Steps to reproduce:

1. Navigate to Administration -> General -> GUI.
2. Change the Working time parameter to 1-5,00:00-24:00 (using 24:00 as the end of the working day).
3. Navigate to Monitoring -> Latest data.
4. Select any item and click on Graph to open a simple graph (chart.php).
5. Change the time period filter to Last 1 day (or any period that crosses a day/weekend boundary, e.g., Last 2 days, Last 7 days).

Result:

• The HTTP request to chart.php hangs indefinitely in a pending state.
• The corresponding PHP-FPM worker process instantly spikes to 100% CPU utilization and stays there until killed or timed out.
• Web interface becomes unresponsive for the user due to PHP session locking or pool exhaustion.
• zabbix_slow.log shows that the execution is stuck in an infinite loop inside graphs.inc.php and CLineGraphDraw.php

Expected:

• The simple graph should render successfully and immediately.

• The time parsing logic should handle 24:00 safely.

Workaround:
Changing Working time to 1-5,00:00-23:59 completely resolves the issue, preventing the infinite loop and allowing graphs to render instantly.