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

Base StructureDefinition for dateTime Type: A date, date-time or partial date (e.g. just year or year + month). If hours and minutes are specified, a time zone SHALL be populated. The format is a union of the schema types gYear, gYearMonth, date and dateTime. Seconds must be provided due to schema type constraints but may be zero-filled and may be ignored. Dates SHALL be valid dates.