Welcome to LQ!
Since you didn't specify the OS, you get several answers (bonus!)
To change the hosts' TZ on Ubuntu: open a terminal and issue:
sudo dpkg-reconfigure tzdata
To change the hosts' TZ on CentOS
Bash Shell only:
export TZ=America/Los_Angeles ; date
for permanence in bash shell add this to your .bashrc
This will not change the system TZ.
and re-source it using
for temporary date manipulation you can use:
in a bash shell.
Hope that helps!