Force absence of offset in xs:date field "Period"

[tomwetjens]

We should introduce a simple type that forces the absence of offset for a xs:date.

E.g. "Period" is always to be interpreted as a local date in the specified "TimeZone". Any offset information could conflict with this.

  <!-- -->
  <!-- Date Type -->
  <!-- -->
  <xs:simpleType name="DateType">
    <xs:restriction base="xs:date">
      <xs:pattern value="\d{4}-\d{2}-\d{2}"/>
    </xs:restriction>
  </xs:simpleType>