1 .\" hwclock.8.in -- man page for util-linux' hwclock
3 .\" 2015-01-07 J William Piggott
4 .\" Authored new section: DATE-TIME CONFIGURATION.
5 .\" Subsections: Keeping Time..., LOCAL vs UTC, POSIX vs 'RIGHT'.
7 .TH HWCLOCK 8 "July 2017" "util-linux" "System Administration"
9 hwclock \- time clocks utility
17 is an administration tool for the time clocks. It can: display the
18 Hardware Clock time; set the Hardware Clock to a specified time; set the
19 Hardware Clock from the System Clock; set the System Clock from the
20 Hardware Clock; compensate for Hardware Clock drift; correct the System
21 Clock timescale; set the kernel's timezone, NTP timescale, and epoch
22 (Alpha only); and predict future
23 Hardware Clock values based on its drift rate.
25 Since v2.26 important changes were made to the
29 option, and a new option
31 was added. See their respective descriptions below.
34 The following functions are mutually exclusive, only one can be given at
35 a time. If none is given, the default is \fB\-\-show\fR.
38 Add or subtract time from the Hardware Clock to account for systematic
39 drift since the last time the clock was set or adjusted. See the
40 discussion below, under
41 .BR "The Adjust Function" .
47 These functions are for Alpha machines only, and are only available
48 through the Linux kernel RTC driver.
50 They are used to read and set the kernel's Hardware Clock epoch value.
51 Epoch is the number of years into AD to which a zero year value in the
52 Hardware Clock refers. For example, if the machine's BIOS sets the year
53 counter in the Hardware Clock to contain the number of full years since
54 1952, then the kernel's Hardware Clock epoch value must be 1952.
56 The \fB\%\-\-setepoch\fR function requires using the
58 option to specify the year. For example:
61 .B hwclock\ \-\-setepoch\ \-\-epoch=1952
63 The RTC driver attempts to guess the correct epoch value, so setting it
66 This epoch value is used whenever
68 reads or sets the Hardware Clock on an Alpha machine. For ISA machines
69 the kernel uses the fixed Hardware Clock epoch of 1900.
74 Predict what the Hardware Clock will read in the future based upon the
77 option and the information in
79 This is useful, for example, to account for drift when setting a
80 Hardware Clock wakeup (aka alarm). See
83 Do not use this function if the Hardware Clock is being modified by
84 anything other than the current operating system's
86 command, such as \%'11\ minute\ mode' or from dual-booting another OS.
93 Read the Hardware Clock and print its time to standard output in the
96 The time shown is always in local time, even if you keep your Hardware Clock
101 Showing the Hardware Clock time is the default when no function is specified.
105 function also applies drift correction to the time read, based upon the
108 Do not use this function if the Hardware Clock is being modified by
109 anything other than the current operating system's
111 command, such as \%'11\ minute\ mode' or from dual-booting another OS.
114 .BR \-s , \ \-\-hctosys
115 Set the System Clock from the Hardware Clock. The time read from the Hardware
116 Clock is compensated to account for systematic drift before using it to set the
117 System Clock. See the discussion below, under
118 .BR "The Adjust Function" .
120 The System Clock must be kept in the UTC timescale for date-time
121 applications to work correctly in conjunction with the timezone configured
122 for the system. If the Hardware Clock is kept in local time then the time read
123 from it must be shifted to the UTC timescale before using it to set the System
126 function does this based upon the information in the
128 file or the command line arguments
129 .BR \%\-\-localtime " and " \-\-utc .
130 Note: no daylight saving adjustment is made. See the discussion below, under
133 The kernel also keeps a timezone value, the
135 function sets it to the timezone configured for the system. The system
136 timezone is configured by the TZ environment variable or the
140 would interpret them.
141 The obsolete tz_dsttime field of the kernel's timezone value is set
142 to zero. (For details on what this field used to mean, see
143 .BR \%settimeofday (2).)
