I had this same issue on my Apache server and I fixed it by making a .htaccess file in the root of my powercms folder with the following code:
php_value date.timezone America/Los_Angeles
Basically, your PHP on your server doesn't have a default timezone so this file gives it a default. If this doesn't work for you, you would have to contact your host and ask them to set a default timezone in PHP for you.
-justin