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 sys‐
30 tematic drift since the last time the clock was set or adjusted.
31 See the discussion below, under The Adjust Function.
32 --getepoch
34 --setepoch
35 These functions are for Alpha machines only, and are only avail‐
36 able through the Linux kernel RTC driver.
37 They are used to read and set the kernel's Hardware Clock epoch
39 value. Epoch is the number of years into AD to which a zero
40 year value in the Hardware Clock refers. For example, if the
41 machine's BIOS sets the year counter in the Hardware Clock to
42 contain the number of full years since 1952, then the kernel's
43 Hardware Clock epoch value must be 1952.
44 The --setepoch function requires using the --epoch option to
46 specify the year. For example:
47 hwclock --setepoch --epoch=1952
49 The RTC driver attempts to guess the correct epoch value, so
51 setting it may not be required.
52 This epoch value is used whenever hwclock reads or sets the
54 Hardware Clock on an Alpha machine. For ISA machines the kernel
55 uses the fixed Hardware Clock epoch of 1900.
56 --predict
58 Predict what the Hardware Clock will read in the future based
59 upon the time given by the --date option and the information in
60 /etc/adjtime. This is useful, for example, to account for drift
61 when setting a Hardware Clock wakeup (aka alarm). See
62 rtcwake(8).
63 Do not use this function if the Hardware Clock is being modified
65 by anything other than the current operating system's hwclock
66 command, such as '11 minute mode' or from dual-booting another
67 OS.
68 -r, --show
70 --get
71 Read the Hardware Clock and print its time to standard output in
72 the ISO 8601 format. The time shown is always in local time,
73 even if you keep your Hardware Clock in UTC. See the
74 --localtime option.
75 Showing the Hardware Clock time is the default when no function
77 is specified.
78 The --get function also applies drift correction to the time
80 read, based upon the information in /etc/adjtime. Do not use
81 this function if the Hardware Clock is being modified by any‐
82 thing other than the current operating system's hwclock command,
83 such as '11 minute mode' or from dual-booting another OS.
84 -s, --hctosys
86 Set the System Clock from the Hardware Clock. The time read
87 from the Hardware Clock is compensated to account for systematic
88 drift before using it to set the System Clock. See the discus‐
89 sion below, under The Adjust Function.
90 The System Clock must be kept in the UTC timescale for date-time
92 applications to work correctly in conjunction with the timezone
93 configured for the system. If the Hardware Clock is kept in
94 local time then the time read from it must be shifted to the UTC
95 timescale before using it to set the System Clock. The
96 --hctosys function does this based upon the information in the
97 /etc/adjtime file or the command line arguments --localtime and
98 --utc. Note: no daylight saving adjustment is made. See the
99 discussion below, under LOCAL vs UTC.
100 The kernel also keeps a timezone value, the --hctosys function
102 sets it to the timezone configured for the system. The system
103 timezone is configured by the TZ environment variable or the
104 /etc/localtime file, as tzset(3) would interpret them. The
105 obsolete tz_dsttime field of the kernel's timezone value is set
106 to zero. (For details on what this field used to mean, see
107 settimeofday(2).)
108 When used in a startup script, making the --hctosys function the
110 first caller of settimeofday(2) from boot, it will set the NTP
111 '11 minute mode' timescale via the persistent_clock_is_local
112 kernel variable. If the Hardware Clock's timescale configura‐
113 tion is changed then a reboot is required to inform the kernel.
114 See the discussion below, under Automatic Hardware Clock Syn‐
115 chronization by the Kernel.
116 This is a good function to use in one of the system startup
118 scripts before the file systems are mounted read/write.
119 This function should never be used on a running system. Jumping
121 system time will cause problems, such as corrupted filesystem
122 timestamps. Also, if something has changed the Hardware Clock,
123 like NTP's '11 minute mode', then --hctosys will set the time
124 incorrectly by including drift compensation.
