1HWCLOCK(8) System Manager's Manual HWCLOCK(8)
2
6 hwclock - query and set the hardware clock (RTC)
7
9 hwclock [functions] [options]
10
13 hwclock is a tool for accessing the Hardware Clock. You can display
14 the current time, set the Hardware Clock to a specified time, set the
15 Hardware Clock to the System Time, and set the System Time from the
16 Hardware Clock.
17 You can also run hwclock periodically to insert or remove time from the
19 Hardware Clock to compensate for systematic drift (where the clock con‐
20 sistently gains or loses time at a certain rate if left to run).
21
24 You need exactly one of the following options to tell hwclock what
25 function to perform:
26 -r, --show
28 Read the Hardware Clock and print the time on Standard Output.
29 The time shown is always in local time, even if you keep your
30 Hardware Clock in Coordinated Universal Time. See the --utc
31 option.
32 --set Set the Hardware Clock to the time given by the --date option.
35 -s, --hctosys
37 Set the System Time from the Hardware Clock.
38 Also set the kernel's timezone value to the local timezone as
40 indicated by the TZ environment variable and/or /usr/share/zone‐
41 info, as tzset(3) would interpret them. The obsolete tz_dsttime
42 field of the kernel's timezone value is set to DST_NONE. (For
43 details on what this field used to mean, see settimeofday(2).)
44 This is a good option to use in one of the system startup
46 scripts.
47 -w, --systohc
49 Set the Hardware Clock to the current System Time.
50 --systz
52 Reset the System Time based on the current timezone.
53 Also set the kernel's timezone value to the local timezone as
55 indicated by the TZ environment variable and/or /usr/share/zone‐
56 info, as tzset(3) would interpret them. The obsolete tz_dsttime
57 field of the kernel's timezone value is set to DST_NONE. (For
58 details on what this field used to mean, see settimeofday(2).)
59 This is an alternate option to --hctosys that does not read the
61 hardware clock, and may be used in system startup scripts for
62 recent 2.6 kernels where you know the System Time contains the
63 Hardware Clock time.
64 --adjust
66 Add or subtract time from the Hardware Clock to account for sys‐
67 tematic drift since the last time the clock was set or adjusted.
68 See discussion below.
69 --getepoch
71 Print the kernel's Hardware Clock epoch value to standard out‐
72 put. This is the number of years into AD to which a zero year
73 value in the Hardware Clock refers. For example, if you are
74 using the convention that the year counter in your Hardware
75 Clock contains the number of full years since 1952, then the
76 kernel's Hardware Counter epoch value must be 1952.
77 This epoch value is used whenever hwclock reads or sets the
79 Hardware Clock.
80 --setepoch
82 Set the kernel's Hardware Clock epoch value to the value speci‐
83 fied by the --epoch option. See the --getepoch option for
84 details.
85 -v, --version
87 Print the version of hwclock on Standard Output.
88 --date=date_string
90 You need this option if you specify the --set option. Other‐
91 wise, it is ignored. This specifies the time to which to set
92 the Hardware Clock. The value of this option is an argument to
93 the date(1) program. For example,
94 hwclock --set --date="9/22/96 16:45:05"
96 The argument is in local time, even if you keep your Hardware
98 Clock in Coordinated Universal time. See the --utc option.
99 --epoch=year
102 Specifies the year which is the beginning of the Hardware
103 Clock's epoch. I.e. the number of years into AD to which a zero
104 value in the Hardware Clock's year counter refers. It is used
105 together with the --setepoch option to set the kernel's idea of
106 the epoch of the Hardware Clock, or otherwise to specify the
107 epoch for use with direct ISA access.
108 For example, on a Digital Unix machine:
110 hwclock --setepoch --epoch=1952
112 --predict
115 Predict what the RTC will read at time given by the --date
116 option based on the adjtime file. This is useful for example if
117 you need to set an RTC wakeup time to distant future and want to
118 account for the RTC drift.
