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

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

SYNOPSIS
     adjkerntz -i
     adjkerntz -a [-s]

DESCRIPTION
     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 in order to maintain proper timestamps for local
     time file systems such as the MS-DOS file system.  The main purpose of
     maintaining these timestamps properly is to keep the timestamps of a
     FreeBSD 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 timestamps.  If the file /etc/wall_cmos_clock exists, it means
     that the 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 in 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.

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

FILES
     /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.

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

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

HISTORY
     The adjkerntz utility appeared in FreeBSD 1.0.

AUTHORS
     Andrey A. Chernov <ache@astral.msk.su>

FreeBSD 13.1-RELEASE-p6          March 8, 2012         FreeBSD 13.1-RELEASE-p6