145 When used in a startup script, making the
147 function the first caller of
148 .BR \%settimeofday (2)
149 from boot, it will set the NTP \%'11\ minute\ mode' timescale via the
150 .I \%persistent_clock_is_local
151 kernel variable. If the Hardware Clock's timescale configuration is
152 changed then a reboot is required to inform the kernel. See the
153 discussion below, under
154 .BR "Automatic Hardware Clock Synchronization by the Kernel" .
156 This is a good function to use in one of the system startup scripts before the
157 file systems are mounted read/write.
159 This function should never be used on a running system. Jumping system time
160 will cause problems, such as corrupted filesystem timestamps. Also, if
161 something has changed the Hardware Clock, like NTP's \%'11\ minute\ mode', then
163 will set the time incorrectly by including drift compensation.
165 Drift compensation can be inhibited by setting the drift factor in
167 to zero. This setting will be persistent as long as the
168 .BR \%\-\-update\-drift " option is not used with " \%\-\-systohc
169 at shutdown (or anywhere else). Another way to inhibit this is by using the
170 .BR \%\-\-noadjfile " option when calling the " \%\-\-hctosys
171 function. A third method is to delete the
172 .IR @ADJTIME_PATH@ " file."
174 will then default to using the UTC timescale for the Hardware Clock. If
175 the Hardware Clock is ticking local time it will need to be defined in
176 the file. This can be done by calling
177 .BR hwclock\ \-\-localtime\ \-\-adjust ;
178 when the file is not present this command will not actually
179 adjust the Clock, but it will create the file with local time
180 configured, and a drift factor of zero.
182 A condition under which inhibiting
184 drift correction may be desired is when dual-booting multiple operating
185 systems. If while this instance of Linux is stopped, another OS changes
186 the Hardware Clock's value, then when this instance is started again the
187 drift correction applied will be incorrect.
189 .RB "For " hwclock 's
190 drift correction to work properly it is imperative that nothing changes
191 the Hardware Clock while its Linux instance is not running.
195 Set the Hardware Clock to the time given by the
197 option, and update the timestamps in
200 .B \%\-\-update-drift
201 option also (re)calculate the drift factor. Try it without the option if
202 .BR \%\-\-set " fails. See " \%\-\-update-drift " below."
206 This is an alternate to the
208 function that does not read the Hardware Clock nor set the System Clock;
209 consequently there is not any drift correction. It is intended to be
210 used in a startup script on systems with kernels above version 2.6 where
211 you know the System Clock has been set from the Hardware Clock by the
214 It does the following things that are detailed above in the
215 .BR \%\-\-hctosys " function:"
218 Corrects the System Clock timescale to UTC as needed. Only instead of
219 accomplishing this by setting the System Clock,
221 simply informs the kernel and it handles the change.
223 Sets the kernel's NTP \%'11\ minute\ mode' timescale.
225 Sets the kernel's timezone.
227 The first two are only available on the first call of
228 .BR \%settimeofday (2)
229 after boot. Consequently this option only makes sense when used in a
230 startup script. If the Hardware Clocks timescale configuration is
231 changed then a reboot would be required to inform the kernel.
235 .BR \-w , \ \-\-systohc
236 Set the Hardware Clock from the System Clock, and update the timestamps in
239 .B \%\-\-update-drift
240 option also (re)calculate the drift factor. Try it without the option if
241 .BR \%\-\-systohc " fails. See " \%\-\-update-drift " below."
244 .BR \-V , \ \-\-version
245 Display version information and exit.
249 Display help text and exit.
254 .BI \-\-adjfile= filename
255 .RI "Override the default " @ADJTIME_PATH@ " file path."
258 .BI \%\-\-date= date_string
259 This option must be used with the
263 functions, otherwise it is ignored.
266 .B "hwclock\ \-\-set\ \-\-date='16:45'"
268 .B "hwclock\ \-\-predict\ \-\-date='2525-08-14\ 07:11:05'"
270 The argument must be in local time, even if you keep your Hardware Clock in
273 option. Therefore, the argument should not include any timezone information.
274 It also should not be a relative time like "+5 minutes", because
276 precision depends upon correlation between the argument's value and when the
277 enter key is pressed. Fractional seconds are silently dropped. This option is
278 capable of understanding many time and date formats, but the previous
279 parameters should be observed.
