My assumption is that you are asking about changing the timezone rather than the time.
The /etc/localtime file contains the actual timezone data, so if you wish to change the timezone, you can use a copy or a symbolic link from one of the /usr/share/zoneinfo files.
However, most distros provide a tool to do the reconfiguration for you, eg redhat-config-date or system-config-date in RedHat/Fedora, and dpkg-reconfigure tzdata in Debian. These may also make other changes (for example, setting the timezone name in an appropriate place).
The /etc/sysconfig/clock controls whether the hardware clock is treated as being in UTC time or not. Historically this has always been UTC. However, some other operating systems could not handle a UTC hardware clock (such as pre-NT versions of Windows), so it was useful to change this behaviour. You wouldn't normally touch it.
If you have a GUI that is able to change the timezone, then you don't need to also do it from the terminal command line. It is up to you if you prefer to use a command line or a GUI. An advantage of using the terminal is that you will understand more about the guts of the system, a disadvantage is that you will need to figure out the details specific to your particular distro.