1HWCLOCK(8) System Administration HWCLOCK(8)
2
6 hwclock - time clocks utility
7
9 hwclock [function] [option...]
10
12 hwclock is an administration tool for the time clocks. It can: display
13 the Hardware Clock time; set the Hardware Clock to a specified time;
14 set the Hardware Clock from the System Clock; set the System Clock from
15 the Hardware Clock; compensate for Hardware Clock drift; correct the
16 System Clock timescale; set the kernel’s timezone, NTP timescale, and
17 epoch (Alpha only); and predict future Hardware Clock values based on
18 its drift rate.
19 Since v2.26 important changes were made to the --hctosys function and
21 the --directisa option, and a new option --update-drift was added. See
22 their respective descriptions below.
23
25 The following functions are mutually exclusive, only one can be given
26 at a time. If none is given, the default is --show.
27 -a, --adjust
29 Add or subtract time from the Hardware Clock to account for
30 systematic drift since the last time the clock was set or adjusted.
31 See the discussion below, under The Adjust Function.
32 --getepoch; --setepoch
34 These functions are for Alpha machines only, and are only available
35 through the Linux kernel RTC driver.
36 They are used to read and set the kernel’s Hardware Clock epoch
38 value. Epoch is the number of years into AD to which a zero year
39 value in the Hardware Clock refers. For example, if the machine’s
40 BIOS sets the year counter in the Hardware Clock to contain the
41 number of full years since 1952, then the kernel’s Hardware Clock
42 epoch value must be 1952.
43 The --setepoch function requires using the --epoch option to
45 specify the year. For example:
46 hwclock --setepoch --epoch=1952
48 The RTC driver attempts to guess the correct epoch value, so
50 setting it may not be required.
51 This epoch value is used whenever hwclock reads or sets the
53 Hardware Clock on an Alpha machine. For ISA machines the kernel
54 uses the fixed Hardware Clock epoch of 1900.
55 --param-get=parameter; --param-set=parameter=value
57 Read and set the RTC’s parameter. This is useful, for example, to
58 retrieve the RTC’s feature or set the RTC’s Backup Switchover Mode.
59 parameter is either a numeric RTC parameter value (see the Kernel’s
61 include/uapi/linux/rtc.h) or an alias. See --help for a list of
62 valid aliases. parameter and value, if prefixed with 0x, are
63 interpreted as hexadecimal, otherwise decimal values.
64 --predict
66 Predict what the Hardware Clock will read in the future based upon
67 the time given by the --date option and the information in
68 /etc/adjtime. This is useful, for example, to account for drift
69 when setting a Hardware Clock wakeup (aka alarm). See rtcwake(8).
70 Do not use this function if the Hardware Clock is being modified by
72 anything other than the current operating system’s hwclock command,
73 such as '11 minute mode' or from dual-booting another OS.
74 -r, --show; --get
76 Read the Hardware Clock and print its time to standard output in
77 the ISO 8601 format. The time shown is always in local time, even
78 if you keep your Hardware Clock in UTC. See the --localtime option.
79 Showing the Hardware Clock time is the default when no function is
81 specified.
82 The --get function also applies drift correction to the time read,
84 based upon the information in /etc/adjtime. Do not use this
85 function if the Hardware Clock is being modified by anything other
86 than the current operating system’s hwclock command, such as '11
87 minute mode' or from dual-booting another OS.
88 -s, --hctosys
90 Set the System Clock from the Hardware Clock. The time read from
91 the Hardware Clock is compensated to account for systematic drift
92 before using it to set the System Clock. See the discussion below,
93 under The Adjust Function.
94 The System Clock must be kept in the UTC timescale for date-time
96 applications to work correctly in conjunction with the timezone
97 configured for the system. If the Hardware Clock is kept in local
98 time then the time read from it must be shifted to the UTC
99 timescale before using it to set the System Clock. The --hctosys
100 function does this based upon the information in the /etc/adjtime
101 file or the command line arguments --localtime and --utc. Note: no
102 daylight saving adjustment is made. See the discussion below, under
103 LOCAL vs UTC.
104 The kernel also keeps a timezone value, the --hctosys function sets
106 it to the timezone configured for the system. The system timezone
107 is configured by the TZ environment variable or the /etc/localtime
108 file, as tzset(3) would interpret them. The obsolete tz_dsttime
109 field of the kernel’s timezone value is set to zero. (For details
110 on what this field used to mean, see settimeofday(2).)
