provide all real-time clock and UTC-lag management
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.
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 command is not normally run from a
shell prompt; it is generally invoked by the system. Commands such as
rdate(1M), which are used to set the
time on a system, invoke
-c to ensure that daylight savings time (DST) is
corrected for properly.
- This option checks for DST and makes corrections to the RTC if necessary.
It is normally run once a day by a
If there is no RTC time zone or
/etc/rtc_config file, this option will do
- 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.
- 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
- 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.
- This option does nothing. It is present for compatibility with Solaris
- 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.
- 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