This document specifies a set of representational primitives and semantic relations required for an unambiguous representation of explicit time-related expressions in health informatics. This document does not introduce or force a specific ontology of time, nor does it force the use of a fixed representation scheme for such an ontology. Rather this document provides a set of principles for syntactic and semantic representation that allow the comparability of specific ontologies on time, and the exchange of time-related information that is expressed explicitly. This document is applicable to: 1) developers of medical information systems in which the need is felt to have explicit timerelated concepts for internal organization (e.g. temporal data bases, temporal reasoning systems); 2) information modellers or knowledge engineers building models for the systems mentioned in (1); 3) experts involved in the development of semantic standards on precise subdomains in health care where time-related information need to be covered, (e.g. in the study of Pathochronology, i.e. the discipline dealing with the time course of specific diseases); 4) developers of interchange formats for messages in which time-related information is embedded. This document is not intended to be used directly for: 1) representing what is true in time; 2) reasoning about time; 3) representation of metrological time (which is covered in other standards).