--adjfile=filename
Override the default /etc/adjtime file path.
--date=date_string
This option must be used with the --set or --predict
functions, otherwise it is ignored.
hwclock --set --date='16:45'
hwclock --predict --date='2525-08-14 07:11:05'
The argument must be in local time, even if you keep your
Hardware Clock in UTC. See the --localtime option. Therefore,
the argument should not include any timezone information. It
also should not be a relative time like "+5 minutes", because
hwclock's precision depends upon correlation between the
argument's value and when the enter key is pressed.
Fractional seconds are silently dropped. This option is
capable of understanding many time and date formats, but the
previous parameters should be observed.
--delay=seconds
This option can be used to overwrite the internally used
delay when setting the clock time. The default is 0.5 (500ms)
for rtc_cmos, for another RTC types the delay is 0. If RTC
type is impossible to determine (from sysfs) then it defaults
also to 0.5 to be backwardly compatible.
The 500ms default is based on commonly used
MC146818A-compatible (x86) hardware clock. This Hardware
Clock can only be set to any integer time plus one half
second. The integer time is required because there is no
interface to set or get a fractional second. The additional
half second delay is because the Hardware Clock updates to
the following second precisely 500 ms after setting the new
time. Unfortunately, this behavior is hardware specific and
in same cases another delay is required.
-D, --debug
Use --verbose. The --debug option has been deprecated and may
be repurposed or removed in a future release.
--directisa
This option is meaningful for ISA compatible machines in the
x86 and x86_64 family. For other machines, it has no effect.
This option tells hwclock to use explicit I/O instructions to
access the Hardware Clock. Without this option, hwclock will
use the rtc device file, which it assumes to be driven by the
Linux RTC device driver. As of v2.26 it will no longer
automatically use directisa when the rtc driver is
unavailable; this was causing an unsafe condition that could
allow two processes to access the Hardware Clock at the same
time. Direct hardware access from userspace should only be
used for testing, troubleshooting, and as a last resort when
all other methods fail. See the --rtc option.
--epoch=year
This option is required when using the --setepoch function.
The minimum year value is 1900. The maximum is system
dependent (ULONG_MAX - 1).
-f, --rtc=filename
Override hwclock's default rtc device file name. Otherwise it
will use the first one found in this order: /dev/rtc0,
/dev/rtc, /dev/misc/rtc. For IA-64: /dev/efirtc
/dev/misc/efirtc
-l, --localtime; -u, --utc
Indicate which timescale the Hardware Clock is set to.
The Hardware Clock may be configured to use either the UTC or
the local timescale, but nothing in the clock itself says
which alternative is being used. The --localtime or --utc
options give this information to the hwclock command. If you
specify the wrong one (or specify neither and take a wrong
default), both setting and reading the Hardware Clock will be
incorrect.
If you specify neither --utc nor --localtime then the one
last given with a set function (--set, --systohc, or
--adjust), as recorded in /etc/adjtime, will be used. If the
adjtime file doesn't exist, the default is UTC.
Note: daylight saving time changes may be inconsistent when
the Hardware Clock is kept in local time. See the discussion
below, under LOCAL vs UTC.
--noadjfile
Disable the facilities provided by /etc/adjtime. hwclock will
not read nor write to that file with this option. Either
--utc or --localtime must be specified when using this
option.
--test
Do not actually change anything on the system, that is, the
Clocks or /etc/adjtime (--verbose is implicit with this
option).
--update-drift
Update the Hardware Clock's drift factor in /etc/adjtime. It
can only be used with --set or --systohc.
A minimum four hour period between settings is required. This
is to avoid invalid calculations. The longer the period, the
more precise the resulting drift factor will be.
This option was added in v2.26, because it is typical for
systems to call hwclock --systohc at shutdown; with the old
behavior this would automatically (re)calculate the drift
factor which caused several problems:
• When using NTP with an '11 minute mode' kernel the drift
factor would be clobbered to near zero.
• It would not allow the use of 'cold' drift correction.
With most configurations using 'cold' drift will yield
favorable results. Cold, means when the machine is turned
off which can have a significant impact on the drift
factor.
• (Re)calculating drift factor on every shutdown delivers
suboptimal results. For example, if ephemeral conditions
cause the machine to be abnormally hot the drift factor
calculation would be out of range.
• Significantly increased system shutdown times (as of
v2.31 when not using --update-drift the RTC is not read).
Having hwclock calculate the drift factor is a good starting
point, but for optimal results it will likely need to be adjusted
by directly editing the /etc/adjtime file. For most
configurations once a machine's optimal drift factor is crafted
it should not need to be changed. Therefore, the old behavior to
automatically (re)calculate drift was changed and now requires
this option to be used. See the discussion below, under The
Adjust Function.
This option requires reading the Hardware Clock before setting
it. If it cannot be read, then this option will cause the set
functions to fail. This can happen, for example, if the Hardware
Clock is corrupted by a power failure. In that case, the clock
must first be set without this option. Despite it not working,
the resulting drift correction factor would be invalid anyway.
-v, --verbose
Display more details about what hwclock is doing internally.