125 Drift compensation can be inhibited by setting the drift factor
127 in /etc/adjtime to zero. This setting will be persistent as
128 long as the --update-drift option is not used with --systohc at
129 shutdown (or anywhere else). Another way to inhibit this is by
130 using the --noadjfile option when calling the --hctosys func‐
131 tion. A third method is to delete the /etc/adjtime file.
132 Hwclock will then default to using the UTC timescale for the
133 Hardware Clock. If the Hardware Clock is ticking local time it
134 will need to be defined in the file. This can be done by call‐
135 ing hwclock --localtime --adjust; when the file is not present
136 this command will not actually adjust the Clock, but it will
137 create the file with local time configured, and a drift factor
138 of zero.
139 A condition under which inhibiting hwclock's drift correction
141 may be desired is when dual-booting multiple operating systems.
142 If while this instance of Linux is stopped, another OS changes
143 the Hardware Clock's value, then when this instance is started
144 again the drift correction applied will be incorrect.
145 For hwclock's drift correction to work properly it is imperative
147 that nothing changes the Hardware Clock while its Linux instance
148 is not running.
149 --set Set the Hardware Clock to the time given by the --date option,
151 and update the timestamps in /etc/adjtime. With the
152 --update-drift option also (re)calculate the drift factor. Try
153 it without the option if --set fails. See --update-drift below.
154 --systz
156 This is an alternate to the --hctosys function that does not
157 read the Hardware Clock nor set the System Clock; consequently
158 there is not any drift correction. It is intended to be used in
159 a startup script on systems with kernels above version 2.6 where
160 you know the System Clock has been set from the Hardware Clock
161 by the kernel during boot.
162 It does the following things that are detailed above in the
164 --hctosys function:
165 · Corrects the System Clock timescale to UTC as needed. Only
167 instead of accomplishing this by setting the System Clock,
168 hwclock simply informs the kernel and it handles the change.
169 · Sets the kernel's NTP '11 minute mode' timescale.
171 · Sets the kernel's timezone.
173 The first two are only available on the first call of
175 settimeofday(2) after boot. Consequently this option only makes
176 sense when used in a startup script. If the Hardware Clocks
177 timescale configuration is changed then a reboot would be
178 required to inform the kernel.
179 -w, --systohc
181 Set the Hardware Clock from the System Clock, and update the
182 timestamps in /etc/adjtime. With the --update-drift option also
183 (re)calculate the drift factor. Try it without the option if
184 --systohc fails. See --update-drift below.
185 -V, --version
187 Display version information and exit.
188 -h, --help
190 Display help text and exit.
191
193 --adjfile=filename
194 Override the default /etc/adjtime file path.
195 --date=date_string
197 This option must be used with the --set or --predict functions,
198 otherwise it is ignored.
199 hwclock --set --date='16:45'
201 hwclock --predict --date='2525-08-14 07:11:05'
203 The argument must be in local time, even if you keep your Hard‐
205 ware Clock in UTC. See the --localtime option. Therefore, the
206 argument should not include any timezone information. It also
207 should not be a relative time like "+5 minutes", because
208 hwclock's precision depends upon correlation between the argu‐
209 ment's value and when the enter key is pressed. Fractional sec‐
210 onds are silently dropped. This option is capable of under‐
211 standing many time and date formats, but the previous parameters
212 should be observed.
213 --delay=seconds
215 This option allows to overwrite internally used delay when set
216 clock time. The default is 0.5 (500ms) for rtc_cmos, for another
217 RTC types the delay is 0. If RTC type is impossible to determine
218 (from sysfs) then it defaults also to 0.5 to be backwardly com‐
219 patible.
220 The 500ms default is based on commonly used MC146818A-compatible
222 (x86) hardware clock. This Hardware Clock can only be set to any
223 integer time plus one half second. The integer time is required
224 because there is no interface to set or get a fractional second.
225 The additional half second delay is because the Hardware Clock
226 updates to the following second precisely 500 ms after setting
227 the new time. Unfortunately, this behavior is hardware specific
228 and in same cases another delay is required.
229 -D, --debug
232 Use --verbose. The --debug option has been deprecated
233 and may be repurposed or removed in a future release.