283 .BI \%\-\-delay= seconds
284 This option allows to overwrite internally used delay when set clock time. The
285 default is 0.5 (500ms) for rtc_cmos, for another RTC types the delay is 0. If
286 RTC type is impossible to determine (from sysfs) then it defaults also to 0.5
287 to be backwardly compatible.
290 The 500ms default is based on commonly used MC146818A-compatible (x86) hardware clock. This
291 Hardware Clock can only be set to any integer time plus one half second. The
292 integer time is required because there is no interface to set or get a
293 fractional second. The additional half second delay is because the Hardware
294 Clock updates to the following second precisely 500 ms after setting the new
295 time. Unfortunately, this behavior is hardware specific and in same cases
296 another delay is required.
301 .BR \-D ", " \-\-debug
302 .RB Use\ \-\-verbose .
303 .RB The\ \%\-\-debug\ option
304 has been deprecated and may be repurposed or removed in a future release.
308 This option is meaningful for ISA compatible machines in the x86 and
309 x86_64 family. For other machines, it has no effect. This option tells
311 to use explicit I/O instructions to access the Hardware Clock.
314 will use the rtc device file, which it assumes to be driven by the Linux
315 RTC device driver. As of v2.26 it will no longer automatically use
316 directisa when the rtc driver is unavailable; this was causing an unsafe
317 condition that could allow two processes to access the Hardware Clock at
318 the same time. Direct hardware access from userspace should only be
319 used for testing, troubleshooting, and as a last resort when all other
320 methods fail. See the
321 .BR \-\-rtc " option."
325 This option is required when using the
326 .BR \%\-\-setepoch \ function.
327 .RI "The minimum " year
328 value is 1900. The maximum is system dependent
329 .RB ( ULONG_MAX\ -\ 1 ).
332 .BR \-f , \ \-\-rtc=\fIfilename\fR
333 .RB "Override " \%hwclock 's
334 default rtc device file name. Otherwise it will
335 use the first one found in this order:
354 .BR \-l , \ \-\-localtime
357 Indicate which timescale the Hardware Clock is set to.
359 The Hardware Clock may be configured to use either the UTC or the local
360 timescale, but nothing in the clock itself says which alternative is
362 .BR \%\-\-localtime " or " \-\-utc
363 options give this information to the
365 command. If you specify the wrong one (or specify neither and take a
366 wrong default), both setting and reading the Hardware Clock will be
369 If you specify neither
370 .BR \-\-utc " nor " \%\-\-localtime
371 then the one last given with a set function
372 .RB ( \-\-set ", " \%\-\-systohc ", or " \%\-\-adjust ),
375 will be used. If the adjtime file doesn't exist, the default is UTC.
377 Note: daylight saving time changes may be inconsistent when the
378 Hardware Clock is kept in local time. See the discussion below, under
383 Disable the facilities provided by
386 will not read nor write to that file with this option. Either
387 .BR \-\-utc " or " \%\-\-localtime
388 must be specified when using this option.
392 Do not actually change anything on the system, that is, the Clocks or
395 is implicit with this option).
399 Update the Hardware Clock's drift factor in
401 It can only be used with
402 .BR \-\-set " or " \%\-\-systohc ,
404 A minimum four hour period between settings is required. This is to
405 avoid invalid calculations. The longer the period, the more precise the
406 resulting drift factor will be.
408 This option was added in v2.26, because
409 it is typical for systems to call
410 .B \%hwclock\ \-\-systohc
411 at shutdown; with the old behaviour this would automatically
412 (re)calculate the drift factor which caused several problems:
415 When using NTP with an \%'11\ minute\ mode' kernel the drift factor
416 would be clobbered to near zero.
418 It would not allow the use of 'cold' drift correction. With most
419 configurations using 'cold' drift will yield favorable results. Cold,
420 means when the machine is turned off which can have a significant impact
423 (Re)calculating drift factor on every shutdown delivers suboptimal
424 results. For example, if ephemeral conditions cause the machine to be
425 abnormally hot the drift factor calculation would be out of range.
