First, provide a way to group events as a set (functionally equivalent to a sub-timeline) that can be manipulated as a unit. The default set would be all events in the project. Explicitly defined event groups could be for scenes, groups of scenes (sections, acts, etc.), plot, subplots, sub- or separate timelines (e.g., a timeline of the movements, leadership changes, battles, and campaigns in a war; a genealogical timeline for a family of characters; a timeline of technology development in a sci-fi novel; etc.).
Second, enable each event group to be moved or shifted forward or backward in time as a unit. So, if I change the start or end date of an event group, all the dates and/or temporal relationships within the event group maintain their proportional relationships (number of days between them).
Last, let event groups be nested.
This functionality would be like that in a project management program that allows re-baselining a project from a new start or end date.
Example use case: I start a hard (technical) sci-fi timeline with the appearance of a technology. Later, I realize that technology's likely window of appearance is much earlier or later, based on real world science and technology research and development trends and constraints. I need to shift everything earlier or later to maintain a sense of fictional plausibility for well-informed hard sci-fi readers.