234 --directisa
236 This option is meaningful for ISA compatible machines in
237 the x86 and x86_64 family. For other machines, it has no
238 effect. This option tells hwclock to use explicit I/O
239 instructions to access the Hardware Clock. Without this
240 option, hwclock will use the rtc device file, which it
241 assumes to be driven by the Linux RTC device driver. As
242 of v2.26 it will no longer automatically use directisa
243 when the rtc driver is unavailable; this was causing an
244 unsafe condition that could allow two processes to access
245 the Hardware Clock at the same time. Direct hardware
246 access from userspace should only be used for testing,
247 troubleshooting, and as a last resort when all other
248 methods fail. See the --rtc option.
249 --epoch=year
251 This option is required when using the
252 --setepoch function. The minimum year value is 1900. The
253 maximum is system dependent (ULONG_MAX - 1).
254 -f, --rtc=filename
256 Override hwclock's default rtc device file name. Other‐
257 wise it will use the first one found in this order:
258 /dev/rtc0
259 /dev/rtc
260 /dev/misc/rtc
261 For IA-64:
262 /dev/efirtc
263 /dev/misc/efirtc
264 -l, --localtime
266 -u, --utc
267 Indicate which timescale the Hardware Clock is set to.
268 The Hardware Clock may be configured to use either the
270 UTC or the local timescale, but nothing in the clock
271 itself says which alternative is being used. The
272 --localtime or --utc options give this information to the
273 hwclock command. If you specify the wrong one (or spec‐
274 ify neither and take a wrong default), both setting and
275 reading the Hardware Clock will be incorrect.
276 If you specify neither --utc nor --localtime then the one
278 last given with a set function (--set, --systohc, or
279 --adjust), as recorded in /etc/adjtime, will be used. If
280 the adjtime file doesn't exist, the default is UTC.
281 Note: daylight saving time changes may be inconsistent
283 when the Hardware Clock is kept in local time. See the
284 discussion below, under LOCAL vs UTC.
285 --noadjfile
287 Disable the facilities provided by /etc/adjtime. hwclock
288 will not read nor write to that file with this option.
289 Either --utc or --localtime must be specified when using
290 this option.
291 --test Do not actually change anything on the system, that is,
293 the Clocks or /etc/adjtime (--verbose is implicit with
294 this option).
295 --update-drift
297 Update the Hardware Clock's drift factor in /etc/adjtime.
298 It can only be used with --set or --systohc,
299 A minimum four hour period between settings is required.
301 This is to avoid invalid calculations. The longer the
302 period, the more precise the resulting drift factor will
303 be.
304 This option was added in v2.26, because it is typical for
306 systems to call hwclock --systohc at shutdown; with the
307 old behaviour this would automatically (re)calculate the
308 drift factor which caused several problems:
309 · When using NTP with an '11 minute mode' kernel the
311 drift factor would be clobbered to near zero.
312 · It would not allow the use of 'cold' drift correction.
314 With most configurations using 'cold' drift will yield
315 favorable results. Cold, means when the machine is
316 turned off which can have a significant impact on the
317 drift factor.
318 · (Re)calculating drift factor on every shutdown delivers
320 suboptimal results. For example, if ephemeral condi‐
321 tions cause the machine to be abnormally hot the drift
322 factor calculation would be out of range.
323 · Significantly increased system shutdown times (as of
325 v2.31 when not using --update-drift the RTC is not
326 read).
327 Having hwclock calculate the drift factor is a good
329 starting point, but for optimal results it will likely
330 need to be adjusted by directly editing the /etc/adjtime
331 file. For most configurations once a machine's optimal
332 drift factor is crafted it should not need to be changed.
333 Therefore, the old behavior to automatically (re)calcu‐
334 late drift was changed and now requires this option to be
335 used. See the discussion below, under The Adjust Func‐
336 tion.
337 This option requires reading the Hardware Clock before
339 setting it. If it cannot be read, then this option will
340 cause the set functions to fail. This can happen, for
341 example, if the Hardware Clock is corrupted by a power
342 failure. In that case, the clock must first be set with‐
343 out this option. Despite it not working, the resulting
344 drift correction factor would be invalid anyway.
345 -v, --verbose
347 Display more details about what hwclock is doing inter‐
348 nally.
349
351 Clocks in a Linux System
352 There are two types of date-time clocks:
353 The Hardware Clock: This clock is an independent hardware
355 device, with its own power domain (battery, capacitor, etc),
356 that operates when the machine is powered off, or even
357 unplugged.
358 On an ISA compatible system, this clock is specified as part of
360 the ISA standard. A control program can read or set this clock
361 only to a whole second, but it can also detect the edges of the
362 1 second clock ticks, so the clock actually has virtually infi‐
363 nite precision.
364 This clock is commonly called the hardware clock, the real time
366 clock, the RTC, the BIOS clock, and the CMOS clock. Hardware
367 Clock, in its capitalized form, was coined for use by hwclock.
368 The Linux kernel also refers to it as the persistent clock.
369 Some non-ISA systems have a few real time clocks with only one
371 of them having its own power domain. A very low power external
372 I2C or SPI clock chip might be used with a backup battery as the
373 hardware clock to initialize a more functional integrated real-
374 time clock which is used for most other purposes.
375 The System Clock: This clock is part of the Linux kernel and is
377 driven by a timer interrupt. (On an ISA machine, the timer
378 interrupt is part of the ISA standard.) It has meaning only
379 while Linux is running on the machine. The System Time is the
380 number of seconds since 00:00:00 January 1, 1970 UTC (or more
381 succinctly, the number of seconds since 1969 UTC). The System
382 Time is not an integer, though. It has virtually infinite pre‐
383 cision.
384 The System Time is the time that matters. The Hardware Clock's
386 basic purpose is to keep time when Linux is not running so that
387 the System Clock can be initialized from it at boot. Note that
388 in DOS, for which ISA was designed, the Hardware Clock is the
389 only real time clock.
390 It is important that the System Time not have any discontinu‐
392 ities such as would happen if you used the date(1) program to
393 set it while the system is running. You can, however, do what‐
394 ever you want to the Hardware Clock while the system is running,
395 and the next time Linux starts up, it will do so with the
396 adjusted time from the Hardware Clock. Note: currently this is
397 not possible on most systems because hwclock --systohc is called
398 at shutdown.
399 The Linux kernel's timezone is set by hwclock. But don't be
401 misled -- almost nobody cares what timezone the kernel thinks it
402 is in. Instead, programs that care about the timezone (perhaps
403 because they want to display a local time for you) almost always
404 use a more traditional method of determining the timezone: They
405 use the TZ environment variable or the /etc/localtime file, as
406 explained in the man page for tzset(3). However, some programs
407 and fringe parts of the Linux kernel such as filesystems use the
408 kernel's timezone value. An example is the vfat filesystem. If
409 the kernel timezone value is wrong, the vfat filesystem will
410 report and set the wrong timestamps on files. Another example
411 is the kernel's NTP '11 minute mode'. If the kernel's timezone
412 value and/or the persistent_clock_is_local variable are wrong,
413 then the Hardware Clock will be set incorrectly by
414 '11 minute mode'. See the discussion below, under Automatic
415 Hardware Clock Synchronization by the Kernel.
416 hwclock sets the kernel's timezone to the value indicated by TZ
418 or /etc/localtime with the --hctosys or --systz functions.
419 The kernel's timezone value actually consists of two parts: 1) a
421 field tz_minuteswest indicating how many minutes local time (not
422 adjusted for DST) lags behind UTC, and 2) a field tz_dsttime
423 indicating the type of Daylight Savings Time (DST) convention
424 that is in effect in the locality at the present time. This
425 second field is not used under Linux and is always zero. See
426 also settimeofday(2).
427 Hardware Clock Access Methods
429 hwclock uses many different ways to get and set Hardware Clock
430 values. The most normal way is to do I/O to the rtc device spe‐
431 cial file, which is presumed to be driven by the rtc device
432 driver. Also, Linux systems using the rtc framework with udev,
433 are capable of supporting multiple Hardware Clocks. This may
434 bring about the need to override the default rtc device by spec‐
435 ifying one with the --rtc option.
