1HWCLOCK(8) System Manager's Manual HWCLOCK(8)
2
6 hwclock - query and set the hardware clock (RTC)
7
9 hwclock -r or hwclock --show
10 hwclock -w or hwclock --systohc
11 hwclock -s or hwclock --hctosys
12 hwclock -a or hwclock --adjust
13 hwclock -v or hwclock --version
14 hwclock --set --date=newdate
15 hwclock --getepoch
16 hwclock --setepoch --epoch=year
17 other options:
19 [-u|--utc] --localtime --noadjfile --directisa --test [-D|--debug]
21 and arcane options for DEC Alpha:
23 [-A|--arc] [-J|--jensen] [-S|--srm] [-F|--funky-toy]
25 Minimum unique abbreviations of all options are acceptable.
27 Also, -h asks for a help message.
29
32 hwclock is a tool for accessing the Hardware Clock. You can display
33 the current time, set the Hardware Clock to a specified time, set the
34 Hardware Clock to the System Time, and set the System Time from the
35 Hardware Clock.
36 You can also run hwclock periodically to insert or remove time from the
38 Hardware Clock to compensate for systematic drift (where the clock con‐
39 sistently gains or loses time at a certain rate if left to run).
40
43 You need exactly one of the following options to tell hwclock what
44 function to perform:
45 --show Read the Hardware Clock and print the time on Standard Output.
47 The time shown is always in local time, even if you keep your
48 Hardware Clock in Coordinated Universal Time. See the --utc
49 option.
50 --set Set the Hardware Clock to the time given by the --date option.
53 --hctosys
55 Set the System Time from the Hardware Clock.
56 Also set the kernel's timezone value to the local timezone as
58 indicated by the TZ environment variable and/or /usr/share/zone‐
59 info, as tzset(3) would interpret them. The obsolete tz_dsttime
60 field of the kernel's timezone value is set to DST_NONE. (For
61 details on what this field used to mean, see settimeofday(2).)
62 This is a good option to use in one of the system startup
64 scripts.
65 --systohc
67 Set the Hardware Clock to the current System Time.
68 --adjust
70 Add or subtract time from the Hardware Clock to account for sys‐
71 tematic drift since the last time the clock was set or adjusted.
72 See discussion below.
73 --getepoch
75 Print the kernel's Hardware Clock epoch value to standard out‐
76 put. This is the number of years into AD to which a zero year
77 value in the Hardware Clock refers. For example, if you are
78 using the convention that the year counter in your Hardware
79 Clock contains the number of full years since 1952, then the
80 kernel's Hardware Counter epoch value must be 1952.
81 This epoch value is used whenever hwclock reads or sets the
83 Hardware Clock.
84 --setepoch
86 Set the kernel's Hardware Clock epoch value to the value speci‐
87 fied by the --epoch option. See the --getepoch option for
88 details.
89 --version
91 Print the version of hwclock on Standard Output.
92 --date=date_string
94 You need this option if you specify the --set option. Other‐
95 wise, it is ignored. This specifies the time to which to set
96 the Hardware Clock. The value of this option is an argument to
97 the date(1) program. For example,
98 hwclock --set --date="9/22/96 16:45:05"
100 The argument is in local time, even if you keep your Hardware
102 Clock in Coordinated Universal time. See the --utc option.
103 --epoch=year
106 Specifies the year which is the beginning of the Hardware
107 Clock's epoch. I.e. the number of years into AD to which a zero
108 value in the Hardware Clock's year counter refers. It is used
109 together with the --setepoch option to set the kernel's idea of
110 the epoch of the Hardware Clock, or otherwise to specify the
111 epoch for use with direct ISA access.
112 For example, on a Digital Unix machine:
114 hwclock --setepoch --epoch=1952
116 The following options apply to most functions.
119 --utc
121 --localtime
123 Indicates that the Hardware Clock is kept in Coordinated Univer‐
124 sal Time or local time, respectively. It is your choice whether
125 to keep your clock in UTC or local time, but nothing in the
126 clock tells which you've chosen. So this option is how you give
127 that information to hwclock.
128 If you specify the wrong one of these options (or specify nei‐
130 ther and take a wrong default), both setting and querying of the
131 Hardware Clock will be messed up.
132 If you specify neither --utc nor --localtime , the default is
134 whichever was specified the last time hwclock was used to set
135 the clock (i.e. hwclock was successfully run with the --set ,
136 --systohc , or --adjust options), as recorded in the adjtime
137 file. If the adjtime file doesn't exist, the default is local
138 time.
139 --noadjfile
142 disables the facilities provided by /etc/adjtime. hwclock will
143 not read nor write to that file with this option. Either --utc
144 or --localtime must be specified when using this option.
