Your xs:restriction
element is closed twice. Here's a fix:
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:simpleType name="validdatetype">
<xs:restriction base="xs:date">
<xs:minInclusive value="1900-01-01"/>
</xs:restriction>
</xs:simpleType>
</xs:schema>