436 However, this method is not always available as older systems do
438 not have an rtc driver. On these systems, the method of access‐
439 ing the Hardware Clock depends on the system hardware.
440 On an ISA compatible system, hwclock can directly access the
442 "CMOS memory" registers that constitute the clock, by doing I/O
443 to Ports 0x70 and 0x71. It does this with actual I/O instruc‐
444 tions and consequently can only do it if running with superuser
445 effective userid. This method may be used by specifying the
446 --directisa option.
447 This is a really poor method of accessing the clock, for all the
449 reasons that userspace programs are generally not supposed to do
450 direct I/O and disable interrupts. hwclock provides it for
451 testing, troubleshooting, and because it may be the only method
452 available on ISA systems which do not have a working rtc device
453 driver.
454 The Adjust Function
456 The Hardware Clock is usually not very accurate. However, much
457 of its inaccuracy is completely predictable - it gains or loses
458 the same amount of time every day. This is called systematic
459 drift. hwclock's --adjust function lets you apply systematic
460 drift corrections to the Hardware Clock.
461 It works like this: hwclock keeps a file, /etc/adjtime, that
463 keeps some historical information. This is called the adjtime
464 file.
465 Suppose you start with no adjtime file. You issue a
467 hwclock --set command to set the Hardware Clock to the true cur‐
468 rent time. hwclock creates the adjtime file and records in it
469 the current time as the last time the clock was calibrated.
470 Five days later, the clock has gained 10 seconds, so you issue a
471 hwclock --set --update-drift command to set it back 10 seconds.
472 hwclock updates the adjtime file to show the current time as the
473 last time the clock was calibrated, and records 2 seconds per
474 day as the systematic drift rate. 24 hours go by, and then you
475 issue a hwclock --adjust command. hwclock consults the adjtime
476 file and sees that the clock gains 2 seconds per day when left
477 alone and that it has been left alone for exactly one day. So
478 it subtracts 2 seconds from the Hardware Clock. It then records
479 the current time as the last time the clock was adjusted.
480 Another 24 hours go by and you issue another hwclock --adjust.
481 hwclock does the same thing: subtracts 2 seconds and updates the
482 adjtime file with the current time as the last time the clock
483 was adjusted.
484 When you use the --update-drift option with --set or --systohc,
486 the systematic drift rate is (re)calculated by comparing the
487 fully drift corrected current Hardware Clock time with the new
488 set time, from that it derives the 24 hour drift rate based on
489 the last calibrated timestamp from the adjtime file. This
490 updated drift factor is then saved in /etc/adjtime.
491 A small amount of error creeps in when the Hardware Clock is
493 set, so --adjust refrains from making any adjustment that is
494 less than 1 second. Later on, when you request an adjustment
495 again, the accumulated drift will be more than 1 second and
496 --adjust will make the adjustment including any fractional
497 amount.
498 hwclock --hctosys also uses the adjtime file data to compensate
500 the value read from the Hardware Clock before using it to set
501 the System Clock. It does not share the 1 second limitation of
502 --adjust, and will correct sub-second drift values immediately.
503 It does not change the Hardware Clock time nor the adjtime file.
504 This may eliminate the need to use --adjust, unless something
505 else on the system needs the Hardware Clock to be compensated.
506 The Adjtime File
508 While named for its historical purpose of controlling adjust‐
509 ments only, it actually contains other information used by
510 hwclock from one invocation to the next.
511 The format of the adjtime file is, in ASCII:
513 Line 1: Three numbers, separated by blanks: 1) the systematic
515 drift rate in seconds per day, floating point decimal; 2) the
516 resulting number of seconds since 1969 UTC of most recent
517 adjustment or calibration, decimal integer; 3) zero (for compat‐
518 ibility with clock(8)) as a decimal integer.