427 Significantly increased system shutdown times (as of v2.31 when not
429 .B \%\-\-update\-drift
430 the RTC is not read).
432 .RB "Having " \%hwclock
433 calculate the drift factor is a good starting point, but for optimal
434 results it will likely need to be adjusted by directly editing the
436 file. For most configurations once a machine's optimal drift factor is
437 crafted it should not need to be changed. Therefore, the old behavior to
438 automatically (re)calculate drift was changed and now requires this
439 option to be used. See the discussion below, under
440 .BR "The Adjust Function" .
442 This option requires reading the Hardware Clock before setting it. If
443 it cannot be read, then this option will cause the set functions to fail.
444 This can happen, for example, if the Hardware Clock is corrupted by a
445 power failure. In that case, the clock must first be set without this
446 option. Despite it not working, the resulting drift correction factor
447 would be invalid anyway.
451 .BR \-v ", " \-\-verbose
452 Display more details about what
458 .SS Clocks in a Linux System
460 There are two types of date-time clocks:
462 .B The Hardware Clock:
463 This clock is an independent hardware device, with its own power domain
464 (battery, capacitor, etc), that operates when the machine is powered off,
467 On an ISA compatible system, this clock is specified as part of the ISA
468 standard. A control program can read or set this clock only to a whole
469 second, but it can also detect the edges of the 1 second clock ticks, so
470 the clock actually has virtually infinite precision.
472 This clock is commonly called the hardware clock, the real time clock,
473 the RTC, the BIOS clock, and the CMOS clock. Hardware Clock, in its
474 capitalized form, was coined for use by
476 The Linux kernel also refers to it as the persistent clock.
478 Some non-ISA systems have a few real time clocks with
479 only one of them having its own power domain.
480 A very low power external I2C or SPI clock chip might be used with a
481 backup battery as the hardware clock to initialize a more functional
482 integrated real-time clock which is used for most other purposes.
485 This clock is part of the Linux kernel and is driven by
486 a timer interrupt. (On an ISA machine, the timer interrupt is part of
487 the ISA standard.) It has meaning only while Linux is running on the
488 machine. The System Time is the number of seconds since 00:00:00
489 January 1, 1970 UTC (or more succinctly, the number of seconds since
490 1969 UTC). The System Time is not an integer, though. It has virtually
493 The System Time is the time that matters. The Hardware Clock's basic
494 purpose is to keep time when Linux is not running so that the System
495 Clock can be initialized from it at boot. Note that in DOS, for which
496 ISA was designed, the Hardware Clock is the only real time clock.
498 It is important that the System Time not have any discontinuities such as
499 would happen if you used the
501 program to set it while the system is running. You can, however, do whatever
502 you want to the Hardware Clock while the system is running, and the next
503 time Linux starts up, it will do so with the adjusted time from the Hardware
504 Clock. Note: currently this is not possible on most systems because
505 .B \%hwclock\ \-\-systohc
506 is called at shutdown.
508 The Linux kernel's timezone is set by
510 But don't be misled -- almost nobody cares what timezone the kernel
511 thinks it is in. Instead, programs that care about the timezone
512 (perhaps because they want to display a local time for you) almost
513 always use a more traditional method of determining the timezone: They
514 use the TZ environment variable or the
516 file, as explained in the man page for
518 However, some programs and fringe parts of the Linux kernel such as filesystems
519 use the kernel's timezone value. An example is the vfat filesystem. If the
520 kernel timezone value is wrong, the vfat filesystem will report and set the
521 wrong timestamps on files. Another example is the kernel's NTP \%'11\ minute\ mode'.
522 If the kernel's timezone value and/or the
523 .I \%persistent_clock_is_local
524 variable are wrong, then the Hardware Clock will be set incorrectly
525 by \%'11\ minute\ mode'. See the discussion below, under
526 .BR "Automatic Hardware Clock Synchronization by the Kernel" .
529 sets the kernel's timezone to the value indicated by TZ or
530 .IR \%/etc/localtime " with the"
531 .BR \%\-\-hctosys " or " \%\-\-systz " functions."
