[to order events relatively, without relying on an arbitrary external axis, yet still allowing for multiple observers]

the goal is to create a tool capable of modelling any historical event or fictional storyline in a manner which is useful for the organization of thoughts during the process of discovering or inventing the events involved

defining the problem is the hardest part which has yet been attempted, so I will start there.

• definition of discrete events. this would seem to be the most fundamental function of a timeline.
• ordering of multiple events into a sequence. event A comes before event B. this would seem to be the second most fundamental function of a timeline.

these appear to be the only actual requirements of calling something a timeline. all models of timelines I have found have ways of defining these two fundamental aspects. but these two features are not enough to be useful in all cases. some additional features I have seen, others I have not, but I shall attempt to list them all here.

• duration of events, or time between events. I won't go any further on this yet.
• multiple accounts of the same event. two people can very easily disagree on what happened when, and how. sometimes there is no definitive version of events.
• disagreements as to what accounts describe what events. there can be disagreement as to whether two accounts are actually describing the same thing, or different things altogether. the example here is that of two historians each looking at two documents describing a battle. one believes the two documents describe the same battle, while another claims that the battles were separate.
• recurring events. there is a sunrise every day, a timeline should be able to denote that.
• variations on other events. "it happened exactly like before, but differently."
• variable time scales across multiple observers, for example due to relativity.
• fictional necessities, such as events happening in different orders for different observers (time travellers), branching of timelines, transport to disconnected timelines (other universes)
• relative duration specification (happened three battles after this one).
• detection of conflicts