519 Line 2: One number: the resulting number of seconds since 1969
521 UTC of most recent calibration. Zero if there has been no cali‐
522 bration yet or it is known that any previous calibration is moot
523 (for example, because the Hardware Clock has been found, since
524 that calibration, not to contain a valid time). This is a deci‐
525 mal integer.
526 Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is
528 set to Coordinated Universal Time or local time. You can always
529 override this value with options on the hwclock command line.
530 You can use an adjtime file that was previously used with the
532 clock(8) program with hwclock.
533 Automatic Hardware Clock Synchronization by the Kernel
535 You should be aware of another way that the Hardware Clock is
536 kept synchronized in some systems. The Linux kernel has a mode
537 wherein it copies the System Time to the Hardware Clock every 11
538 minutes. This mode is a compile time option, so not all kernels
539 will have this capability. This is a good mode to use when you
540 are using something sophisticated like NTP to keep your System
541 Clock synchronized. (NTP is a way to keep your System Time syn‐
542 chronized either to a time server somewhere on the network or to
543 a radio clock hooked up to your system. See RFC 1305.)
544 If the kernel is compiled with the '11 minute mode' option it
546 will be active when the kernel's clock discipline is in a syn‐
547 chronized state. When in this state, bit 6 (the bit that is set
548 in the mask 0x0040) of the kernel's time_status variable is
549 unset. This value is output as the 'status' line of the
550 adjtimex --print or ntptime commands.
551 It takes an outside influence, like the NTP daemon to put the
553 kernel's clock discipline into a synchronized state, and there‐
554 fore turn on '11 minute mode'. It can be turned off by running
555 anything that sets the System Clock the old fashioned way,
556 including hwclock --hctosys. However, if the NTP daemon is
557 still running, it will turn '11 minute mode' back on again the
558 next time it synchronizes the System Clock.
559 If your system runs with '11 minute mode' on, it may need to use
561 either --hctosys or --systz in a startup script, especially if
562 the Hardware Clock is configured to use the local timescale.
563 Unless the kernel is informed of what timescale the Hardware
564 Clock is using, it may clobber it with the wrong one. The kernel
565 uses UTC by default.
566 The first userspace command to set the System Clock informs the
568 kernel what timescale the Hardware Clock is using. This happens
569 via the persistent_clock_is_local kernel variable. If --hctosys
570 or --systz is the first, it will set this variable according to
571 the adjtime file or the appropriate command-line argument. Note
572 that when using this capability and the Hardware Clock timescale
573 configuration is changed, then a reboot is required to notify
574 the kernel.
575 hwclock --adjust should not be used with NTP '11 minute mode'.
577 ISA Hardware Clock Century value
579 There is some sort of standard that defines CMOS memory Byte 50
580 on an ISA machine as an indicator of what century it is.
581 hwclock does not use or set that byte because there are some
582 machines that don't define the byte that way, and it really
583 isn't necessary anyway, since the year-of-century does a good
584 job of implying which century it is.
585 If you have a bona fide use for a CMOS century byte, contact the
587 hwclock maintainer; an option may be appropriate.
588 Note that this section is only relevant when you are using the
590 "direct ISA" method of accessing the Hardware Clock. ACPI pro‐
591 vides a standard way to access century values, when they are
592 supported by the hardware.
593
595 Keeping Time without External Synchronization
596 This discussion is based on the following conditions:
597 · Nothing is running that alters the date-time clocks, such as
599 NTP daemon or a cron job."
600 · The system timezone is configured for the correct local time.
602 See below, under POSIX vs 'RIGHT'.
603 · Early during startup the following are called, in this order:
605 adjtimex --tick value --frequency value
606 hwclock --hctosys
607 · During shutdown the following is called:
609 hwclock --systohc
610 * Systems without adjtimex may use ntptime.
612 Whether maintaining precision time with NTP daemon or not, it
614 makes sense to configure the system to keep reasonably good
615 date-time on its own.
616 The first step in making that happen is having a clear under‐
618 standing of the big picture. There are two completely separate
619 hardware devices running at their own speed and drifting away
620 from the 'correct' time at their own rates. The methods and
621 software for drift correction are different for each of them.