533 The kernel's timezone value actually consists of two parts: 1) a field
534 tz_minuteswest indicating how many minutes local time (not adjusted
535 for DST) lags behind UTC, and 2) a field tz_dsttime indicating
536 the type of Daylight Savings Time (DST) convention that is in effect
537 in the locality at the present time.
538 This second field is not used under Linux and is always zero.
540 .BR \%settimeofday (2).
542 .SS Hardware Clock Access Methods
545 uses many different ways to get and set Hardware Clock values. The most
546 normal way is to do I/O to the rtc device special file, which is
547 presumed to be driven by the rtc device driver. Also, Linux systems
548 using the rtc framework with udev, are capable of supporting multiple
549 Hardware Clocks. This may bring about the need to override the default
550 rtc device by specifying one with the
551 .BR \-\-rtc " option."
553 However, this method is not always available as older systems do not
554 have an rtc driver. On these systems, the method of accessing the
555 Hardware Clock depends on the system hardware.
557 On an ISA compatible system,
559 can directly access the "CMOS memory" registers that
560 constitute the clock, by doing I/O to Ports 0x70 and 0x71. It does
561 this with actual I/O instructions and consequently can only do it if
562 running with superuser effective userid. This method may be used by
564 .BR \%\-\-directisa " option."
566 This is a really poor method of accessing the clock, for all the
567 reasons that userspace programs are generally not supposed to do
568 direct I/O and disable interrupts.
570 provides it for testing, troubleshooting, and because it may be the
571 only method available on ISA systems which do not have a working rtc
573 .SS The Adjust Function
575 The Hardware Clock is usually not very accurate. However, much of its
576 inaccuracy is completely predictable - it gains or loses the same amount
577 of time every day. This is called systematic drift.
578 .BR \%hwclock "'s " \%\-\-adjust
579 function lets you apply systematic drift corrections to the
583 .BR \%hwclock " keeps a file,"
585 that keeps some historical information. This is called the adjtime file.
587 Suppose you start with no adjtime file. You issue a
588 .B \%hwclock\ \-\-set
589 command to set the Hardware Clock to the true current time.
591 creates the adjtime file and records in it the current time as the
592 last time the clock was calibrated.
593 Five days later, the clock has gained 10 seconds, so you issue a
594 .B \%hwclock\ \-\-set\ \-\-update\-drift
595 command to set it back 10 seconds.
597 updates the adjtime file to show the current time as the last time the
598 clock was calibrated, and records 2 seconds per day as the systematic
599 drift rate. 24 hours go by, and then you issue a
600 .B \%hwclock\ \-\-adjust
603 consults the adjtime file and sees that the clock gains 2 seconds per
604 day when left alone and that it has been left alone for exactly one
605 day. So it subtracts 2 seconds from the Hardware Clock. It then
606 records the current time as the last time the clock was adjusted.
607 Another 24 hours go by and you issue another
608 .BR \%hwclock\ \-\-adjust .
610 does the same thing: subtracts 2 seconds and updates the adjtime file
611 with the current time as the last time the clock was adjusted.
614 .BR \%\-\-update\-drift " option with " \-\-set " or " \%\-\-systohc ,
615 the systematic drift rate is (re)calculated by comparing the fully drift
616 corrected current Hardware Clock time with the new set time, from that
617 it derives the 24 hour drift rate based on the last calibrated timestamp
618 from the adjtime file. This updated drift factor is then saved in
621 A small amount of error creeps in when
622 the Hardware Clock is set, so
624 refrains from making any adjustment that is less
625 than 1 second. Later on, when you request an adjustment again, the accumulated
626 drift will be more than 1 second and
628 will make the adjustment including any fractional amount.
630 .B \%hwclock\ \-\-hctosys
631 also uses the adjtime file data to compensate the value read from the Hardware
632 Clock before using it to set the System Clock. It does not share the 1 second
635 and will correct sub-second drift values immediately. It does not
636 change the Hardware Clock time nor the adjtime file. This may eliminate
639 unless something else on the system needs the Hardware Clock to be
643 While named for its historical purpose of controlling adjustments only,
644 it actually contains other information used by
646 from one invocation to the next.
