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In the Debian system, the system time zone is determined
by the symbolic link
This link points at a time zone data file that describes
the local time zone. The time zone data files are stored in
/usr/lib/zoneinfo. Other Linux distributions
may do this differently.
A user can change his private time zone by setting the
TZ environment variable. If it is unset, the system time zone
is assumed. The syntax of the TZ variable is described in the
tzset manual page.
date only shows or sets the software clock. The clock commands synchronises the hardware and software clocks. It is used when the system boots, to read the hardware clock and set the software clock. If you need to set both clocks, you first set the software clock with date, and then the hardware clock with clock -w.
-u option to clock
tells it that the hardware clock is in universal time.
You must use the
option correctly. If you don't, your computer will be quite
confused about what the time is.
The clocks should be changed with care. Many parts of a Unix system require the clocks to work correctly. For example, the cron daemon runs commands periodically. If you change the clock, it can be confused of whether it needs to run the commands or not. On one early Unix system, someone set the clock twenty years into the future, and cron wanted to run all the periodic commands for twenty years all at once. Current versions of cron can handle this correctly, but you should still be careful. Big jumps or backward jumps are more dangerous than smaller or forward ones.
Beware of the time command, which does not show the current time.