"A time period defined by a start and end date and optionally time." HL7 FHIR, Period This data type represents an Interval, where the Low and High Limits are Points In Time. An Interval is a "set of consecutive values of an ordered base data type." - HL7 V3 A PointInTime is "a quantity specifying a point on the axis of natural time. A point in time is most often represented as a calendar expression." - HL7 V3