The value domain of the metadata elements referred to in points 5.1 to 5.4 is a set of dates. Each date shall refer to a temporal reference system and shall be expressed in a form compatible with that system. The default referencesystem shall be the Gregorian calendar, with dates expressed in accordance with ISO 8601.5.1. Temporal extentThe temporal extent defines the time period covered by the content of the resource. This time period may be expressed as any of the following:— an individual date,— an interval of dates expressed through the starting date and end date of the interval,— a mix of individual dates and intervals of dates.5.2. Date of publicationThis is the date of publication of the resource when available, or the date of entry into force. There may be more than one date of publication.5.3. Date of last revisionThis is the date of last revision of the resource, if the resource has been revised. There shall not be more than one date of last revision.5.4. Date of creationThis is the date of creation of the resource. There shall not be more than one date of creation.