Referenziert ein anderes Element. Pflicht: Die ID des referenzierten Elements. Optional: Ist die Referenz unsicher? Erlaubt: true | false (date) contains a date in any format. supplies the value of the date or time in a standard form, e.g. yyyy-mm-dd. specifies the earliest possible date for the event in standard form, e.g. yyyy-mm-dd. specifies the latest possible date for the event in standard form, e.g. yyyy-mm-dd. indicates the starting point of the period in standard form, e.g. yyyy-mm-dd. indicates the ending point of the period in standard form, e.g. yyyy-mm-dd.