adjtimex - display or set the kernel time variables
This program gives you raw access to the kernel time variables. Anyone
may print out the time variables, but only the superuser may change
Your computer has two clocks - the "hardware clock" that runs all the
time, and the system clock that runs only while the computer is on.
Normally, "hwclock --hctosys" should be run at startup to initialize
the system clock. The system clock has much better precision
(approximately 1 usec), but the hardware clock probably has better
long-term stability. There are three basic strategies for managing
For a machine connected to the Internet, or equipped with a precision
oscillator or radio clock, the best way is to regulate the system clock
with ntpd(8). The kernel will automatically update the hardware clock
every eleven minutes.
In addition, hwclock(8) can be used to approximately correct for a
constant drift in the hardware clock. In this case, "hwclock --adjust"
is run occasionally. hwclock notes how long it has been since the last
adjustment, and nudges the hardware clock forward or back by the
appropriate amount. The user needs to set the time with "hwclock
--set" several times over the course of a few days so hwclock can
estimate the drift rate. During that time, ntpd should not be running,
or else hwclock will conclude the hardware clock does not drift at all.
After you have run "hwclock --set" for the last time, it’s okay to
start ntpd. Then, "hwclock --systohc" should be run when the machine
is shut down. (To see why, suppose the machine runs for a week with
ntpd, is shut down for a day, is restarted, and "hwclock --adjust" is
run by a startup script. It should only correct for one day’s worth of
drift. However, it has no way of knowing that ntpd has been adjusting
the hardware clock, so it bases its adjustment on the last time hwclock
For a standalone or intermittently connected machine, where it’s not
possible to run ntpd, you may use adjtimex instead to correct the
system clock for systematic drift.
There are several ways to estimate the drift rate. If your computer
can be connected to the net, you might run ntpd for at least several
hours and run "adjtimex --print" to learn what values of tick and freq
it settled on. Alternately, you could estimate values using as a
reference the CMOS clock (see the --compare and --adjust switches),
another host (see --host and --review), or some other source of time
(see --watch and --review). You could then add a line to rc.local
invoking adjtimex, or configure /etc/init.d/adjtimex or
/etc/default/adjtimex, to set those parameters each time you reboot.
Options may be introduced by either - or --, and unique abbreviations
may be used.
Here is a summary of the options, grouped by type. Explanations
Get/Set Kernel Time Parameters
-p --print -t --tick val -f newfreq --frequency newfreq -o val
--offset val -s adjustment --singleshot adjustment -S status
--status status -m val -R --reset --maxerror val -e val
--esterror val -T val --timeconstant val -a[count]
Estimate Systematic Drifts
-c[count] --compare[=count] -i tim --interval tim -l file
--logfile file -h timeserver --host timeserver -w --watch
-r[file] --review[=file] -u --utc -d --directisa -n
--help -v --version -V --verbose
Print the current values of the kernel time variables. NOTE:
The time is "raw", and may be off by up to one timer tick (10
msec). "status" gives the value of the time_status variable in
the kernel. For Linux 1.0 and 1.2 kernels, the value is as
0 clock is synchronized (so the kernel should
periodically set the CMOS clock to match the
1 inserting a leap second at midnight
2 deleting a leap second at midnight
3 leap second in progress
4 leap second has occurred
5 clock not externally synchronized (so the
kernel should leave the CMOS clock alone)
For Linux kernels 2.0 through 2.6, the value is a sum of these:
1 PLL updates enabled
2 PPS freq discipline enabled
4 PPS time discipline enabled
8 frequency-lock mode enabled
16 inserting leap second
32 deleting leap second
64 clock unsynchronized
128 holding frequency
256 PPS signal present
512 PPS signal jitter exceeded
1024 PPS signal wander exceeded
2048 PPS signal calibration error
4096 clock hardware fault
-t val, --tick val
Set the number of microseconds that should be added to the
system time for each kernel tick interrupt. For a kernel with
USER_HZ=100, there are supposed to be 100 ticks per second, so
val should be close to 10000. Increasing val by 1 speeds up the
system clock by about 100 ppm, or 8.64 sec/day. tick must be in
the range 900000/USER_HZ...1100000/USER_HZ. If val is rejected
by the kernel, adjtimex will determine the acceptable range
through trial and error and print it. (After completing the
search, it will restore the original value.)
-f newfreq, --frequency newfreq
Set the system clock frequency offset to newfreq. newfreq can
be negative or positive, and gives a much finer adjustment than
the --tick switch. When USER_HZ=100, the value is scaled such
that newfreq = 65536 speeds up the system clock by about 1 ppm,
or .0864 sec/day. Thus, all of these are about the same:
--tick 9995 --frequency 32768000
--tick 10000 --frequency 6553600
--tick 10001 --frequency 0
--tick 10002 --frequency -6553600
--tick 10005 --frequency -32768000
To see the acceptable range for newfreq, use --print and look at
"tolerance", or try an illegal value (e.g. --tick 0).
-s adj, --singleshot adj
Slew the system clock by adj usec. (Its rate is changed
temporarily by about 1 part in 2000.)
-o adj, --offset adj
Add a time offset of adj usec. The kernel code adjusts the time
gradually by adj, notes how long it has been since the last time
offset, and then adjusts the frequency offset to correct for the
apparent drift. adj must be in the range -512000...512000.
