You need to add the following lines in the php.ini file in the MODX root. If there isn't one, just create one and add these lines:
[Date]
; Defines the default timezone used by the date functions
date.timezone = "America/Los_Angeles"
If you're in another timezone, there's a list of supported timezone settings here:
http://www.php.net/manual/en/timezones.php
If you don't have the ability to create and edit files on the server, you can download the php.ini file in the MODX root, edit it, and upload it. Make a backup copy of the file before you start. If there isn't one, you can create it locally in any text editor and upload it.
Just be careful not to overwrite an existing php.ini on the server with one you created yourself.