622 However, most systems are configured to exchange values between
623 these two clocks at startup and shutdown. Now the individual
624 device's time keeping errors are transferred back and forth
625 between each other. Attempt to configure drift correction for
626 only one of them, and the other's drift will be overlaid upon
627 it.
628 This problem can be avoided when configuring drift correction
630 for the System Clock by simply not shutting down the machine.
631 This, plus the fact that all of hwclock's precision (including
632 calculating drift factors) depends upon the System Clock's rate
633 being correct, means that configuration of the System Clock
634 should be done first.
635 The System Clock drift is corrected with the adjtimex(8) com‐
637 mand's --tick and --frequency options. These two work together:
638 tick is the coarse adjustment and frequency is the fine adjust‐
639 ment. (For systems that do not have an adjtimex package,
640 ntptime -f ppm may be used instead.)
641 Some Linux distributions attempt to automatically calculate the
643 System Clock drift with adjtimex's compare operation. Trying to
644 correct one drifting clock by using another drifting clock as a
645 reference is akin to a dog trying to catch its own tail. Suc‐
646 cess may happen eventually, but great effort and frustration
647 will likely precede it. This automation may yield an improve‐
648 ment over no configuration, but expecting optimum results would
649 be in error. A better choice for manual configuration would be
650 adjtimex's --log options.
651 It may be more effective to simply track the System Clock drift
653 with sntp, or date -Ins and a precision timepiece, and then cal‐
654 culate the correction manually.
655 After setting the tick and frequency values, continue to test
657 and refine the adjustments until the System Clock keeps good
658 time. See adjtimex(8) for more information and the example
659 demonstrating manual drift calculations.
660 Once the System Clock is ticking smoothly, move on to the Hard‐
662 ware Clock.
663 As a rule, cold drift will work best for most use cases. This
665 should be true even for 24/7 machines whose normal downtime con‐
666 sists of a reboot. In that case the drift factor value makes
667 little difference. But on the rare occasion that the machine is
668 shut down for an extended period, then cold drift should yield
669 better results.
670 Steps to calculate cold drift:
672 1 Ensure that NTP daemon will not be launched at startup.
674 2 The System Clock time must be correct at shutdown!
676 3 Shut down the system.
678 4 Let an extended period pass without changing the Hardware
680 Clock.
681 5 Start the system.
683 6 Immediately use hwclock to set the correct time, adding the
685 --update-drift option.
686 Note: if step 6 uses --systohc, then the System Clock must be
688 set correctly (step 6a) just before doing so.
689 Having hwclock calculate the drift factor is a good starting
691 point, but for optimal results it will likely need to be
692 adjusted by directly editing the /etc/adjtime file. Continue to
693 test and refine the drift factor until the Hardware Clock is
694 corrected properly at startup. To check this, first make sure
695 that the System Time is correct before shutdown and then use
696 sntp, or date -Ins and a precision timepiece, immediately after
697 startup.
698 LOCAL vs UTC
700 Keeping the Hardware Clock in a local timescale causes inconsis‐
701 tent daylight saving time results:
702 · If Linux is running during a daylight saving time change, the
704 time written to the Hardware Clock will be adjusted for the
705 change.
706 · If Linux is NOT running during a daylight saving time change,
708 the time read from the Hardware Clock will NOT be adjusted for
709 the change.
710 The Hardware Clock on an ISA compatible system keeps only a date
712 and time, it has no concept of timezone nor daylight saving.
713 Therefore, when hwclock is told that it is in local time, it
714 assumes it is in the 'correct' local time and makes no adjust‐
715 ments to the time read from it.
716 Linux handles daylight saving time changes transparently only
718 when the Hardware Clock is kept in the UTC timescale. Doing so
719 is made easy for system administrators as hwclock uses local
720 time for its output and as the argument to the --date option.
721 POSIX systems, like Linux, are designed to have the System Clock
723 operate in the UTC timescale. The Hardware Clock's purpose is to
724 initialize the System Clock, so also keeping it in UTC makes
725 sense.
