<simpleType name="CalDate">
    <annotation>
        <documentation xml:lang="en">Calendar dates may be indicated with varying degrees of precision, 
      using year, year-month, date. 
      When used with non-Gregorian calendars based on years, months, days, 
      the same lexical representation should still be used, with leading zeros added if the 
      year value would otherwise have fewer than four digits.  
      time is used for a position that recurs daily (see clause 5.4.4.2 of ISO 19108:2002).</documentation>
    </annotation>
    <union memberTypes="date gYearMonth gYear" />
</simpleType>