MinOccurs 0 and nillable true

<xsd:element minOccurs="0" name="birthDate" nillable="true" type="xsd:dateTime"/>

<birthDate/>

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Setting nillable="true" means that the <birthDate> tag can appear as follows:

<birthDate xsi:nil="true"/>

However, since you also set minOccurs="0", you could also omit the <birthDate> tag completely from the XML and it would also still validate against your XSD.

Note that <birthDate/> or <birthDate></birthDate> is not considered null according to XSD rules.

https://stackoverflow.com/questions/36732264/minoccurs-0-and-nillable-true/36732347