2 - Raspberry PI Community Projects

The timezone, configured during initial installation, is a configuration item for the tzdata package.
To modify it, use the dpkg-reconfigure tzdata command, which allows you to choose the
timezone to be used in an interactive manner (until lenny, the command to use was tzconfig).
Its configuration is stored in the /etc/timezone file. Additionally, the corresponding file in
the /usr/share/zoneinfo directory is copied in /etc/localtime; this file contains the rules
governing the dates where daylight saving time is active, for countries that use it.
When you need to temporarily change the timezone, use the TZ environment variable, which
takes priority over the configured system default:
$ date
Wed Mar 28 15:51:19 CEST 2012
$ TZ="Pacific/Honolulu" date
Wed Mar 28 03:51:21 HST 2012
NOTE
System clock, hardware
clock
There are two time sources in a computer. A computer's motherboard has a
hardware clock, called the “CMOS clock”. This clock is not very precise, and
provides rather slow access times. The operating system kernel has its own,
the soware clock, which it keeps up to date with its own means (possibly
with the help of time servers, see the “Time Synchronization” section). This
system clock is generally more accurate, especially since it doesn't need access
to hardware variables. However, since it only exists in live memory, it is zeroed
out every time the machine is booted, contrary to the CMOS clock, which has
a baery and therefore “survives” rebooting or halting of the machine. The
system clock is, thus, set from the CMOS clock during boot, and the CMOS
clock is updated on shutdown (to take into account possible changes or corrections
if it has been improperly adjusted).
In practice, there is a problem, since the CMOS clock is nothing more than
a counter and contains no information regarding the time zone. There is a
choice to make regarding its interpretation: either the system considers it
runs in universal time (UTC, formerly GMT), or in local time. This choice
could be a simple shi, but things are actually more complicated: as a result
of daylight saving time, this offset is not constant. The result is that the system
has no way to determine whether the offset is correct, especially around
periods of time change. Since it is always possible to reconstruct local time
from universal time and the timezone information, we strongly recommend
using the CMOS clock in universal time.
Unfortunately, Windows systems in their default configuration ignore this
recommendation; they keep the CMOS clock on local time, applying time
changes when booting the computer by trying to guess during time changes
if the change has already been applied or not. This works relatively well, as
long as the system has only Windows running on it. But when a computer has
several systems (whether it be a “dual-boot” configuration or running other
systems via virtual machine), chaos ensues, with no means to determine if
the time is correct. If you absolutely must retain Windows on a computer,
you should either configure it to keep the CMOS clock as UTC, or deactivate
UTC in the /etc/default/rcS file on the Debian system (and make sure to
manually check your clock in spring and autumn).
168 The Debian Administrator's Handbook