Tribblix: 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