The reasoning needed for diagnosis and patient management in a medical domain requires the ability to determine both the aspects of ttle patient state that are definitely known and those that are possible given what is known about the patient. This paper discusses a mechanism for including the time constraints of causal relationships in the representation and the increased discriminatory power of the reasoning mechanisms when the time relationships are used appropriately. It is further argued that in such a domain where time bounds are weak there is often more information in the relationships between times than in the time values themselves. Thus, it is often necessary to reason from the relationships rather than by comparing time values. A program utilizing the mechanisms outlined is currently under development.