111 When used in a startup script, making the --hctosys function the
113 first caller of settimeofday(2) from boot, it will set the NTP '11
114 minute mode' timescale via the persistent_clock_is_local kernel
115 variable. If the Hardware Clock’s timescale configuration is
116 changed then a reboot is required to inform the kernel. See the
117 discussion below, under Automatic Hardware Clock Synchronization by
118 the Kernel.
119 This is a good function to use in one of the system startup scripts
121 before the file systems are mounted read/write.
122 This function should never be used on a running system. Jumping
124 system time will cause problems, such as corrupted filesystem
125 timestamps. Also, if something has changed the Hardware Clock, like
126 NTP’s '11 minute mode', then --hctosys will set the time
127 incorrectly by including drift compensation.
128 Drift compensation can be inhibited by setting the drift factor in
130 /etc/adjtime to zero. This setting will be persistent as long as
131 the --update-drift option is not used with --systohc at shutdown
132 (or anywhere else). Another way to inhibit this is by using the
133 --noadjfile option when calling the --hctosys function. A third
134 method is to delete the /etc/adjtime file. Hwclock will then
135 default to using the UTC timescale for the Hardware Clock. If the
136 Hardware Clock is ticking local time it will need to be defined in
137 the file. This can be done by calling hwclock --localtime --adjust;
138 when the file is not present this command will not actually adjust
139 the Clock, but it will create the file with local time configured,
140 and a drift factor of zero.
141 A condition under which inhibiting hwclock's drift correction may
143 be desired is when dual-booting multiple operating systems. If
144 while this instance of Linux is stopped, another OS changes the
145 Hardware Clock’s value, then when this instance is started again
146 the drift correction applied will be incorrect.
147 For hwclock's drift correction to work properly it is imperative
149 that nothing changes the Hardware Clock while its Linux instance is
150 not running.
151 --set
153 Set the Hardware Clock to the time given by the --date option, and
154 update the timestamps in /etc/adjtime. With the --update-drift
155 option also (re)calculate the drift factor. Try it without the
156 option if --set fails. See --update-drift below.
157 --systz
159 This is an alternate to the --hctosys function that does not read
160 the Hardware Clock nor set the System Clock; consequently there is
161 not any drift correction. It is intended to be used in a startup
162 script on systems with kernels above version 2.6 where you know the
163 System Clock has been set from the Hardware Clock by the kernel
164 during boot.
165 It does the following things that are detailed above in the
167 --hctosys function:
168 • Corrects the System Clock timescale to UTC as needed. Only
170 instead of accomplishing this by setting the System Clock,
171 hwclock simply informs the kernel and it handles the change.
172 • Sets the kernel’s NTP '11 minute mode' timescale.
174 • Sets the kernel’s timezone.
176 The first two are only available on the first call of settimeofday(2)
178 after boot. Consequently this option only makes sense when used in a
179 startup script. If the Hardware Clocks timescale configuration is
180 changed then a reboot would be required to inform the kernel.
181 -w, --systohc
183 Set the Hardware Clock from the System Clock, and update the
184 timestamps in /etc/adjtime. With the --update-drift option also
185 (re)calculate the drift factor. Try it without the option if
186 --systohc fails. See --update-drift below.
187 -h, --help
189 Display help text and exit.
190 -V, --version
192 Print version and exit.
193
195 --adjfile=filename
196 Override the default /etc/adjtime file path.
197 --date=date_string
199 This option must be used with the --set or --predict functions,
200 otherwise it is ignored.
201 hwclock --set --date='16:45'
203 hwclock --predict --date='2525-08-14 07:11:05'
205 The argument must be in local time, even if you keep your Hardware
207 Clock in UTC. See the --localtime option. Therefore, the argument
208 should not include any timezone information. It also should not be
209 a relative time like "+5 minutes", because hwclock's precision
210 depends upon correlation between the argument’s value and when the
211 enter key is pressed. Fractional seconds are silently dropped. This
212 option is capable of understanding many time and date formats, but
213 the previous parameters should be observed.
