I recently bought a new VPS from a hosting company in New York and I installed Ubuntu 13.04 on the new machine. As expected, it chose Americ/New_York as the default time zone for the new server. To change it, it is as easy as run the following command:
sudo dpkg-reconfigure tzdata
And the just follow the prompts to choose your currently region and city.
If you want your cron jobs to be using the updated time zone, you will also need to restart the cron demaeon.
sudo service rsyslog restart
Or maybe just reboot the server to make sure everything is in synced.