If you set a dojo date field to required and set disableClientSideValidation="false", the client side validation is still in control.
Use the XPages source code below to test:
<?xml version="1.0" encoding="UTF-8"?>
<xp:view xmlns:xp="http://www.ibm.com/xsp/core" xmlns:xe="http://www.ibm.com/xsp/coreex">
<xe:djDateTextBox id="date" value="#{viewScope.date}"
disableClientSideValidation="true">
<xe:this.validators>
<xp:validateRequired message="This field is required"></xp:validateRequired>
</xe:this.validators></xe:djDateTextBox>
<xp:message id="msgDate" for="date"></xp:message>
<xp:button value="Submit" id="btnSubmit">
<xp:eventHandler event="onclick" submit="true"
refreshMode="complete" immediate="false" save="true" />
</xp:button>
</xp:view>