214 --delay=seconds
216 This option can be used to overwrite the internally used delay when
217 setting the clock time. The default is 0.5 (500ms) for rtc_cmos,
218 for another RTC types the delay is 0. If RTC type is impossible to
219 determine (from sysfs) then it defaults also to 0.5 to be
220 backwardly compatible.
221 The 500ms default is based on commonly used MC146818A-compatible
223 (x86) hardware clock. This Hardware Clock can only be set to any
224 integer time plus one half second. The integer time is required
225 because there is no interface to set or get a fractional second.
226 The additional half second delay is because the Hardware Clock
227 updates to the following second precisely 500 ms after setting the
228 new time. Unfortunately, this behavior is hardware specific and in
229 some cases another delay is required.
230 -D, --debug
232 Use --verbose. The --debug option has been deprecated and may be
233 repurposed or removed in a future release.
234 --directisa
236 This option is meaningful for ISA compatible machines in the x86
237 and x86_64 family. For other machines, it has no effect. This
238 option tells hwclock to use explicit I/O instructions to access the
239 Hardware Clock. Without this option, hwclock will use the rtc
240 device file, which it assumes to be driven by the Linux RTC device
241 driver. As of v2.26 it will no longer automatically use directisa
242 when the rtc driver is unavailable; this was causing an unsafe
243 condition that could allow two processes to access the Hardware
244 Clock at the same time. Direct hardware access from userspace
245 should only be used for testing, troubleshooting, and as a last
246 resort when all other methods fail. See the --rtc option.
247 --epoch=year
249 This option is required when using the --setepoch function. The
250 minimum year value is 1900. The maximum is system dependent
251 (ULONG_MAX - 1).
252 -f, --rtc=filename
254 Override hwclock's default rtc device file name. Otherwise it will
255 use the first one found in this order: /dev/rtc0, /dev/rtc,
256 /dev/misc/rtc. For IA-64: /dev/efirtc /dev/misc/efirtc
257 -l, --localtime; -u, --utc
259 Indicate which timescale the Hardware Clock is set to.
260 The Hardware Clock may be configured to use either the UTC or the
262 local timescale, but nothing in the clock itself says which
263 alternative is being used. The --localtime or --utc options give
264 this information to the hwclock command. If you specify the wrong
265 one (or specify neither and take a wrong default), both setting and
266 reading the Hardware Clock will be incorrect.
267 If you specify neither --utc nor --localtime then the one last
269 given with a set function (--set, --systohc, or --adjust), as
270 recorded in /etc/adjtime, will be used. If the adjtime file doesn’t
271 exist, the default is UTC.
272 Note: daylight saving time changes may be inconsistent when the
274 Hardware Clock is kept in local time. See the discussion below,
275 under LOCAL vs UTC.
276 --noadjfile
278 Disable the facilities provided by /etc/adjtime. hwclock will not
279 read nor write to that file with this option. Either --utc or
280 --localtime must be specified when using this option.
281 --test
283 Do not actually change anything on the system, that is, the Clocks
284 or /etc/adjtime (--verbose is implicit with this option).
285 --update-drift
287 Update the Hardware Clock’s drift factor in /etc/adjtime. It can
288 only be used with --set or --systohc.
289 A minimum four hour period between settings is required. This is to
291 avoid invalid calculations. The longer the period, the more precise
292 the resulting drift factor will be.
293 This option was added in v2.26, because it is typical for systems
295 to call hwclock --systohc at shutdown; with the old behavior this
296 would automatically (re)calculate the drift factor which caused
297 several problems:
298 • When using NTP with an '11 minute mode' kernel the drift factor
300 would be clobbered to near zero.
301 • It would not allow the use of 'cold' drift correction. With
303 most configurations using 'cold' drift will yield favorable
304 results. Cold, means when the machine is turned off which can
305 have a significant impact on the drift factor.
306 • (Re)calculating drift factor on every shutdown delivers
308 suboptimal results. For example, if ephemeral conditions cause
309 the machine to be abnormally hot the drift factor calculation
310 would be out of range.
