In Centos 5.x there is the text mode interface of system-config-date to set the time zone. Although that utility still exists in Centos 6 the text mode is gone. Is there anything similar in Centos 6?
/etc/localtime controls the system timezone. It's just a symlink to one of the timezone files in
ln -sf /usr/share/zoneinfo/your/zone /etc/localtime