Проблемы с date (PHP)

$today=date("Y-m-d");

При вызове этой функции получаю такую ошибку:

Warning: date() [function.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/Helsinki' for '3.0/DST' instead in

Почему она возникает? Раньше такого не было..