ESMA has recommended that period elements be presented in the format yyyy-mm-dd and without a time component as a time component is not expected to be necessary in annual reports.
Because of this, you may see an error message specifying that "The xbrli:startDate, xbrli:endDate and xbrli:instant elements MUST identify periods using whole days specified without a time content and time zone" if a time component is included in a period element.