PHP Warning: mktime(): 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 /var/www/...
If you encounter similar warning, then it means that you need to set default timezone used by all date/time functions.
Open php.ini configuration file and edit date.timezone setting:
date.timezone = "Europe/Warsaw"
In case of doubt check out list of supported timezones.