Como resolver este problema de DATE chato no PHP?

Ae galera o PHP fica toda hora me avisando sobre isto:

2048 error date() [function.date]: It is not safe to rely on the system's timezone settings. Please use the date.timezone setting, the TZ environment variable or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'America/Sao_Paulo' for '-2.0/DST' instead in line 128 of file class.Cache.php

Como resolver esta chatura?

Na linha 123 não tem nada de mais, apenas:

return current($array = explode(date('O'),date( "r",$time))) . ' GMT';

[code=auto:0]

Obrigado