rtc —
provide
all real-time clock and UTC-lag management
| rtc |
[-csuvw]
[-z
zone-name] |
The Real Time Clock (RTC) is the hardware device on x86 computers that maintains
the date and time. The RTC is battery-powered, so that it keeps running when
the computer is shut down. It can be set from the BIOS and also from the
operating system running on the computer. The RTC has no setting for the time
zone or for Daylight Saving Time (DST). It relies on the operating system for
these facilities and for automatic changes between standard time and DST.
On x86 systems, the
rtc command reconciles the
difference in the way that time is established between UNIX and Windows
systems. The internal clock on UNIX systems utilizes Universal Coordinated
Time (UTC) while Windows systems usually expect the RTC to run in local time,
including DST changes.
Without arguments,
rtc displays the currently
configured time zone string for the RTC. The currently configured time zone
string is based on what was last recorded by
rtc
-z zone-name.
The
rtc command is not normally run from a shell
prompt; it is generally invoked by the system. Commands such as
date(1) and
rdate(1M), which are used to set the time on a
system, invoke
rtc
-c to ensure that daylight savings time (DST) is
corrected for properly.
-
-
- -c
- This option checks for DST and makes corrections to the RTC
if necessary. It is normally run once a day by a
cron(1M) job.
If there is no RTC time zone or /etc/rtc_config
file, this option will do nothing.
-
-
- -s
- This option specifies that the RTC runs in local standard
time all year round. It is incompatible with Windows, but is convenient if
only one operating system is to be run on the computer. The
cron(1M) command is not necessary, and should
not be run.
-
-
- -u
- This option specifies that the RTC runs in UTC time. As a
side effect, it sets the time zone in
/etc/rtc_config to UTC. Windows can operate
in UTC time, but requires a registry change to do so. The
cron(1M) command is not necessary.
-
-
- -v
- This option specifies that the RTC tracks local time,
including DST changes. This is the default. It accomodates Windows with no
changes. The cron(1M) command is necessary to
change the RTC when DST is in effect.
-
-
- -w
- This option does nothing. It is present for compatibility
with Solaris 11.
-
-
- -z
zone-name
- This option, which is normally run by the system at
software installation time, is used to specify the time zone in which the
RTC is to be maintained. It updates the configuration file
/etc/rtc_config with the name of the
specified zone and the current UTC lag for that zone. If there is an
existing /etc/rtc_config file, this command
will update it. If not, this command will create it.
-
-
- /etc/rtc_config
- The data file used to record the time zone and UTC lag.
This file is completely managed by rtc. At
boot time, the kernel reads the UTC lag from this file, and uses it to set
the system time.
x86
date(1),
cron(1M),
rdate(1M),
attributes(5)