XML schema construct for "any one or more of these elements but must be at least one"

I'm trying to set up part of a schema that's like a "Sequence" where all child elements are optional, but at least one of the elements must be present, and there could be more than one of them.

I tried doing the following, but XMLSpy complains that "The content model contains the elements <element name="DateConstant"> and <element name="DateConstant"> which cannot be uniquely determined.":

    <xs:choice>
        <xs:sequence>
            <xs:element name="DateConstant"/>
            <xs:element name="TimeConstant"/>
        </xs:sequence>
        <xs:element name="DateConstant"/>
        <xs:element name="TimeConstant"/>
    </xs:choice>

Can this be done (and if so, how)?

Some clarification: I only want to allow one of each element of the same name. There can be one "DateConstant" and/or one "TimeConstant", but not two of either. Gizmo's answer matches my requirements, but it's impractical for a larger number of elements. Hurst's answer allows two or more elements of the same name, which I don't want.