The current date/time classes (data_types.quantity.date_time package) are
defined with a number of data attributes, e.g. DATE is defined to have
year, month and day as separate INTEGER attributes. This is fine for
in-memory representation and processing, but is fairly inconvenient for
persistence, particularly in XML, since it would prevent the use of the
XML standard ISO8601 based date/time types.
A single attribute of type string in ISO 8601 standard syntax would be
more efficient and would have no effect on the semantics of the classes.
ISO 8601 provides syntax for all the date/time types defined in openEHR,
including the partial ones.