Warning: date_create(): It is not safe to rely on the system's timezone settings. After upgrade to php 5.3.1

After upgrading to php 5.3.1 I get error as below on admin/config/regional/settings

Warning: date_create(): 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 'CET/1.0/no DST' instead in format_date() (line 2322 of /var/www/drupal7/includes/common.inc).