145 --directisa
148 is meaningful only on an ISA machine or an Alpha (which imple‐
149 ments enough of ISA to be, roughly speaking, an ISA machine for
150 hwclock's purposes). For other machines, it has no effect.
151 This option tells hwclock to use explicit I/O instructions to
152 access the Hardware Clock. Without this option, hwclock will
153 try to use the /dev/rtc device (which it assumes to be driven by
154 the rtc device driver). If it is unable to open the device (for
155 read), it will use the explicit I/O instructions anyway.
156 The rtc device driver was new in Linux Release 2.
158 --badyear
160 Indicates that the Hardware Clock is incapable of storing years
161 outside the range 1994-1999. There is a problem in some BIOSes
162 (almost all Award BIOSes made between 4/26/94 and 5/31/95)
163 wherein they are unable to deal with years after 1999. If one
164 attempts to set the year-of-century value to something less than
165 94 (or 95 in some cases), the value that actually gets set is 94
166 (or 95). Thus, if you have one of these machines, hwclock can‐
167 not set the year after 1999 and cannot use the value of the
168 clock as the true time in the normal way.
169 To compensate for this (without your getting a BIOS update,
171 which would definitely be preferable), always use --badyear if
172 you have one of these machines. When hwclock knows it's working
173 with a brain-damaged clock, it ignores the year part of the
174 Hardware Clock value and instead tries to guess the year based
175 on the last calibrated date in the adjtime file, by assuming
176 that that date is within the past year. For this to work, you
177 had better do a hwclock --set or hwclock --systohc at least once
178 a year!
179 Though hwclock ignores the year value when it reads the Hardware
181 Clock, it sets the year value when it sets the clock. It sets
182 it to 1995, 1996, 1997, or 1998, whichever one has the same
183 position in the leap year cycle as the true year. That way, the
184 Hardware Clock inserts leap days where they belong. Again, if
185 you let the Hardware Clock run for more than a year without set‐
186 ting it, this scheme could be defeated and you could end up los‐
187 ing a day.
188 hwclock warns you that you probably need --badyear whenever it
190 finds your Hardware Clock set to 1994 or 1995.
191 --srm This option is equivalent to --epoch=1900 and is used to specify
194 the most common epoch on Alphas with SRM console.
195 --arc This option is equivalent to --epoch=1980 and is used to specify
197 the most common epoch on Alphas with ARC console (but Ruffians
198 have epoch 1900).
199 --jensen
201 --funky-toy
203 These two options specify what kind of Alpha machine you have.
204 They are invalid if you don't have an Alpha and are usually
205 unnecessary if you do, because hwclock should be able to deter‐
206 mine by itself what it's running on, at least when /proc is
207 mounted. (If you find you need one of these options to make
208 hwclock work, contact the maintainer to see if the program can
209 be improved to detect your system automatically. Output of
210 `hwclock --debug' and `cat /proc/cpuinfo' may be of interest.)
211 --jensen means you are running on a Jensen model.
213 --funky-toy means that on your machine, one has to use the UF
215 bit instead of the UIP bit in the Hardware Clock to detect a
216 time transition. "Toy" in the option name refers to the Time Of
217 Year facility of the machine.
218 --test Do everything except actually updating the Hardware Clock or
222 anything else. This is useful, especially in conjunction with
223 --debug, in learning about hwclock.
224 --debug
226 Display a lot of information about what hwclock is doing inter‐
227 nally. Some of its function is complex and this output can help
228 you understand how the program works.
229
234 There are two main clocks in a Linux system:
235 The Hardware Clock: This is a clock that runs independently of any con‐
237 trol program running in the CPU and even when the machine is powered
238 off.
239 On an ISA system, this clock is specified as part of the ISA standard.
241 The control program can read or set this clock to a whole second, but
242 the control program can also detect the edges of the 1 second clock
243 ticks, so the clock actually has virtually infinite precision.
244 This clock is commonly called the hardware clock, the real time clock,
246 the RTC, the BIOS clock, and the CMOS clock. Hardware Clock, in its
247 capitalized form, was coined for use by hwclock because all of the
248 other names are inappropriate to the point of being misleading.
249 The System Time: This is the time kept by a clock inside the Linux ker‐
251 nel and driven by a timer interrupt. (On an ISA machine, the timer
252 interrupt is part of the ISA standard). It has meaning only while
253 Linux is running on the machine. The System Time is the number of sec‐
254 onds since 00:00:00 January 1, 1970 UTC (or more succinctly, the number
255 of seconds since 1969). The System Time is not an integer, though. It
256 has virtually infinite precision.
