change timezone in a linux system

/etc/localtime stores the default timezone of the host.

one can observe this file by
# zdump /etc/localtime

to change the default timezone, one can do
1. keep a copy of origirnal one
# cd /etc
# mv localtime localtime.orig

2. select the proper zone file. e.g.
# cp /usr/share/zoneinfo/Asia/Taipei /etc/localtime

now, the default timezone is changed.