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Manual Pages  — ADJKERNTZ


adjkerntz – adjust the local time CMOS clock to reflect time zone changes and keep the 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 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:
  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.


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).
  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), zic(8)


The adjkerntz utility appeared in FreeBSD 1.0 .


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

ADJKERNTZ (8) March 8, 2012

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