http://hl7.org/fhir/StructureDefinition/Period|4.0.1

Base StructureDefinition for Period Type: A time period defined by a start and end date and optionally time.

Name	Flags	Card	Type	Description & Constraints
Period		0..*		Time range defined by start and end date/time
start	Σ	0..1	dateTime	Starting time with inclusive boundary
end	Σ	0..1	dateTime	End time with inclusive boundary, if not ongoing