311 • Significantly increased system shutdown times (as of v2.31 when
313 not using --update-drift the RTC is not read).
314 Having hwclock calculate the drift factor is a good starting point, but
316 for optimal results it will likely need to be adjusted by directly
317 editing the /etc/adjtime file. For most configurations once a machine’s
318 optimal drift factor is crafted it should not need to be changed.
319 Therefore, the old behavior to automatically (re)calculate drift was
320 changed and now requires this option to be used. See the discussion
321 below, under The Adjust Function.
322 This option requires reading the Hardware Clock before setting it. If
324 it cannot be read, then this option will cause the set functions to
325 fail. This can happen, for example, if the Hardware Clock is corrupted
326 by a power failure. In that case, the clock must first be set without
327 this option. Despite it not working, the resulting drift correction
328 factor would be invalid anyway.
329 -v, --verbose
331 Display more details about what hwclock is doing internally.
332
334 Clocks in a Linux System
335 There are two types of date-time clocks:
336 The Hardware Clock: This clock is an independent hardware device, with
338 its own power domain (battery, capacitor, etc), that operates when the
339 machine is powered off, or even unplugged.
340 On an ISA compatible system, this clock is specified as part of the ISA
342 standard. A control program can read or set this clock only to a whole
343 second, but it can also detect the edges of the 1 second clock ticks,
344 so the clock actually has virtually infinite precision.
345 This clock is commonly called the hardware clock, the real time clock,
347 the RTC, the BIOS clock, and the CMOS clock. Hardware Clock, in its
348 capitalized form, was coined for use by hwclock. The Linux kernel also
349 refers to it as the persistent clock.
350 Some non-ISA systems have a few real time clocks with only one of them
352 having its own power domain. A very low power external I2C or SPI clock
353 chip might be used with a backup battery as the hardware clock to
354 initialize a more functional integrated real-time clock which is used
355 for most other purposes.
356 The System Clock: This clock is part of the Linux kernel and is driven
358 by a timer interrupt. (On an ISA machine, the timer interrupt is part
359 of the ISA standard.) It has meaning only while Linux is running on the
360 machine. The System Time is the number of seconds since 00:00:00
361 January 1, 1970 UTC (or more succinctly, the number of seconds since
362 1969 UTC). The System Time is not an integer, though. It has virtually
363 infinite precision.
364 The System Time is the time that matters. The Hardware Clock’s basic
366 purpose is to keep time when Linux is not running so that the System
367 Clock can be initialized from it at boot. Note that in DOS, for which
368 ISA was designed, the Hardware Clock is the only real time clock.
369 It is important that the System Time not have any discontinuities such
371 as would happen if you used the date(1) program to set it while the
372 system is running. You can, however, do whatever you want to the
373 Hardware Clock while the system is running, and the next time Linux
374 starts up, it will do so with the adjusted time from the Hardware
375 Clock. Note: currently this is not possible on most systems because
376 hwclock --systohc is called at shutdown.
377 The Linux kernel’s timezone is set by hwclock. But don’t be misled —
379 almost nobody cares what timezone the kernel thinks it is in. Instead,
380 programs that care about the timezone (perhaps because they want to
381 display a local time for you) almost always use a more traditional
382 method of determining the timezone: They use the TZ environment
383 variable or the /etc/localtime file, as explained in the man page for
384 tzset(3). However, some programs and fringe parts of the Linux kernel
385 such as filesystems use the kernel’s timezone value. An example is the
386 vfat filesystem. If the kernel timezone value is wrong, the vfat
387 filesystem will report and set the wrong timestamps on files. Another
388 example is the kernel’s NTP '11 minute mode'. If the kernel’s timezone
389 value and/or the persistent_clock_is_local variable are wrong, then the
390 Hardware Clock will be set incorrectly by '11 minute mode'. See the
391 discussion below, under Automatic Hardware Clock Synchronization by the
392 Kernel.
393 hwclock sets the kernel’s timezone to the value indicated by TZ or
395 /etc/localtime with the --hctosys or --systz functions.