257 The System Time is the time that matters. The Hardware Clock's basic
259 purpose in a Linux system is to keep time when Linux is not running.
260 You initialize the System Time to the time from the Hardware Clock when
261 Linux starts up, and then never use the Hardware Clock again. Note
262 that in DOS, for which ISA was designed, the Hardware Clock is the only
263 real time clock.
264 It is important that the System Time not have any discontinuities such
266 as would happen if you used the date(1L) program to set it while the
267 system is running. You can, however, do whatever you want to the Hard‐
268 ware Clock while the system is running, and the next time Linux starts
269 up, it will do so with the adjusted time from the Hardware Clock. You
270 can also use the program adjtimex(8) to smoothly adjust the System Time
271 while the system runs.
272 A Linux kernel maintains a concept of a local timezone for the system.
274 But don't be misled -- almost nobody cares what timezone the kernel
275 thinks it is in. Instead, programs that care about the timezone (per‐
276 haps because they want to display a local time for you) almost always
277 use a more traditional method of determining the timezone: They use the
278 TZ environment variable and/or the /usr/share/zoneinfo directory, as
279 explained in the man page for tzset(3). However, some programs and
280 fringe parts of the Linux kernel such as filesystems use the kernel
281 timezone value. An example is the vfat filesystem. If the kernel
282 timezone value is wrong, the vfat filesystem will report and set the
283 wrong timestamps on files.
284 hwclock sets the kernel timezone to the value indicated by TZ and/or
286 /usr/share/zoneinfo when you set the System Time using the --hctosys
287 option.
288 The timezone value actually consists of two parts: 1) a field tz_min‐
290 uteswest indicating how many minutes local time (not adjusted for DST)
291 lags behind UTC, and 2) a field tz_dsttime indicating the type of Day‐
292 light Savings Time (DST) convention that is in effect in the locality
293 at the present time. This second field is not used under Linux and is
294 always zero. (See also settimeofday(2).)
295
298 hwclock Uses many different ways to get and set Hardware Clock values.
299 The most normal way is to do I/O to the device special file /dev/rtc,
300 which is presumed to be driven by the rtc device driver. However, this
301 method is not always available. For one thing, the rtc driver is a
302 relatively recent addition to Linux. Older systems don't have it.
303 Also, though there are versions of the rtc driver that work on DEC
304 Alphas, there appear to be plenty of Alphas on which the rtc driver
305 does not work (a common symptom is hwclock hanging).
306 On older systems, the method of accessing the Hardware Clock depends on
308 the system hardware.
309 On an ISA system, hwclock can directly access the "CMOS memory" regis‐
311 ters that constitute the clock, by doing I/O to Ports 0x70 and 0x71.
312 It does this with actual I/O instructions and consequently can only do
313 it if running with superuser effective userid. (In the case of a
314 Jensen Alpha, there is no way for hwclock to execute those I/O instruc‐
315 tions, and so it uses instead the /dev/port device special file, which
316 provides almost as low-level an interface to the I/O subsystem).
317 This is a really poor method of accessing the clock, for all the rea‐
319 sons that user space programs are generally not supposed to do direct
320 I/O and disable interrupts. Hwclock provides it because it is the only
321 method available on ISA and Alpha systems which don't have working rtc
322 device drivers available.
323 On an m68k system, hwclock can access the clock via the console driver,
326 via the device special file /dev/tty1.
327 hwclock tries to use /dev/rtc. If it is compiled for a kernel that
329 doesn't have that function or it is unable to open /dev/rtc, hwclock
330 will fall back to another method, if available. On an ISA or Alpha
331 machine, you can force hwclock to use the direct manipulation of the
332 CMOS registers without even trying /dev/rtc by specifying the --direc‐
333 tisa option.
334
338 The Hardware Clock is usually not very accurate. However, much of its
339 inaccuracy is completely predictable - it gains or loses the same
340 amount of time every day. This is called systematic drift. hwclock's
341 "adjust" function lets you make systematic corrections to correct the
342 systematic drift.
343 It works like this: hwclock keeps a file, /etc/adjtime, that keeps some
345 historical information. This is called the adjtime file.
