The XML schema defines dateTime ISO8601 with a few exceptions, and you must adhere to this, otherwise you will run into serious compatibility issues. If you want to send / receive the date in a different format, use a simpleTyperegular expression constraint and analyze / format the date in your application code:
<xs:simpleType name="CustomDate">
<xs:restriction base="xs:string">
<xs:pattern value="\d{8}"/>
</xs:restriction>
</xs:simpleType>
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