Provides the structure of a Date element, which allows a choice between single, simple dates (of BaseDateType) or date ranges. If the Date element contains a range, Cycle may be used to indicate occurrence of this range within a series of ranges as an integer identifying the cycle, i.e. the 4th wave of a data collection cycle would have 4. ISO-formatted dates are required for all standard date fields; a historically-formatted date may also be provided to reflect the specific content of a legacy document or common practice within an agency or for publication. All uses of DateType should be optional.