making an xsd:date field optional

Hi,

I have a form with an input field which asks for a date (using
xsd:date). However, if it's left empty, the form cannot be sent. How
to make it optional? Here is what I have:

Model part:

<limiteValidite xsi:type="xsd:date" />

Body part:

<xforms:input ref="/Nouvelle/limiteValidite">
   <xforms:label>Date limite de la validite :&nbsp;</xforms:label>
   <xforms:message level="ephemeral" ev:event="DOMFocusIn">Saisissez
une date.</xforms:message>
</xforms:input>

Thanks,

Laszlo