726 Linux does, however, attempt to accommodate the Hardware Clock
728 being in the local timescale. This is primarily for dual-booting
729 with older versions of MS Windows. From Windows 7 on, the Real‐
730 TimeIsUniversal registry key is supposed to be working properly
731 so that its Hardware Clock can be kept in UTC.
732 POSIX vs 'RIGHT'
734 A discussion on date-time configuration would be incomplete
735 without addressing timezones, this is mostly well covered by
736 tzset(3). One area that seems to have no documentation is the
737 'right' directory of the Time Zone Database, sometimes called tz
738 or zoneinfo.
739 There are two separate databases in the zoneinfo system, posix
741 and 'right'. 'Right' (now named zoneinfo-leaps) includes leap
742 seconds and posix does not. To use the 'right' database the Sys‐
743 tem Clock must be set to (UTC + leap seconds), which is equiva‐
744 lent to (TAI - 10). This allows calculating the exact number of
745 seconds between two dates that cross a leap second epoch. The
746 System Clock is then converted to the correct civil time,
747 including UTC, by using the 'right' timezone files which sub‐
748 tract the leap seconds. Note: this configuration is considered
749 experimental and is known to have issues.
750 To configure a system to use a particular database all of the
752 files located in its directory must be copied to the root of
753 /usr/share/zoneinfo. Files are never used directly from the
754 posix or 'right' subdirectories, e.g., TZ='right/Europe/Dublin'.
755 This habit was becoming so common that the upstream zoneinfo
756 project restructured the system's file tree by moving the posix
757 and 'right' subdirectories out of the zoneinfo directory and
758 into sibling directories:
759 /usr/share/zoneinfo
761 /usr/share/zoneinfo-posix
762 /usr/share/zoneinfo-leaps
763 Unfortunately, some Linux distributions are changing it back to
765 the old tree structure in their packages. So the problem of sys‐
766 tem administrators reaching into the 'right' subdirectory per‐
767 sists. This causes the system timezone to be configured to
768 include leap seconds while the zoneinfo database is still con‐
769 figured to exclude them. Then when an application such as a
770 World Clock needs the South_Pole timezone file; or an email MTA,
771 or hwclock needs the UTC timezone file; they fetch it from the
772 root of /usr/share/zoneinfo , because that is what they are sup‐
773 posed to do. Those files exclude leap seconds, but the System
774 Clock now includes them, causing an incorrect time conversion.
775 Attempting to mix and match files from these separate databases
777 will not work, because they each require the System Clock to use
778 a different timescale. The zoneinfo database must be configured
779 to use either posix or 'right', as described above, or by
780 assigning a database path to the TZDIR environment variable.
781
783 One of the following exit values will be returned:
784 EXIT_SUCCESS ('0' on POSIX systems)
786 Successful program execution.
787 EXIT_FAILURE ('1' on POSIX systems)
789 The operation failed or the command syntax was not valid.
790
792 TZ If this variable is set its value takes precedence over
793 the system configured timezone.
794 TZDIR If this variable is set its value takes precedence over
796 the system configured timezone database directory path.
797
799 /etc/adjtime
800 The configuration and state file for hwclock.
801 /etc/localtime
803 The system timezone file.
804 /usr/share/zoneinfo/
806 The system timezone database directory.
807 Device files hwclock may try for Hardware Clock access:
809 /dev/rtc0
810 /dev/rtc
811 /dev/misc/rtc
812 /dev/efirtc
813 /dev/misc/efirtc
814
816 date(1), adjtimex(8), gettimeofday(2), settimeofday(2),
817 crontab(1), tzset(3)
818
820 Written by Bryan Henderson, September 1996 (bryanh@giraffe-
821 data.com), based on work done on the clock(8) program by Charles
822 Hedrick, Rob Hooft, and Harald Koenig. See the source code for
823 complete history and credits.
824
826 The hwclock command is part of the util-linux package and is
827 available from https://www.kernel.org/pub/linux/utils/util-
828 linux/.
829util-linux July 2017 HWCLOCK(8)