648 The format of the adjtime file is, in ASCII:
650 Line 1: Three numbers, separated by blanks: 1) the systematic drift rate
651 in seconds per day, floating point decimal; 2) the resulting number of
652 seconds since 1969 UTC of most recent adjustment or calibration,
653 decimal integer; 3) zero (for compatibility with
655 as a decimal integer.
657 Line 2: One number: the resulting number of seconds since 1969 UTC of most
658 recent calibration. Zero if there has been no calibration yet or it
659 is known that any previous calibration is moot (for example, because
660 the Hardware Clock has been found, since that calibration, not to
661 contain a valid time). This is a decimal integer.
663 Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set to
664 Coordinated Universal Time or local time. You can always override this
665 value with options on the
669 You can use an adjtime file that was previously used with the
670 .BR \%clock "(8) program with " \%hwclock .
672 .SS Automatic Hardware Clock Synchronization by the Kernel
674 You should be aware of another way that the Hardware Clock is kept
675 synchronized in some systems. The Linux kernel has a mode wherein it
676 copies the System Time to the Hardware Clock every 11 minutes. This mode
677 is a compile time option, so not all kernels will have this capability.
678 This is a good mode to use when you are using something sophisticated
679 like NTP to keep your System Clock synchronized. (NTP is a way to keep
680 your System Time synchronized either to a time server somewhere on the
681 network or to a radio clock hooked up to your system. See RFC 1305.)
683 If the kernel is compiled with the \%'11\ minute\ mode' option it will
684 be active when the kernel's clock discipline is in a synchronized state.
685 When in this state, bit 6 (the bit that is set in the mask 0x0040)
688 variable is unset. This value is output as the 'status' line of the
689 .BR \%adjtimex\ --print " or " \%ntptime " commands."
691 It takes an outside influence, like the NTP daemon
692 to put the kernel's clock discipline into a synchronized state, and
693 therefore turn on \%'11\ minute\ mode'.
694 It can be turned off by running anything that sets the System Clock the old
695 fashioned way, including
696 .BR "\%hwclock\ \-\-hctosys" .
697 However, if the NTP daemon is still running, it will turn \%'11\ minute\ mode'
698 back on again the next time it synchronizes the System Clock.
700 If your system runs with \%'11\ minute\ mode' on, it may need to use either
701 .BR \%\-\-hctosys " or " \%\-\-systz
702 in a startup script, especially if the Hardware Clock is configured to use
703 the local timescale. Unless the kernel is informed of what timescale the
704 Hardware Clock is using, it may clobber it with the wrong one. The kernel
707 The first userspace command to set the System Clock informs the
708 kernel what timescale the Hardware Clock is using. This happens via the
709 .I \%persistent_clock_is_local
711 .BR \%\-\-hctosys " or " \%\-\-systz
712 is the first, it will set this variable according to the adjtime file or the
713 appropriate command-line argument. Note that when using this capability and the
714 Hardware Clock timescale configuration is changed, then a reboot is required to
717 .B \%hwclock\ \-\-adjust
718 should not be used with NTP \%'11\ minute\ mode'.
720 .SS ISA Hardware Clock Century value
722 There is some sort of standard that defines CMOS memory Byte 50 on an ISA
723 machine as an indicator of what century it is.
725 does not use or set that byte because there are some machines that
726 don't define the byte that way, and it really isn't necessary anyway,
727 since the year-of-century does a good job of implying which century it
730 If you have a bona fide use for a CMOS century byte, contact the
732 maintainer; an option may be appropriate.
734 Note that this section is only relevant when you are using the "direct
735 ISA" method of accessing the Hardware Clock.
736 ACPI provides a standard way to access century values, when they
737 are supported by the hardware.
739 .SH DATE-TIME CONFIGURATION
741 .SS Keeping Time without External Synchronization
744 This discussion is based on the following conditions:
746 Nothing is running that alters the date-time clocks, such as NTP daemon or a cron job."
748 The system timezone is configured for the correct local time. See below, under
749 .BR "POSIX vs 'RIGHT'" .
