---
name: "adjkerntz"
section: "8"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/adjkerntz"
---

# adjkerntz(8)

adjust real-time clock (rtc) and kernel timezone offset

## NAME

**adjkerntz** – adjust real-time clock (rtc) and kernel timezone offset

## 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 real-time 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 real-time clock keeps local time (MS-DOS and MS-Windows compatible mode). If that file does not exist, it means that the real-time 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 real-time 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 real-time 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)](/init/8.md) 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 real-time 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 real-time clock and *machdep.adjkerntz* kernel 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 real-time 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)](/crontab/5.md) 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)](/crontab/5.md) 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)](/tzset/3.md); not needed when [tzsetup(8)](/tzsetup/8.md) or [zic(8)](/zic/8.md) is used.

## FILES

- */etc/localtime*

  Current zoneinfo file, see [tzsetup(8)](/tzsetup/8.md) and [zic(8)](/zic/8.md).

- */etc/wall\_cmos\_clock*

  Empty file. Its presence indicates that the machine's real-time clock is set to local time, while its absence indicates a UTC real-time clock.

## DIAGNOSTICS

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

## SEE ALSO

[tzset(3)](/tzset/3.md), [crontab(5)](/crontab/5.md), [mount\_msdosfs(8)](/mount_msdosfs/8.md), [ntpd(8)](/ntpd/8.md), [rc(8)](/rc/8.md), [sysctl(8)](/sysctl/8.md), [tzsetup(8)](/tzsetup/8.md), [zic(8)](/zic/8.md)

## HISTORY

The **adjkerntz** utility appeared in FreeBSD 1.0.

## AUTHORS

Andrey A. Chernov ⟨*ache@astral.msk.su*⟩