Scope note: This class comprises abstract temporal extents, in the sense of Galilean physics, having a beginning, an end and a duration. Time Span has no other semantic connotations. Time-Spans are used to define the temporal extent of instances of E4 Period, E5 Event and any other phenomena valid for a certain time. An E52 Time-Span may be identified by one or more instances of E49 Time Appellation. Since our knowledge of history is imperfect, instances of E52 Time-Span can best be considered as approximations of the actual Time-Spans of temporal entities. The properties of E52 Time-Span are intended to allow these approximations to be expressed precisely. An extreme case of approximation, might, for example, define an E52 Time-Span having unknown beginning, end and duration. Used as a common E52 Time-Span for two events, it would nevertheless define them as being simultaneous, even if nothing else was known. Automatic processing and querying of instances of E52 Time-Span is facilitated if data can be parsed into an E61 Time Primitive. Examples: - 1961 - from 12-17-1993 to 12-8-1996 - 14h30 - 16h22 4th July 1945 - 9.30 am 1.1.1999 to 2.00 pm 1.1.1999 - duration of the Ming Dynasty