"The <calendarDesc> element is used within the <profileDesc> element to document objects referenced by means of either the @calendar attribute on date or the @dateScheme attribute on any member of the att.datable class. "
Except that there is no such thing as a dateScheme attribute sfaics. att.datable.custom does supply one called datingMethod which is probably what was intended, though the name seems a bit odd.
I take this as an indication that we probably need some way of checking all <att> elements in the source. Again.