396 The kernel’s timezone value actually consists of two parts: 1) a field
398 tz_minuteswest indicating how many minutes local time (not adjusted for
399 DST) lags behind UTC, and 2) a field tz_dsttime indicating the type of
400 Daylight Savings Time (DST) convention that is in effect in the
401 locality at the present time. This second field is not used under Linux
402 and is always zero. See also settimeofday(2).
403 Hardware Clock Access Methods
405 hwclock uses many different ways to get and set Hardware Clock values.
406 The most normal way is to do I/O to the rtc device special file, which
407 is presumed to be driven by the rtc device driver. Also, Linux systems
408 using the rtc framework with udev, are capable of supporting multiple
409 Hardware Clocks. This may bring about the need to override the default
410 rtc device by specifying one with the --rtc option.
411 However, this method is not always available as older systems do not
413 have an rtc driver. On these systems, the method of accessing the
414 Hardware Clock depends on the system hardware.
415 On an ISA compatible system, hwclock can directly access the "CMOS
417 memory" registers that constitute the clock, by doing I/O to Ports 0x70
418 and 0x71. It does this with actual I/O instructions and consequently
419 can only do it if running with superuser effective userid. This method
420 may be used by specifying the --directisa option.
421 This is a really poor method of accessing the clock, for all the
423 reasons that userspace programs are generally not supposed to do direct
424 I/O and disable interrupts. hwclock provides it for testing,
425 troubleshooting, and because it may be the only method available on ISA
426 systems which do not have a working rtc device driver.
427 The Adjust Function
429 The Hardware Clock is usually not very accurate. However, much of its
430 inaccuracy is completely predictable - it gains or loses the same
431 amount of time every day. This is called systematic drift. hwclock's
432 --adjust function lets you apply systematic drift corrections to the
433 Hardware Clock.
434 It works like this: hwclock keeps a file, /etc/adjtime, that keeps some
436 historical information. This is called the adjtime file.
437 Suppose you start with no adjtime file. You issue a hwclock --set
439 command to set the Hardware Clock to the true current time. hwclock
440 creates the adjtime file and records in it the current time as the last
441 time the clock was calibrated. Five days later, the clock has gained 10
442 seconds, so you issue a hwclock --set --update-drift command to set it
443 back 10 seconds. hwclock updates the adjtime file to show the current
444 time as the last time the clock was calibrated, and records 2 seconds
445 per day as the systematic drift rate. 24 hours go by, and then you
446 issue a hwclock --adjust command. hwclock consults the adjtime file and
447 sees that the clock gains 2 seconds per day when left alone and that it
448 has been left alone for exactly one day. So it subtracts 2 seconds from
449 the Hardware Clock. It then records the current time as the last time
450 the clock was adjusted. Another 24 hours go by and you issue another
451 hwclock --adjust. hwclock does the same thing: subtracts 2 seconds and
452 updates the adjtime file with the current time as the last time the
453 clock was adjusted.
454 When you use the --update-drift option with --set or --systohc, the
456 systematic drift rate is (re)calculated by comparing the fully drift
457 corrected current Hardware Clock time with the new set time, from that
458 it derives the 24 hour drift rate based on the last calibrated
459 timestamp from the adjtime file. This updated drift factor is then
460 saved in /etc/adjtime.
461 A small amount of error creeps in when the Hardware Clock is set, so
463 --adjust refrains from making any adjustment that is less than 1
464 second. Later on, when you request an adjustment again, the accumulated
465 drift will be more than 1 second and --adjust will make the adjustment
466 including any fractional amount.
467 hwclock --hctosys also uses the adjtime file data to compensate the
469 value read from the Hardware Clock before using it to set the System
470 Clock. It does not share the 1 second limitation of --adjust, and will
471 correct sub-second drift values immediately. It does not change the
472 Hardware Clock time nor the adjtime file. This may eliminate the need
473 to use --adjust, unless something else on the system needs the Hardware
474 Clock to be compensated.
475 The Adjtime File
477 While named for its historical purpose of controlling adjustments only,
478 it actually contains other information used by hwclock from one
479 invocation to the next.
