default parameters

Does anyone know how to set the default value of a parameter to an attribute
value, like an id, from an element whose date attribute is the most recent
out of all similar date attributes?

For example, I have a group of elements with date attributes...

<div status="archive">
		<section id="default">
			<source volume="7">AIArchitect</source>
			<date>
				<month number="12">December</month>
				<day>Sunday</day>
				<year>2000</year>
			</date>




 and I need to find the most recent date of a group of elements like above,
and set its section id attribute value as the default value of a global
parameter.  Is there any simple way to do this without resorting to
extension functions? I cant figure out how to go about getting the most
recent date...Any ideas?

Thanks


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