119
122 The following options apply to most functions.
123 -u, --utc
125 --localtime
127 Indicates that the Hardware Clock is kept in Coordinated Univer‐
128 sal Time or local time, respectively. It is your choice whether
129 to keep your clock in UTC or local time, but nothing in the
130 clock tells which you've chosen. So this option is how you give
131 that information to hwclock.
132 If you specify the wrong one of these options (or specify nei‐
134 ther and take a wrong default), both setting and querying of the
135 Hardware Clock will be messed up.
136 If you specify neither --utc nor --localtime , the default is
138 whichever was specified the last time hwclock was used to set
139 the clock (i.e. hwclock was successfully run with the --set,
140 --systohc, or --adjust options), as recorded in the adjtime
141 file. If the adjtime file doesn't exist, the default is local
142 time.
143 --noadjfile
146 disables the facilities provided by /etc/adjtime. hwclock will
147 not read nor write to that file with this option. Either --utc
148 or --localtime must be specified when using this option.
149 --adjfile=filename
152 overrides the default /etc/adjtime.
153 -f, --rtc=filename
156 overrides the default /dev file name, which is /dev/rtc on many
157 platforms but may be /dev/rtc0, /dev/rtc1, and so on.
158 --directisa
161 is meaningful only on an ISA machine or an Alpha (which imple‐
162 ments enough of ISA to be, roughly speaking, an ISA machine for
163 hwclock's purposes). For other machines, it has no effect.
164 This option tells hwclock to use explicit I/O instructions to
165 access the Hardware Clock. Without this option, hwclock will
166 try to use the /dev/rtc device (which it assumes to be driven by
167 the rtc device driver). If it is unable to open the device (for
168 read), it will use the explicit I/O instructions anyway.
169 The rtc device driver was new in Linux Release 2.
171 --badyear
173 Indicates that the Hardware Clock is incapable of storing years
174 outside the range 1994-1999. There is a problem in some BIOSes
175 (almost all Award BIOSes made between 4/26/94 and 5/31/95)
176 wherein they are unable to deal with years after 1999. If one
177 attempts to set the year-of-century value to something less than
178 94 (or 95 in some cases), the value that actually gets set is 94
179 (or 95). Thus, if you have one of these machines, hwclock can‐
180 not set the year after 1999 and cannot use the value of the
181 clock as the true time in the normal way.
182 To compensate for this (without your getting a BIOS update,
184 which would definitely be preferable), always use --badyear if
185 you have one of these machines. When hwclock knows it's working
186 with a brain-damaged clock, it ignores the year part of the
187 Hardware Clock value and instead tries to guess the year based
188 on the last calibrated date in the adjtime file, by assuming
189 that that date is within the past year. For this to work, you
190 had better do a hwclock --set or hwclock --systohc at least once
191 a year!
192 Though hwclock ignores the year value when it reads the Hardware
194 Clock, it sets the year value when it sets the clock. It sets
195 it to 1995, 1996, 1997, or 1998, whichever one has the same
196 position in the leap year cycle as the true year. That way, the
197 Hardware Clock inserts leap days where they belong. Again, if
198 you let the Hardware Clock run for more than a year without set‐
199 ting it, this scheme could be defeated and you could end up los‐
200 ing a day.
201 hwclock warns you that you probably need --badyear whenever it
203 finds your Hardware Clock set to 1994 or 1995.
204 --srm This option is equivalent to --epoch=1900 and is used to specify
207 the most common epoch on Alphas with SRM console.
208 --arc This option is equivalent to --epoch=1980 and is used to specify
210 the most common epoch on Alphas with ARC console (but Ruffians
211 have epoch 1900).
212 --jensen
214 --funky-toy
216 These two options specify what kind of Alpha machine you have.
217 They are invalid if you don't have an Alpha and are usually
218 unnecessary if you do, because hwclock should be able to deter‐
219 mine by itself what it's running on, at least when /proc is
220 mounted. (If you find you need one of these options to make
221 hwclock work, contact the maintainer to see if the program can
222 be improved to detect your system automatically. Output of
223 `hwclock --debug' and `cat /proc/cpuinfo' may be of interest.)