480 The format of the adjtime file is, in ASCII:
482 Line 1: Three numbers, separated by blanks: 1) the systematic drift
484 rate in seconds per day, floating point decimal; 2) the resulting
485 number of seconds since 1969 UTC of most recent adjustment or
486 calibration, decimal integer; 3) zero (for compatibility with clock(8))
487 as a floating point decimal.
488 Line 2: One number: the resulting number of seconds since 1969 UTC of
490 most recent calibration. Zero if there has been no calibration yet or
491 it is known that any previous calibration is moot (for example, because
492 the Hardware Clock has been found, since that calibration, not to
493 contain a valid time). This is a decimal integer.
494 Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set to
496 Coordinated Universal Time or local time. You can always override this
497 value with options on the hwclock command line.
498 You can use an adjtime file that was previously used with the clock(8)
500 program with hwclock.
501 Automatic Hardware Clock Synchronization by the Kernel
503 You should be aware of another way that the Hardware Clock is kept
504 synchronized in some systems. The Linux kernel has a mode wherein it
505 copies the System Time to the Hardware Clock every 11 minutes. This
506 mode is a compile time option, so not all kernels will have this
507 capability. This is a good mode to use when you are using something
508 sophisticated like NTP to keep your System Clock synchronized. (NTP is
509 a way to keep your System Time synchronized either to a time server
510 somewhere on the network or to a radio clock hooked up to your system.
511 See RFC 1305.)
512 If the kernel is compiled with the '11 minute mode' option it will be
514 active when the kernel’s clock discipline is in a synchronized state.
515 When in this state, bit 6 (the bit that is set in the mask 0x0040) of
516 the kernel’s time_status variable is unset. This value is output as the
517 'status' line of the adjtimex --print or ntptime commands.
518 It takes an outside influence, like the NTP daemon to put the kernel’s
520 clock discipline into a synchronized state, and therefore turn on '11
521 minute mode'. It can be turned off by running anything that sets the
522 System Clock the old fashioned way, including hwclock --hctosys.
523 However, if the NTP daemon is still running, it will turn '11 minute
524 mode' back on again the next time it synchronizes the System Clock.
525 If your system runs with '11 minute mode' on, it may need to use either
527 --hctosys or --systz in a startup script, especially if the Hardware
528 Clock is configured to use the local timescale. Unless the kernel is
529 informed of what timescale the Hardware Clock is using, it may clobber
530 it with the wrong one. The kernel uses UTC by default.
531 The first userspace command to set the System Clock informs the kernel
533 what timescale the Hardware Clock is using. This happens via the
534 persistent_clock_is_local kernel variable. If --hctosys or --systz is
535 the first, it will set this variable according to the adjtime file or
536 the appropriate command-line argument. Note that when using this
537 capability and the Hardware Clock timescale configuration is changed,
538 then a reboot is required to notify the kernel.
539 hwclock --adjust should not be used with NTP '11 minute mode'.
541 ISA Hardware Clock Century value
543 There is some sort of standard that defines CMOS memory Byte 50 on an
544 ISA machine as an indicator of what century it is. hwclock does not use
545 or set that byte because there are some machines that don’t define the
546 byte that way, and it really isn’t necessary anyway, since the
547 year-of-century does a good job of implying which century it is.
548 If you have a bona fide use for a CMOS century byte, contact the
550 hwclock maintainer; an option may be appropriate.
551 Note that this section is only relevant when you are using the "direct
553 ISA" method of accessing the Hardware Clock. ACPI provides a standard
554 way to access century values, when they are supported by the hardware.
555
557 Keeping Time without External Synchronization
558 This discussion is based on the following conditions:
559 • Nothing is running that alters the date-time clocks, such as NTP
561 daemon or a cron job."
562 • The system timezone is configured for the correct local time. See
564 below, under POSIX vs 'RIGHT'.
565 • Early during startup the following are called, in this order:
567 adjtimex --tick value --frequency value hwclock --hctosys
568 • During shutdown the following is called: hwclock --systohc
570 • Systems without adjtimex may use ntptime.
572 Whether maintaining precision time with NTP daemon or not, it makes
574 sense to configure the system to keep reasonably good date-time on its
575 own.