346 Suppose you start with no adjtime file. You issue a hwclock --set com‐
348 mand to set the Hardware Clock to the true current time. Hwclock cre‐
349 ates the adjtime file and records in it the current time as the last
350 time the clock was calibrated. 5 days later, the clock has gained 10
351 seconds, so you issue another hwclock --set command to set it back 10
352 seconds. Hwclock updates the adjtime file to show the current time as
353 the last time the clock was calibrated, and records 2 seconds per day
354 as the systematic drift rate. 24 hours go by, and then you issue a
355 hwclock --adjust command. Hwclock consults the adjtime file and sees
356 that the clock gains 2 seconds per day when left alone and that it has
357 been left alone for exactly one day. So it subtracts 2 seconds from
358 the Hardware Clock. It then records the current time as the last time
359 the clock was adjusted. Another 24 hours goes by and you issue another
360 hwclock --adjust. Hwclock does the same thing: subtracts 2 seconds and
361 updates the adjtime file with the current time as the last time the
362 clock was adjusted.
363 Every time you calibrate (set) the clock (using --set or --systohc ),
365 hwclock recalculates the systematic drift rate based on how long it has
366 been since the last calibration, how long it has been since the last
367 adjustment, what drift rate was assumed in any intervening adjustments,
368 and the amount by which the clock is presently off.
369 A small amount of error creeps in any time hwclock sets the clock, so
371 it refrains from making an adjustment that would be less than 1 second.
372 Later on, when you request an adjustment again, the accumulated drift
373 will be more than a second and hwclock will do the adjustment then.
374 It is good to do a hwclock --adjust just before the hwclock --hctosys
376 at system startup time, and maybe periodically while the system is run‐
377 ning via cron.
378 The adjtime file, while named for its historical purpose of controlling
380 adjustments only, actually contains other information for use by
381 hwclock in remembering information from one invocation to the next.
382 The format of the adjtime file is, in ASCII:
384 Line 1: 3 numbers, separated by blanks: 1) systematic drift rate in
386 seconds per day, floating point decimal; 2) Resulting number of seconds
387 since 1969 UTC of most recent adjustment or calibration, decimal inte‐
388 ger; 3) zero (for compatibility with clock(8)) as a decimal integer.
389 Line 2: 1 number: Resulting number of seconds since 1969 UTC of most
391 recent calibration. Zero if there has been no calibration yet or it is
392 known that any previous calibration is moot (for example, because the
393 Hardware Clock has been found, since that calibration, not to contain a
394 valid time). This is a decimal integer.
395 Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set to
397 Coordinated Universal Time or local time. You can always override this
398 value with options on the hwclock command line.
399 You can use an adjtime file that was previously used with the clock(8)
401 program with hwclock.
402
406 You should be aware of another way that the Hardware Clock is kept syn‐
407 chronized in some systems. The Linux kernel has a mode wherein it
408 copies the System Time to the Hardware Clock every 11 minutes. This is
409 a good mode to use when you are using something sophisticated like ntp
410 to keep your System Time synchronized. (ntp is a way to keep your Sys‐
411 tem Time synchronized either to a time server somewhere on the network
412 or to a radio clock hooked up to your system. See RFC 1305).
413 This mode (we'll call it "11 minute mode") is off until something turns
415 it on. The ntp daemon xntpd is one thing that turns it on. You can
416 turn it off by running anything, including hwclock --hctosys, that sets
417 the System Time the old fashioned way.
418 To see if it is on or off, use the command adjtimex --print and look at
420 the value of "status". If the "64" bit of this number (expressed in
421 binary) equal to 0, 11 minute mode is on. Otherwise, it is off.
422 If your system runs with 11 minute mode on, don't use hwclock --adjust
424 or hwclock --hctosys. You'll just make a mess. It is acceptable to
425 use a hwclock --hctosys at startup time to get a reasonable System Time
426 until your system is able to set the System Time from the external
427 source and start 11 minute mode.
428
432 There is some sort of standard that defines CMOS memory Byte 50 on an
433 ISA machine as an indicator of what century it is. hwclock does not
434 use or set that byte because there are some machines that don't define
435 the byte that way, and it really isn't necessary anyway, since the
436 year-of-century does a good job of implying which century it is.
437 If you have a bona fide use for a CMOS century byte, contact the
439 hwclock maintainer; an option may be appropriate.
440 Note that this section is only relevant when you are using the "direct
442 ISA" method of accessing the Hardware Clock.
443
448 TZ
449
452 /etc/adjtime /usr/share/zoneinfo/ (/usr/lib/zoneinfo on old systems)
453 /dev/rtc /dev/port /dev/tty1 /proc/cpuinfo
454
457 adjtimex(8), date(1), gettimeofday(2), settimeofday(2), crontab(1),
458 tzset(3)
459
462 Written by Bryan Henderson, September 1996 (bryanh@giraffe-data.com),
463 based on work done on the clock program by Charles Hedrick, Rob Hooft,
464 and Harald Koenig. See the source code for complete history and cred‐
465 its.
466 02 March 1998 HWCLOCK(8)