ad_cache php5.3 date issue

Warning: date(): It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting 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 'Europe/Berlin' for 'CEST/2.0/DST' instead in alll/modules/ad/cache/file/ad_cache_file.inc on line 171

This is happening due to PHP 5.3
help about this issue in date http://drupal.org/node/630972