224 --jensen means you are running on a Jensen model.
226 --funky-toy means that on your machine, one has to use the UF
228 bit instead of the UIP bit in the Hardware Clock to detect a
229 time transition. "Toy" in the option name refers to the Time Of
230 Year facility of the machine.
231 --test Do everything except actually updating the Hardware Clock or
235 anything else. This is useful, especially in conjunction with
236 --debug, in learning about hwclock.
237 --debug
239 Display a lot of information about what hwclock is doing inter‐
240 nally. Some of its function is complex and this output can help
241 you understand how the program works.
242
247 There are two main clocks in a Linux system:
248 The Hardware Clock: This is a clock that runs independently of any con‐
250 trol program running in the CPU and even when the machine is powered
251 off.
252 On an ISA system, this clock is specified as part of the ISA standard.
254 The control program can read or set this clock to a whole second, but
255 the control program can also detect the edges of the 1 second clock
256 ticks, so the clock actually has virtually infinite precision.
257 This clock is commonly called the hardware clock, the real time clock,
259 the RTC, the BIOS clock, and the CMOS clock. Hardware Clock, in its
260 capitalized form, was coined for use by hwclock because all of the
261 other names are inappropriate to the point of being misleading.
262 So for example, some non-ISA systems have a few real time clocks with
264 only one of them having its own power domain. A very low power exter‐
265 nal I2C or SPI clock chip might be used with a backup battery as the
266 hardware clock to initialize a more functional integrated real-time
267 clock which is used for most other purposes.
268 The System Time: This is the time kept by a clock inside the Linux ker‐
270 nel and driven by a timer interrupt. (On an ISA machine, the timer
271 interrupt is part of the ISA standard). It has meaning only while
272 Linux is running on the machine. The System Time is the number of sec‐
273 onds since 00:00:00 January 1, 1970 UTC (or more succinctly, the number
274 of seconds since 1969). The System Time is not an integer, though. It
275 has virtually infinite precision.
276 The System Time is the time that matters. The Hardware Clock's basic
278 purpose in a Linux system is to keep time when Linux is not running.
279 You initialize the System Time to the time from the Hardware Clock when
280 Linux starts up, and then never use the Hardware Clock again. Note
281 that in DOS, for which ISA was designed, the Hardware Clock is the only
282 real time clock.
283 It is important that the System Time not have any discontinuities such
285 as would happen if you used the date(1L) program to set it while the
286 system is running. You can, however, do whatever you want to the Hard‐
287 ware Clock while the system is running, and the next time Linux starts
288 up, it will do so with the adjusted time from the Hardware Clock. You
289 can also use the program adjtimex(8) to smoothly adjust the System Time
290 while the system runs.
291 A Linux kernel maintains a concept of a local timezone for the system.
293 But don't be misled -- almost nobody cares what timezone the kernel
294 thinks it is in. Instead, programs that care about the timezone (per‐
295 haps because they want to display a local time for you) almost always
296 use a more traditional method of determining the timezone: They use the
297 TZ environment variable and/or the /usr/share/zoneinfo directory, as
298 explained in the man page for tzset(3). However, some programs and
299 fringe parts of the Linux kernel such as filesystems use the kernel
300 timezone value. An example is the vfat filesystem. If the kernel
301 timezone value is wrong, the vfat filesystem will report and set the
302 wrong timestamps on files.
303 hwclock sets the kernel timezone to the value indicated by TZ and/or
305 /usr/share/zoneinfo when you set the System Time using the --hctosys
306 option.
307 The timezone value actually consists of two parts: 1) a field tz_min‐
309 uteswest indicating how many minutes local time (not adjusted for DST)
310 lags behind UTC, and 2) a field tz_dsttime indicating the type of Day‐
311 light Savings Time (DST) convention that is in effect in the locality
312 at the present time. This second field is not used under Linux and is
313 always zero. (See also settimeofday(2).)
