utility maintains the proper relationship between the kernel clock, which
is always set to UTC and the CMOS clock, which may be set to local time.
utility also informs the kernel about machine timezone shifts in order to
maintain proper timestamps for local time file systems such as the MS-DOS
The main purpose of maintaining these timestamps properly is to keep the
timestamps of a
MS-DOS file system and an MS-DOS operating system synchronized when they are
installed on the same system rather than fixing broken MS-DOS file
If the file
exists, it means that the CMOS clock keeps local time (MS-DOS and MS-Windows
If that file does not exist, it means that the CMOS clock keeps UTC time.
utility passes this state to the
Adjustments may be needed at system startup and shutdown, and
whenever a time zone change occurs.
To handle these different situations,
is invoked in two ways:
This form handles system startups and shutdowns.
utility is invoked with this option from
on entry to multi-user mode, before any other daemons have been started.
utility puts itself into the background.
Then, for a local time CMOS clock,
reads the local time from it
and sets the kernel clock to the corresponding UTC time.
utility also stores the local time zone offset in the
kernel variable, for use by subsequent invocations of
and by local time file systems.
For a local time CMOS clock
pauses and remains inactive as a background daemon until it
receives a SIGTERM.
The SIGTERM will normally be sent by
when the system leaves multi-user mode (usually, because the system
is being shut down).
After receiving the SIGTERM,
reads the UTC kernel clock and updates the CMOS clock, if necessary,
to ensure that it reflects the current local time zone.
| -a [-s]
This form is used to update the local time CMOS clock and kernel
variable when time zone changes occur,
e.g., when entering or leaving daylight savings time.
utility uses the kernel clock's UTC time,
the previously stored
time zone offset, and the changed time zone rule to
calculate a new time zone offset.
It stores the new offset into the
kernel variable and updates the wall CMOS clock to the new local time.
was started at a nonexistent time (during a timezone change), it exits
with a warning diagnostic unless the
option was used, in which case
sleeps 30 minutes and tries again.
This form should be invoked from root's
every half hour between midnight and 5am, when most modern time
zone changes occur.
Warning: do not use the
option in a
command line, or multiple
instances could conflict with each other.
utility clears the kernel timezone structure and makes the kernel clock run
in the UTC time zone.
Super-user privileges are required for all operations.