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ADJKERNTZ(8)            FreeBSD 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
     kernel 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
           invocations 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
           timezone 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 <>

FreeBSD 11.0-PRERELEASE          April 4, 1996         FreeBSD 11.0-PRERELEASE


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