The frontend time zone can be set globally in the frontend and adjusted for individual users.
If System is selected, the web server time zone will be used for the frontend (including the value of 'date.timezone' of php.ini, if set), while Zabbix server will use the time zone of the machine it is running on.
Zabbix server will only use the specified global/user timezone when expanding macros in notifications (e.g. {EVENT.TIME} can expand to a different time zone per user) and for the time limit when notifications are sent (see "When active" setting in user media configuration).
The global timezone:
User-level time zone: