The "timeline-utc" setting:

If the timeline-utc setting is true, then Fossil tries to understand and display all time values using UTC. If this setting is false, Fossil tries to understand and display time values using the local timezone.

The word "timeline" in the name of this setting is historical. This setting applies to all user interfaces of Fossil, not just the timeline.

Note that when accessing Fossil using the web interface, the localtime used is the localtime on the server, not on the client.