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This concept has the following major specialisations within the Foundation package:
The TimePoint specialisation of MESSAGE-entity embraces concepts that refer to a particular point in time. A mapping between instances of a TimePoint class and points on the history timeline of the system is established by the atTime relationship. Note that the main reason for the atTime relationship is to help establish a temporal semantics, and that often atTime will not appear explicitly in MESSAGE models. Note, however, that many agent-oriented languages (from an AI research tradition) and knowledge representation formalisms are based on some form of temporal logic.
It is important to maintain a mental distinction between incarnations of this concept at the 'model' (or class) level and at the 'information' (or 'object' or 'data') level. For clarity, the former will be suffixed with 'class' and the latter with 'instance' where ambiguity of interpretation is possible.
A TimePoint instance occurs at a definite point in time (see atTime). The actual time of some Event or Action is unlikely to be known until run-time. However, during analyis and design it is necessary to reason about the possible values this attibute might take at run-time. Thus, analysis and design models, the value will normally be symbolic, or else a variable constrained by TemporalRelations.
The concept of class is similar to that of set. Thus a TimePoint class defines the set of possible instances of that class. It is possible for a TimePoint class definition to restrict (explicitly or implicitly) the possible time values of its instances. Such restrictions imply an association between the class and a set of intervals (i.e. segments of the systems time line bounded by a pair of TimePoints) defining the possible values.