314
317 hwclock uses many different ways to get and set Hardware Clock values.
318 The most normal way is to do I/O to the device special file /dev/rtc,
319 which is presumed to be driven by the rtc device driver. However, this
320 method is not always available. For one thing, the rtc driver is a
321 relatively recent addition to Linux. Older systems don't have it.
322 Also, though there are versions of the rtc driver that work on DEC
323 Alphas, there appear to be plenty of Alphas on which the rtc driver
324 does not work (a common symptom is hwclock hanging). Moreover, recent
325 Linux systems have more generic support for RTCs, even systems that
326 have more than one, so you might need to override the default by speci‐
327 fying /dev/rtc0 or /dev/rtc1 instead.
328 On older systems, the method of accessing the Hardware Clock depends on
330 the system hardware.
331 On an ISA system, hwclock can directly access the "CMOS memory" regis‐
333 ters that constitute the clock, by doing I/O to Ports 0x70 and 0x71.
334 It does this with actual I/O instructions and consequently can only do
335 it if running with superuser effective userid. (In the case of a
336 Jensen Alpha, there is no way for hwclock to execute those I/O instruc‐
337 tions, and so it uses instead the /dev/port device special file, which
338 provides almost as low-level an interface to the I/O subsystem).
339 This is a really poor method of accessing the clock, for all the rea‐
341 sons that user space programs are generally not supposed to do direct
342 I/O and disable interrupts. Hwclock provides it because it is the only
343 method available on ISA and Alpha systems which don't have working rtc
344 device drivers available.
345 On an m68k system, hwclock can access the clock via the console driver,
348 via the device special file /dev/tty1.
349 hwclock tries to use /dev/rtc. If it is compiled for a kernel that
351 doesn't have that function or it is unable to open /dev/rtc (or the
352 alternative special file you've defined on the command line) hwclock
353 will fall back to another method, if available. On an ISA or Alpha
354 machine, you can force hwclock to use the direct manipulation of the
355 CMOS registers without even trying /dev/rtc by specifying the --direc‐
356 tisa option.
357
361 The Hardware Clock is usually not very accurate. However, much of its
362 inaccuracy is completely predictable - it gains or loses the same
363 amount of time every day. This is called systematic drift. hwclock's
364 "adjust" function lets you make systematic corrections to correct the
365 systematic drift.
366 It works like this: hwclock keeps a file, /etc/adjtime, that keeps some
368 historical information. This is called the adjtime file.
369 Suppose you start with no adjtime file. You issue a hwclock --set com‐
371 mand to set the Hardware Clock to the true current time. Hwclock cre‐
372 ates the adjtime file and records in it the current time as the last
373 time the clock was calibrated. 5 days later, the clock has gained 10
374 seconds, so you issue another hwclock --set command to set it back 10
375 seconds. Hwclock updates the adjtime file to show the current time as
376 the last time the clock was calibrated, and records 2 seconds per day
377 as the systematic drift rate. 24 hours go by, and then you issue a
378 hwclock --adjust command. Hwclock consults the adjtime file and sees
379 that the clock gains 2 seconds per day when left alone and that it has
380 been left alone for exactly one day. So it subtracts 2 seconds from
381 the Hardware Clock. It then records the current time as the last time
382 the clock was adjusted. Another 24 hours goes by and you issue another
383 hwclock --adjust. Hwclock does the same thing: subtracts 2 seconds and
384 updates the adjtime file with the current time as the last time the
385 clock was adjusted.
386 Every time you calibrate (set) the clock (using --set or --systohc),
388 hwclock recalculates the systematic drift rate based on how long it has
389 been since the last calibration, how long it has been since the last
390 adjustment, what drift rate was assumed in any intervening adjustments,
391 and the amount by which the clock is presently off.