-S status, --status status
Set kernel system clock status register to value status. Look
here above at the --print switch section for the meaning of
status, depending on your kernel.
Reset clock status after setting a clock parameter. For early
Linux kernels, using the adjtimex(2) system call to set any time
parameter the kernel think the clock is synchronized with an
external time source, so it sets the kernel variable time_status
to TIME_OK. Thereafter, at 11 minute intervals, it will adjust
the CMOS clock to match. We prevent this "eleven minute mode"
by setting the clock, because that has the side effect of
resetting time_status to TIME_BAD. We try not to actually
change the clock setting. Kernel versions 2.0.40 and later
apparently don’t need this. If your kernel does require it, use
this option with: -t -T -t -e -m -f -s -o -c -r.
-m val, --maxerror val
Set maximum error (usec).
-e val, --esterror val
Set estimated error (usec). The maximum and estimated error are
not used by the kernel. They are merely made available to user
processes via the adjtimex(2) system call.
-T val, --timeconstant val
Set phase locked loop (PLL) time constant. val determines the
bandwidth or "stiffness" of the PLL. The effective PLL time
constant will be a multiple of (2^val). For room-temperature
quartz oscillators, David Mills recommends the value 2, which
corresponds to a PLL time constant of about 900 sec and a
maximum update interval of about 64 sec. The maximum update
interval scales directly with the time constant, so that at the
maximum time constant of 6, the update interval can be as large
as 1024 sec.
Values of val between zero and 2 give quick convergence; values
between 2 and 6 can be used to reduce network load, but at a
modest cost in accuracy.
Periodically compare the system clock with the CMOS clock.
After the first two calls, print values for tick and frequency
offset that would bring the system clock into approximate
agreement with the CMOS clock. CMOS clock readings are adjusted
for systematic drift using using the correction in /etc/adjtime
— see hwclock(8). The interval between comparisons is 10
seconds, unless changed by the --interval switch. The optional
argument is the number of comparisons. (If the argument is
supplied, the "=" is required.) If the CMOS clock and the
system clock differ by more than six minutes, adjtimex will try
shifting the time from the CMOS clock by some multiple of one
hour, up to plus or minus 13 hours in all. This should allow
correct operation, including logging, if the --utc switch was
used when the CMOS clock is set to local time (or vice-versa),
or if summer time has started or stopped since the CMOS clock
was last set.
By itself, same as --compare, except the recommended values are
actually installed after every third comparison. With --review,
the tick and frequency are set to the least-squares estimates.
(In the latter case, any count value is ignored.)
Override the sanity check that prevents changing the clock rate
by more than 500 ppm.
-i tim, --interval tim
Set the interval in seconds between clock comparisons for the
--compare and --adjust options.
The CMOS clock is set to UTC (universal time) rather than local
To read the CMOS clock accurately, adjtimex usually accesses the
clock via the /dev/rtc device driver of the kernel, and makes
use of its CMOS update-ended interrupt to detect the beginning
of seconds. It will also try /dev/rtc0 (for udev), /dev/misc/rtc
(for the obsolete devfs) and possibly others. When the /dev/rtc
driver is absent, or when the interrupt is not available,
adjtimex can sometimes automatically fallback to a direct access
method. This method detects the start of seconds by polling the
update-in-progress (UIP) flag of the CMOS clock. You can force
this direct access to the CMOS chip with the --directisa switch.
Note that the /dev/rtc interrupt method is more accurate, less
sensible to perturbations due to system load, cleaner, cheaper,
and is generally better than the direct access method. It is
advisable to not use the --directisa switch, unless the CMOS
chip or the motherboard don’t properly provide the necessary
Force immediate use of busywait access method, without first
waiting for the interrupt timeout.
Save the current values of the system and CMOS clocks, and
optionally a reference time, to file (default
/var/log/clocks.log). The reference time is taken from a
network timeserver (see the --host switch) or supplied by the
user (see the --watch switch).
-h timeserver, --host timeserver
Use ntpdate to query the given timeserver for the current time.
This will fail if timeserver is not running a Network Time
Protocol (NTP) server, or if that server is not synchronized.
Ask for a keypress when the user knows the time, then ask what
that time was, and its approximate accuracy. Implies --log.
Review the clock log file (default /var/log/clocks.log) and
estimate, if possible, the rates of the CMOS and system clocks.
Calculate least-squares rates using all suitable log entries.
Suggest corrections to adjust for systematic drift. With
--adjust, the frequency and tick are set to the suggested
values. (The CMOS clock correction is not changed.)
--help Print the program options.
Print the program version.
If your system clock gained 8 seconds in 24 hours, you could set the
tick to 9999, and then it would lose 0.64 seconds a day (that is, 1
tick unit = 8.64 seconds per day). To correct the rest of the error,
you could set the frequency offset to (2^16)*0.64/.0864 = 485452.
Thus, putting the following in rc.local would approximately correct the
adjtimex --tick 9999 --freq 485452
adjtimex adjusts only the system clock — the one that runs while the
computer is powered up. To set or regulate the CMOS clock, see
Steven S. Dick <ssd at nevets.oau.org>, Jim Van Zandt <jrv at
date(1L), gettimeofday(2), settimeofday(2), hwclock(8), ntpdate(8),
March 11, 2009