576 The first step in making that happen is having a clear understanding of
578 the big picture. There are two completely separate hardware devices
579 running at their own speed and drifting away from the 'correct' time at
580 their own rates. The methods and software for drift correction are
581 different for each of them. However, most systems are configured to
582 exchange values between these two clocks at startup and shutdown. Now
583 the individual device’s time keeping errors are transferred back and
584 forth between each other. Attempt to configure drift correction for
585 only one of them, and the other’s drift will be overlaid upon it.
586 This problem can be avoided when configuring drift correction for the
588 System Clock by simply not shutting down the machine. This, plus the
589 fact that all of hwclock's precision (including calculating drift
590 factors) depends upon the System Clock’s rate being correct, means that
591 configuration of the System Clock should be done first.
592 The System Clock drift is corrected with the adjtimex(8) command’s
594 --tick and --frequency options. These two work together: tick is the
595 coarse adjustment and frequency is the fine adjustment. (For systems
596 that do not have an adjtimex package, ntptime -f ppm may be used
597 instead.)
598 Some Linux distributions attempt to automatically calculate the System
600 Clock drift with adjtimex's compare operation. Trying to correct one
601 drifting clock by using another drifting clock as a reference is akin
602 to a dog trying to catch its own tail. Success may happen eventually,
603 but great effort and frustration will likely precede it. This
604 automation may yield an improvement over no configuration, but
605 expecting optimum results would be in error. A better choice for manual
606 configuration would be adjtimex's --log options.
607 It may be more effective to simply track the System Clock drift with
609 sntp, or date -Ins and a precision timepiece, and then calculate the
610 correction manually.
611 After setting the tick and frequency values, continue to test and
613 refine the adjustments until the System Clock keeps good time. See
614 adjtimex(2) for more information and the example demonstrating manual
615 drift calculations.
616 Once the System Clock is ticking smoothly, move on to the Hardware
618 Clock.
619 As a rule, cold drift will work best for most use cases. This should be
621 true even for 24/7 machines whose normal downtime consists of a reboot.
622 In that case the drift factor value makes little difference. But on the
623 rare occasion that the machine is shut down for an extended period,
624 then cold drift should yield better results.
625 Steps to calculate cold drift:
627 1
629 Ensure that NTP daemon will not be launched at startup.
630 2
632 The System Clock time must be correct at shutdown!
633 3
635 Shut down the system.
636 4
638 Let an extended period pass without changing the Hardware Clock.
639 5
641 Start the system.
642 6
644 Immediately use hwclock to set the correct time, adding the
645 --update-drift option.
646 Note: if step 6 uses --systohc, then the System Clock must be set
648 correctly (step 6a) just before doing so.
649 Having hwclock calculate the drift factor is a good starting point, but
651 for optimal results it will likely need to be adjusted by directly
652 editing the /etc/adjtime file. Continue to test and refine the drift
653 factor until the Hardware Clock is corrected properly at startup. To
654 check this, first make sure that the System Time is correct before
655 shutdown and then use sntp, or date -Ins and a precision timepiece,
656 immediately after startup.
657 LOCAL vs UTC
659 Keeping the Hardware Clock in a local timescale causes inconsistent
660 daylight saving time results:
661 • If Linux is running during a daylight saving time change, the time
663 written to the Hardware Clock will be adjusted for the change.
664 • If Linux is NOT running during a daylight saving time change, the
666 time read from the Hardware Clock will NOT be adjusted for the
667 change.
668 The Hardware Clock on an ISA compatible system keeps only a date and
670 time, it has no concept of timezone nor daylight saving. Therefore,
671 when hwclock is told that it is in local time, it assumes it is in the
672 'correct' local time and makes no adjustments to the time read from it.
673 Linux handles daylight saving time changes transparently only when the
675 Hardware Clock is kept in the UTC timescale. Doing so is made easy for
676 system administrators as hwclock uses local time for its output and as
677 the argument to the --date option.
678 POSIX systems, like Linux, are designed to have the System Clock
680 operate in the UTC timescale. The Hardware Clock’s purpose is to
681 initialize the System Clock, so also keeping it in UTC makes sense.