392 A small amount of error creeps in any time hwclock sets the clock, so
394 it refrains from making an adjustment that would be less than 1 second.
395 Later on, when you request an adjustment again, the accumulated drift
396 will be more than a second and hwclock will do the adjustment then.
397 It is good to do a hwclock --adjust just before the hwclock --hctosys
399 at system startup time, and maybe periodically while the system is run‐
400 ning via cron.
401 The adjtime file, while named for its historical purpose of controlling
403 adjustments only, actually contains other information for use by
404 hwclock in remembering information from one invocation to the next.
405 The format of the adjtime file is, in ASCII:
407 Line 1: 3 numbers, separated by blanks: 1) systematic drift rate in
409 seconds per day, floating point decimal; 2) Resulting number of seconds
410 since 1969 UTC of most recent adjustment or calibration, decimal inte‐
411 ger; 3) zero (for compatibility with clock(8)) as a decimal integer.
412 Line 2: 1 number: Resulting number of seconds since 1969 UTC of most
414 recent calibration. Zero if there has been no calibration yet or it is
415 known that any previous calibration is moot (for example, because the
416 Hardware Clock has been found, since that calibration, not to contain a
417 valid time). This is a decimal integer.
418 Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set to
420 Coordinated Universal Time or local time. You can always override this
421 value with options on the hwclock command line.
422 You can use an adjtime file that was previously used with the clock(8)
424 program with hwclock.
425
429 You should be aware of another way that the Hardware Clock is kept syn‐
430 chronized in some systems. The Linux kernel has a mode wherein it
431 copies the System Time to the Hardware Clock every 11 minutes. This is
432 a good mode to use when you are using something sophisticated like ntp
433 to keep your System Time synchronized. (ntp is a way to keep your Sys‐
434 tem Time synchronized either to a time server somewhere on the network
435 or to a radio clock hooked up to your system. See RFC 1305).
436 This mode (we'll call it "11 minute mode") is off until something turns
438 it on. The ntp daemon xntpd is one thing that turns it on. You can
439 turn it off by running anything, including hwclock --hctosys, that sets
440 the System Time the old fashioned way.
441 To see if it is on or off, use the command adjtimex --print and look at
443 the value of "status". If the "64" bit of this number (expressed in
444 binary) equal to 0, 11 minute mode is on. Otherwise, it is off.
445 If your system runs with 11 minute mode on, don't use hwclock --adjust
447 or hwclock --hctosys. You'll just make a mess. It is acceptable to
448 use a hwclock --hctosys at startup time to get a reasonable System Time
449 until your system is able to set the System Time from the external
450 source and start 11 minute mode.
451
455 There is some sort of standard that defines CMOS memory Byte 50 on an
456 ISA machine as an indicator of what century it is. hwclock does not
457 use or set that byte because there are some machines that don't define
458 the byte that way, and it really isn't necessary anyway, since the
459 year-of-century does a good job of implying which century it is.
460 If you have a bona fide use for a CMOS century byte, contact the
462 hwclock maintainer; an option may be appropriate.
463 Note that this section is only relevant when you are using the "direct
465 ISA" method of accessing the Hardware Clock. ACPI provides a standard
466 way to access century values, when they are supported by the hardware.
467
470 TZ
471
474 /etc/adjtime /usr/share/zoneinfo/ (/usr/lib/zoneinfo on old systems)
475 /dev/rtc /dev/rtc0 /dev/port /dev/tty1 /proc/cpuinfo
476
479 adjtimex(8), date(1), gettimeofday(2), settimeofday(2), crontab(1),
480 tzset(3)
481
484 Written by Bryan Henderson, September 1996 (bryanh@giraffe-data.com),
485 based on work done on the clock program by Charles Hedrick, Rob Hooft,
486 and Harald Koenig. See the source code for complete history and cred‐
487 its.
488
491 The hwclock command is part of the util-linux-ng package and is avail‐
492 able from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.
493 06 August 2008 HWCLOCK(8)