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adjkerntz - adjust local time CMOS clock to reflect time zone changes and



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


      adjkerntz -i
      adjkerntz -a [-s]


      Adjkerntz 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.  Adjkerntz 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 times‐
      tamps 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 compati‐
      ble mode).  If that file does not exist, it means that the CMOS clock
      keeps UTC time.  Adjkerntz 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.  Adjkerntz is
            invoked with this option from /etc/rc on entry to multi-user mode,
            before any other daemons have been started.  Adjkerntz 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 cor‐
            responding UTC time.  Adjkerntz also stores the local time zone
            offset into the machdep.adjkerntz kernel variable, for use by sub‐
            sequent invocations of ’adjkerntz -a’ and by local time file sys‐
            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.  Adjkerntz uses the ker‐
            nel 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
            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: don’t use the -s option in a crontab(5) command line, or
            multiple ’adjkerntz -a’ instances could conflict with each other.
      Adjkerntz clears the kernel timezone structure and makes the kernel clock
      run in the UTC time zone.  Super-user privileges are required for all


      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.
      tzset(3), crontab(5), mount_msdos(8), rc(8), sysctl(8), tzsetup(8),


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


      Andrey A. Chernov 〈〉


      The adjkerntz command appeared in FreeBSD 1.0.


Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.

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