PHP Warning: date() after upgrading to 2.2.0

Today I upgraded my Piwik installation and after updating my cron job of archiving, which now is “console core:archive”, I got the following warning:

$ sudo su www-data -c "/usr/bin/php5 /var/www/piwik/console core:archive --url=http://X.X.X.X:8081/piwik/"
PHP 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 the timezone 'UTC' for now, but please set date.timezone to select your timezone. in /var/www/piwik/core/Log.php on line 520

Despite of this, the process continues and reports seems to be working fine, but I don’t know if this warning is affecting something.