adjtimex - display or set the kernel time variables
This program gives you raw access to the kernel time variables. For a machine connected
to the Internet, or equipped with a precision oscillator or radio clock, the best way to
regulate the system clock is with ntpd(8). For a standalone or intermittently connected
machine, you may use adjtimex instead to at least correct for systematic drift.
Anyone may print out the time variables, but only the superuser may change them.
If your computer can be connected to the net, you might run ntpd for at least several
hours and use adjtimex --print to learn what values of tick and freq it settled on.
Alternately, you could estimate values using the CMOS clock as a reference (see the --com-
pare and --adjust switches). You could then add a line to rc.local invoking 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 follow.
Get/Set Kernel Time Parameters
-p --print -t --tick val -f newfreq --frequency newfreq -o val --offset val -s
adjustment --singleshot adjustment -m val --maxerr val -e val --esterror val -T val
--timeconstant val -a[count] --adjust[=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
--help -v --version
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 2.0 kernels, 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. 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 9000...11000 on Intel systems,
or 900...1100 on Alpha systems.
-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. The value is
scaled such that newfreq = 1<<16 speeds up the system clock by about 1 ppm, or
.0864 sec/day. Thus, --tick 10000 --frequency 6553600 is about the same as --tick
10001 --frequency 0. newfreq must be in the range -6553600...6553600, allowing
maximum adjustments of plus or minus 100 ppm.
-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
-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)
-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 (1
<< 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.)
By itself, same as --compare, except the recommended values are actually installed
after every other comparison. With --review, the tick and frequency are set to the
least-squares estimates. (In the latter case, any count value is ignored.)
-i tim, --interval tim
Set the interval in seconds between clock comparisons for the --compare and
The CMOS clock is set to UTC (universal time) rather than local time.
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
-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. Implies --log.
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.)
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 (1<<16)*0.64/.0864 =
485452. Thus, putting the following in rc.local would approximately correct the system
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 hwclock(8).
Steven S. Dick <firstname.lastname@example.org>, Jim Van Zandt <email@example.com>.
date(1L), gettimeofday(2), settimeofday(2), hwclock(8), ntpdate(8), ntpd(8),
October 24, 1998 ADJTIMEX(8)