3.8.2 Timing

As mentioned, the occurrence time of a change is treated as its attribute. The change is placed in a history, which is a sequence of changes for an entity. The order of those changes conforms to the incremental order of their time attributes. In this way, changes are carried out one after another, with the invariant that the first change (always at time $0$) is the creation of the entity, and the last change is its destruction.

As time is an attribute of changes, one can imagine that even the time (or, the sequence) of changes is mutable at run-time. The basic action semantics gives no narration on this issue. If this is permitted, interesting features will exhibit themselves in various models, because the execution of a model is capable of dynamically modifying the structure of the model itself, which affects the execution in return. Modifying the time attribute of another change to a value larger than the current local time is just like scheduling an event in the future; modifying it to be smaller makes no sense, because the time has already elapsed and the past is cannot be altered.