751 Early during startup the following are called, in this order:
753 .BI \%adjtimex\ \-\-tick \ value\ \-\-frequency \ value
755 .B \%hwclock\ \-\-hctosys
757 During shutdown the following is called:
759 .B \%hwclock\ \-\-systohc
762 .BR * " Systems without " adjtimex " may use " ntptime .
765 Whether maintaining precision time with NTP daemon
766 or not, it makes sense to configure the system to keep reasonably good
767 date-time on its own.
769 The first step in making that happen is having a clear understanding of
770 the big picture. There are two completely separate hardware devices
771 running at their own speed and drifting away from the 'correct' time at
772 their own rates. The methods and software for drift correction are
773 different for each of them. However, most systems are configured to
774 exchange values between these two clocks at startup and shutdown. Now
775 the individual device's time keeping errors are transferred back and
776 forth between each other. Attempt to configure drift correction for only
777 one of them, and the other's drift will be overlaid upon it.
779 This problem can be avoided when configuring drift correction for the
780 System Clock by simply not shutting down the machine. This, plus the
783 precision (including calculating drift factors) depends upon the System
784 Clock's rate being correct, means that configuration of the System Clock
785 should be done first.
787 The System Clock drift is corrected with the
788 .BR \%adjtimex "(8) command's " \-\-tick " and " \%\-\-frequency
789 options. These two work together: tick is the coarse adjustment and
790 frequency is the fine adjustment. (For systems that do not have an
791 .BR \%adjtimex " package,"
792 .BI \%ntptime\ \-f\ ppm
793 may be used instead.)
795 Some Linux distributions attempt to automatically calculate the System
798 compare operation. Trying to correct one
799 drifting clock by using another drifting clock as a reference is akin to
800 a dog trying to catch its own tail. Success may happen eventually, but
801 great effort and frustration will likely precede it. This automation may
802 yield an improvement over no configuration, but expecting optimum
803 results would be in error. A better choice for manual configuration
805 .BR \%adjtimex 's " \-\-log " options.
807 It may be more effective to simply track the System Clock drift with
808 .BR \%sntp ", or " \%date\ \-Ins
809 and a precision timepiece, and then calculate the correction manually.
811 After setting the tick and frequency values, continue to test and refine the
812 adjustments until the System Clock keeps good time. See
814 for more information and the example demonstrating manual drift
817 Once the System Clock is ticking smoothly, move on to the Hardware Clock.
819 As a rule, cold drift will work best for most use cases. This should be
820 true even for 24/7 machines whose normal downtime consists of a reboot.
821 In that case the drift factor value makes little difference. But on the
822 rare occasion that the machine is shut down for an extended period, then
823 cold drift should yield better results.
825 .B Steps to calculate cold drift:
827 .B "Ensure that NTP daemon will not be launched at startup."
829 .RI The " System Clock " "time must be correct at shutdown!"
831 Shut down the system.
833 Let an extended period pass without changing the Hardware Clock.
837 .RB "Immediately use " hwclock " to set the correct time, adding the"
838 .BR \%\-\-update\-drift " option."
842 then the System Clock must be set correctly (step 6a) just before doing so.
844 .RB "Having " hwclock
845 calculate the drift factor is a good starting point, but for optimal
846 results it will likely need to be adjusted by directly editing the
848 file. Continue to test and refine the drift factor until the Hardware
849 Clock is corrected properly at startup. To check this, first make sure
850 that the System Time is correct before shutdown and then use
851 .BR \%sntp ", or " \%date\ \-Ins
852 and a precision timepiece, immediately after startup.
854 Keeping the Hardware Clock in a local timescale causes inconsistent
855 daylight saving time results:
857 If Linux is running during a daylight saving time change, the time
858 written to the Hardware Clock will be adjusted for the change.
860 If Linux is NOT running during a daylight saving time change, the time
861 read from the Hardware Clock will NOT be adjusted for the change.
863 The Hardware Clock on an ISA compatible system keeps only a date and time,
864 it has no concept of timezone nor daylight saving. Therefore, when
866 is told that it is in local time, it assumes it is in the 'correct'
867 local time and makes no adjustments to the time read from it.
