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[edit] What is it

The TIMEZONE variable is specified in the /etc/rc.conf file. It defines what timezone is the computer clock showing.

[edit] What can it be

The complete list of possibilities can be shown using this command:

pacman -Ql tzdata | grep zoneinfo | grep -v posix | grep -v right | grep -v .tab | grep -v /$ | sed "s@tzdata /usr/share/zoneinfo/@ @"

[edit] UTC vs. localtime

The hardware clock can be set to UTC or to local time. Usually hardware clock on Linux machines is set to UTC time. If you use Linux with Windows on the same machine, then your clock is usually set to local time.

Setting hardware clock mode is done via HARDWARECLOCK variable:

HARDWARECLOCK="UTC"

or

HARDWARECLOCK="localtime"

NOTE: As of current version of initscripts setting HARDWARECLOCK to any value other than "UTC" or just leaving it empty is the same as HARDWARECLOCK="localtime".

NOTE: Linux will change to and from DST when the HARDWARECLOCK setting is set to UTC, regardless of whether Linux was running at the time DST is entered or left.

IMPORTANT: When the HARDWARECLOCK setting is set to localtime, Linux will not adjust the time, operating under the assumption that your system may be a dual boot system at that time and that the other OS takes care of the DST switch. If that was not the case, the DST change needs to be made manually.