adjkerntz man page on FreeBSD

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ADJKERNTZ(8)		  BSD System Manager's Manual		  ADJKERNTZ(8)

     adjkerntz — adjust local time CMOS clock to reflect time zone changes and
     keep current timezone offset for the kernel

     adjkerntz -i
     adjkerntz -a [-s]

     The adjkerntz utility maintains the proper relationship between the ker‐
     nel clock, which is always set to UTC, and the CMOS clock, which may be
     set to local time.	 The adjkerntz utility also informs the kernel about
     machine timezone shifts to maintain proper timestamps for local time file
     systems such as the MS-DOS file system.  The main purpose of this thing
     is not general fixing of initially broken MS-DOS file timestamp idea but
     keeping the same timestamps between FreeBSD MS-DOS file system and MS-DOS
     operating system installed on the same machine.  If the file
     /etc/wall_cmos_clock exists, it means that CMOS clock keeps local time
     (MS-DOS and MS-Windows compatible mode).  If that file does not exist, it
     means that the CMOS clock keeps UTC time.	The adjkerntz utility passes
     this state to the machdep.wall_cmos_clock kernel variable.

     Adjustments may be needed at system startup and shutdown, and whenever a
     time zone change occurs.  To handle these different situations, adjkerntz
     is invoked in two ways:

     -i	   This form handles system startups and shutdowns.  The adjkerntz
	   utility is invoked with this option from /etc/rc on entry to multi-
	   user mode, before any other daemons have been started.  The
	   adjkerntz utility puts itself into the background.  Then, for a
	   local time CMOS clock, adjkerntz reads the local time from it and
	   sets the kernel clock to the corresponding UTC time.	 The adjkerntz
	   utility also stores the local time zone offset into the
	   machdep.adjkerntz kernel variable, for use by subsequent invoca‐
	   tions of 'adjkerntz -a' and by local time file systems.

	   For a local time CMOS clock 'adjkerntz -i' pauses, and remains
	   inactive as a background daemon until it receives a SIGTERM.	 The
	   SIGTERM will normally be sent by init(8) when the system leaves
	   multi-user mode (usually, because the system is being shut down).
	   After receiving the SIGTERM, adjkerntz reads the UTC kernel clock
	   and updates the CMOS clock, if necessary, to ensure that it
	   reflects the current local time zone.  Then adjkerntz exits.

     -a [-s]
	   This form is used to update the local time CMOS clock and kernel
	   machdep.adjkerntz variable when time zone changes occur, e.g., when
	   entering or leaving daylight savings time.  The adjkerntz 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 machdep.adjkerntz kernel
	   variable, and updates the wall CMOS clock to the new local time.
	   If 'adjkerntz -a' was started at a nonexistent time (during a time‐
	   zone change), it exits with a warning diagnostic unless the -s
	   option was used, in which case adjkerntz sleeps 30 minutes and
	   tries again.

	   This form should be invoked from root's crontab(5) every half hour
	   between midnight and 5am, when most modern time zone changes occur.
	   Warning: do not use the -s option in a crontab(5) command line, or
	   multiple 'adjkerntz -a' instances could conflict with each other.

     The adjkerntz 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.

     TZ		 Time zone change rule, see tzset(3); not needed when
		 tzsetup(8) or zic(8) is used.

     /etc/localtime	   Current zoneinfo file, see tzsetup(8) and zic(8).
     /etc/wall_cmos_clock  Empty file.	Its presence indicates that the
			   machine's CMOS clock is set to local time, while
			   its absence indicates a UTC CMOS clock.

     No diagnostics.  If an error occurs, adjkerntz logs an error message via
     syslog(3) and exits with a nonzero return code.

     tzset(3), crontab(5), mount_msdosfs(8), rc(8), sysctl(8), tzsetup(8),

     The adjkerntz utility appeared in FreeBSD 1.0.

     Andrey A. Chernov ⟨⟩

BSD				 April 4, 1996				   BSD

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