Name	Flags	Card.	Type	Description & Constraints
Period	Σ I N		Element	Time range defined by start and end date/time + Rule: If present, start SHALL have a lower value than end Elements defined in Ancestors: id, extension
start	Σ I	0..1	dateTime	Starting time with inclusive boundary
end	Σ I	0..1	dateTime	End time with inclusive boundary, if not ongoing
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