The PHP timezone must be configured before the user is able to start the zabbix server web interface. Both the instructions for Ubuntu on the Zabbix download page:

https://www.zabbix.com/download?zabbix=4.0&os_distribution=ubuntu&os_version=18.04_bionic&db=mysql

and within the manual:

https://www.zabbix.com/documentation/4.0/manual/installation/install_from_packages/debian_ubuntu

Are incorrect, for Ubuntu 18.04 at least but I suspect its wrong for all versions of Ubuntu. The current instructions tell the user to edit /etc/zabbix/apache.conf but the correct path of the file to edit is /etc/php/PHP_VERSION/apache2/php.ini. On my current Ubuntu 18.04 install the file to edit is /etc/php/7.2/apache2/php.ini . Apache needs to be restarted after editing this file, but that is already mentioned.