According to my brief google search, it sounds like your PHP installation does not have a default timezone set, which is highly unsual.
You need to set the time zone, either using php.ini (which you may need to ask customer support to do for you if you don't have access to edit that yourself) or with the php function date_default_timezone_set() in the file where the error is happening.
Via php.ini (this will fix it everywhere):
PHP Code:
[Date]
; Defines the default timezone used by the date functions
; http://php.net/date.timezone
date.timezone = Europe/London
Or using php (before the line that has the error):
PHP Code:
date_default_timezone_set("Europe/London");
For a list of example timezones,
https://secure.php.net/manual/en/timezones.php has the list of supported timezones in php. (Click a content, and then paste in a value found on one of these pages.)