Change directory to /usr/lib/zoneinfo/. Get the time zone package if you don't have this directory. The source is available in ftp://metalab.unc.edu/pub/Linux/system/admin/time/.
Then make a symbolic link named localtime pointing to one of the files in this directory (or a subdirectory), and one called posixrules pointing to localtime. For example:
$ ln -sf US/Mountain localtime
$ ln -sf localtime posixrules
This change will take effect immediately—try date.
If the system uses Red Hat-style configuration files, the respective time zone info files are /usr/share/zoneinfo and /etc/localtime.
The manual pages for tzset or tzselect describe setting the time zone. Some programs recognize the "TZ" environment variable, but this is not POSIX-correct.
You should also make sure that your Linux kernel clock is set to the correct GMT time. Type date -u and check that the correct UTC time is displayed. ("The Computer Has the Wrong Time.")