
       including UTC, by using the 'right' timezone files which subtract the leap seconds. Note: this configuration is
       considered experimental and is known to have issues.

       To configure a system to use a particular database all of the files located in its directory must be copied to
       the root of /usr/share/zoneinfo. Files are never used directly from the posix or 'right' subdirectories, e.g.,
       TZ='right/Europe/Dublin'. This habit was becoming so common that the upstream zoneinfo project restructured the
       system’s file tree by moving the posix and 'right' subdirectories out of the zoneinfo directory and into
       sibling directories:

       /usr/share/zoneinfo, /usr/share/zoneinfo-posix, /usr/share/zoneinfo-leaps

       Unfortunately, some Linux distributions are changing it back to the old tree structure in their packages. So
       the problem of system administrators reaching into the 'right' subdirectory persists. This causes the system
       timezone to be configured to include leap seconds while the zoneinfo database is still configured to exclude
       them. Then when an application such as a World Clock needs the South_Pole timezone file; or an email MTA, or
       hwclock needs the UTC timezone file; they fetch it from the root of /usr/share/zoneinfo , because that is what
       they are supposed to do. Those files exclude leap seconds, but the System Clock now includes them, causing an
       incorrect time conversion.

       Attempting to mix and match files from these separate databases will not work, because they each require the
       System Clock to use a different timescale. The zoneinfo database must be configured to use either posix or
       'right', as described above, or by assigning a database path to the TZDIR environment variable.

EXIT STATUS
       One of the following exit values will be returned:

       EXIT_SUCCESS ('0' on POSIX systems)
           Successful program execution.

       EXIT_FAILURE ('1' on POSIX systems)
           The operation failed or the command syntax was not valid.

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

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

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

       /etc/localtime
           The system timezone file.

       /usr/share/zoneinfo/
           The system timezone database directory.

       Device files hwclock may try for Hardware Clock access: /dev/rtc0 /dev/rtc /dev/misc/rtc /dev/efirtc
       /dev/misc/efirtc

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

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

REPORTING BUGS
       For bug reports, use the issue tracker at https://github.com/karelzak/util-linux/issues.

AVAILABILITY
       The hwclock command is part of the util-linux package which can be downloaded from Linux Kernel Archive
       <https://www.kernel.org/pub/linux/utils/util-linux/>.

util-linux 2.37.2                                     2021-06-02                                            HWCLOCK(8)