With Zabbix 5.2 it is possible to choose timezone directly in the web-interface. It has new "Timezone" field globally and in user preferences. Global one has "System" value, it is not clear which system value it uses: 1 - system-wide or 2 - PHP date.timezone.

Upgrade notes do not contain such information: https://www.zabbix.com/documentation/5.2/manual/installation/upgrade_notes_520
while https://www.zabbix.com/documentation/5.2/manual/web_interface/time_zone#overview section contains information:

Since Zabbix 5.2 the time zone can be set globally and also adjusted for different users.

The System option for time zone means that the web server time zone will be used for frontend (including the value of 'date.timezone' of php.ini, if set) and Zabbix server will use the time zone of the machine it is running on.

It would be great to mention such information in upgrade notes as well!