According to this /r thread (https://www.reddit.com/r/opnsense/comments/1f0vh4v/system_is_ahead_by_4_hours/), the subject is not sufficiently covered by documentation, so I thought I'd prepare this carefully crafted howto:
Configure a timezone on your OPNsense firewall:
1/ Go to System - Settings - General.
2/ From the Time zone dropdown, select one that's closely matching your location.
3/ Click Save.
Hints: Setting up a NTP server (even a local one on your LAN) and syncing the time with that server will NOT set a proper timezone for you. NTP does not recognize time zones, it manages all time information based on UTC.
Reference:
- https://docs.opnsense.org/manual/settingsmenu.html#general
- https://en.wikipedia.org/wiki/Coordinated_Universal_Time
Hope this helps and clears some doubts. ;D ;D ;D