This document specifies a set of representational primitives and semantic relations needed 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 applies to both the representation of actual phenomena occurring in the real world (e.g. registrations in medical records) and to the description of concepts (e.g. medical knowledge bases).This document is applicable to developers of medical information systems where there might be a need for explicit time-related concepts for internal organization (e.g. temporal data bases, temporal reasoning systems), information modellers or knowledge engineers building models for the systems mentioned in a), experts involved in the development of semantic standards on precise subdomains in health care where time-related information needs to be covered, (e.g. in the study of pathochronology, i.e. the discipline dealing with the time course of specific diseases), and developers of interchange formats for messages in which time-related information is embedded. This document is not intended to be used directly for representing what is true in time, reasoning about time, or representation of metrological time.