XML/XML Schema/gDay

xs:minInclusive defines a minimum value that can be reached based on gDay

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.wbex.ru" xmlns="http://www.wbex.ru"
  elementFormDefault="qualified">
  <xsd:element name="data" type="afterOrOnThe20th"/>
  <xsd:simpleType name="afterOrOnThe20th">
    <xsd:restriction base="xsd:gDay">
      <xsd:minInclusive value="---20" />
    </xsd:restriction>
  </xsd:simpleType>

</xsd:schema>