869 Linux handles daylight saving time changes transparently only when the
870 Hardware Clock is kept in the UTC timescale. Doing so is made easy for
871 system administrators as
873 uses local time for its output and as the argument to the
874 .BR \%\-\-date " option."
876 POSIX systems, like Linux, are designed to have the System Clock operate
877 in the UTC timescale. The Hardware Clock's purpose is to initialize the
878 System Clock, so also keeping it in UTC makes sense.
880 Linux does, however, attempt to accommodate the Hardware Clock being in
881 the local timescale. This is primarily for dual-booting with older
882 versions of MS Windows. From Windows 7 on, the RealTimeIsUniversal
883 registry key is supposed to be working properly so that its Hardware
884 Clock can be kept in UTC.
887 A discussion on date-time configuration would be incomplete without
888 addressing timezones, this is mostly well covered by
890 One area that seems to have no documentation is the 'right'
891 directory of the Time Zone Database, sometimes called tz or zoneinfo.
893 There are two separate databases in the zoneinfo system, posix
894 and 'right'. 'Right' (now named zoneinfo\-leaps) includes leap seconds and posix
895 does not. To use the 'right' database the System Clock must be set to
896 \%(UTC\ +\ leap seconds), which is equivalent to \%(TAI\ \-\ 10). This
897 allows calculating the
898 exact number of seconds between two dates that cross a leap second
899 epoch. The System Clock is then converted to the correct civil time,
900 including UTC, by using the 'right' timezone files which subtract the
901 leap seconds. Note: this configuration is considered experimental and is
902 known to have issues.
904 To configure a system to use a particular database all of the files
905 located in its directory must be copied to the root of
906 .IR \%/usr/share/zoneinfo .
907 Files are never used directly from the posix or 'right' subdirectories, e.g.,
908 .RI \%TZ=' right/Europe/Dublin '.
909 This habit was becoming so common that the upstream zoneinfo project
910 restructured the system's file tree by moving the posix and 'right'
911 subdirectories out of the zoneinfo directory and into sibling directories:
914 .I /usr/share/zoneinfo
916 .I /usr/share/zoneinfo\-posix
918 .I /usr/share/zoneinfo\-leaps
920 Unfortunately, some Linux distributions are changing it back to the old
921 tree structure in their packages. So the problem of system
922 administrators reaching into the 'right' subdirectory persists. This
923 causes the system timezone to be configured to include leap seconds
924 while the zoneinfo database is still configured to exclude them. Then
925 when an application such as a World Clock needs the South_Pole timezone
926 file; or an email MTA, or
928 needs the UTC timezone file; they fetch it from the root of
929 .I \%/usr/share/zoneinfo
930 , because that is what they are supposed to do. Those files exclude leap
931 seconds, but the System Clock now includes them, causing an incorrect
934 Attempting to mix and match files from these separate databases will not
935 work, because they each require the System Clock to use a different
936 timescale. The zoneinfo database must be configured to use either posix
937 or 'right', as described above, or by assigning a database path to the
939 environment variable.
941 One of the following exit values will be returned:
943 .BR EXIT_SUCCESS " ('0' on POSIX systems)"
944 Successful program execution.
946 .BR EXIT_FAILURE " ('1' on POSIX systems)"
947 The operation failed or the command syntax was not valid.
951 If this variable is set its value takes precedence over the system
955 If this variable is set its value takes precedence over the system
956 configured timezone database directory path.
960 The configuration and state file for hwclock.
963 The system timezone file.
965 .I /usr/share/zoneinfo/
966 The system timezone database directory.
970 may try for Hardware Clock access:
984 .BR gettimeofday (2),
985 .BR settimeofday (2),
990 Written by Bryan Henderson, September 1996 (bryanh@giraffe-data.com),
991 based on work done on the
993 program by Charles Hedrick, Rob Hooft, and Harald Koenig.
994 See the source code for complete history and credits.
997 The hwclock command is part of the util-linux package and is available from
998 https://www.kernel.org/pub/linux/utils/util-linux/.
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