time zone for php is not set (configuration parameter "date.timezone").
I had the same issue on Ubuntu 14.04.4 and I have the set the values in /etc/php5/apache2/php.ini as below.
You can also edit /etc/apache2/conf-enabled/zabbix.conf which is simlink to /etc/zabbix/apache.conf:
php_value max_execution_time 300
php_value memory_limit 128M
php_value post_max_size 16M
php_value upload_max_filesize 2M
php_value max_input_time 300
php_value always_populate_raw_post_data -1
php_value date.timezone Greece/Athens