http://hl7.org/fhir/StructureDefinition/artifact-periodDuration|5.2.0

Specifies the duration of the period, based on the starting date, rather than specifying the end date of the period.

element_id	types
Extension
Extension.extension
Extension.url
Extension.value[x]	Duration
Extension
Extension.id	http://hl7.org/fhirpath/System.String
Extension.extension	Extension
Extension.url	http://hl7.org/fhirpath/System.String
Extension.value[x]	Duration