682 Linux does, however, attempt to accommodate the Hardware Clock being in
684 the local timescale. This is primarily for dual-booting with older
685 versions of MS Windows. From Windows 7 on, the RealTimeIsUniversal
686 registry key is supposed to be working properly so that its Hardware
687 Clock can be kept in UTC.
688 POSIX vs 'RIGHT'
690 A discussion on date-time configuration would be incomplete without
691 addressing timezones, this is mostly well covered by tzset(3). One area
692 that seems to have no documentation is the 'right' directory of the
693 Time Zone Database, sometimes called tz or zoneinfo.
694 There are two separate databases in the zoneinfo system, posix and
696 'right'. 'Right' (now named zoneinfo-leaps) includes leap seconds and
697 posix does not. To use the 'right' database the System Clock must be
698 set to (UTC + leap seconds), which is equivalent to (TAI - 10). This
699 allows calculating the exact number of seconds between two dates that
700 cross a leap second epoch. The System Clock is then converted to the
701 correct civil time, including UTC, by using the 'right' timezone files
702 which subtract the leap seconds. Note: this configuration is considered
703 experimental and is known to have issues.
704 To configure a system to use a particular database all of the files
706 located in its directory must be copied to the root of
707 /usr/share/zoneinfo. Files are never used directly from the posix or
708 'right' subdirectories, e.g., TZ='right/Europe/Dublin'. This habit was
709 becoming so common that the upstream zoneinfo project restructured the
710 system’s file tree by moving the posix and 'right' subdirectories out
711 of the zoneinfo directory and into sibling directories:
712 /usr/share/zoneinfo, /usr/share/zoneinfo-posix,
714 /usr/share/zoneinfo-leaps
715 Unfortunately, some Linux distributions are changing it back to the old
717 tree structure in their packages. So the problem of system
718 administrators reaching into the 'right' subdirectory persists. This
719 causes the system timezone to be configured to include leap seconds
720 while the zoneinfo database is still configured to exclude them. Then
721 when an application such as a World Clock needs the South_Pole timezone
722 file; or an email MTA, or hwclock needs the UTC timezone file; they
723 fetch it from the root of /usr/share/zoneinfo , because that is what
724 they are supposed to do. Those files exclude leap seconds, but the
725 System Clock now includes them, causing an incorrect time conversion.
726 Attempting to mix and match files from these separate databases will
728 not work, because they each require the System Clock to use a different
729 timescale. The zoneinfo database must be configured to use either posix
730 or 'right', as described above, or by assigning a database path to the
731 TZDIR environment variable.
732
734 One of the following exit values will be returned:
735 EXIT_SUCCESS ('0' on POSIX systems)
737 Successful program execution.
738 EXIT_FAILURE ('1' on POSIX systems)
740 The operation failed or the command syntax was not valid.
741
743 TZ
744 If this variable is set its value takes precedence over the system
745 configured timezone.
746 TZDIR
748 If this variable is set its value takes precedence over the system
749 configured timezone database directory path.
750
752 /etc/adjtime
753 The configuration and state file for hwclock. See also
754 adjtime_config(5).
755 /etc/localtime
757 The system timezone file.
758 /usr/share/zoneinfo/
760 The system timezone database directory.
761 Device files hwclock may try for Hardware Clock access: /dev/rtc0
763 /dev/rtc /dev/misc/rtc /dev/efirtc /dev/misc/efirtc
764
766 date(1), adjtime_config(5), adjtimex(8), gettimeofday(2),
767 settimeofday(2), crontab(1p), tzset(3)
768
770 Written by Bryan Henderson <bryanh@giraffe-data.com>, September 1996,
771 based on work done on the clock(8) program by Charles Hedrick, Rob
772 Hooft, and Harald Koenig. See the source code for complete history and
773 credits.
774
776 For bug reports, use the issue tracker at
777 https://github.com/util-linux/util-linux/issues.
778
780 The hwclock command is part of the util-linux package which can be
781 downloaded from Linux Kernel Archive
782 <https://www.kernel.org/pub/linux/utils/util-linux/>.
783util-linux 2.39.2 2023-08-17 HWCLOCK(8)