Except for the first and the last day off the timeline range, the DayStartTime and DayEndTime properties have no effect in TimelineView.
This issue is known and approved here
in your feedback portal, but I could not find any hint of a release date.
Is this feature on your roadmap for 2015?
This feature is vastly asked for among our customers and they often complain about the timeline view being quite useless as it is now.