illumos: manual page: rtc.8

RTC(8) Maintenance Commands and Procedures RTC(8)

NAME

rtc - provide all real-time clock and UTC-lag management

SYNOPSIS

rtc [-csuvw] [-z zone-name]

DESCRIPTION

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(8), which are
used to set the time on a system, invoke rtc -c to ensure that daylight
savings time (DST) is corrected for properly.

OPTIONS

-c This option checks for DST and makes corrections to the RTC if
necessary. It is normally run once a day by a cron(8) 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(8) 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(8) 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(8) 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.

FILES

/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.

ARCHITECTURE