I needed to sort a document collection by a date value. I found this XSnippet:
http://openntf.org/XSnippets.nsf/snippet.xsp?id=sort-notesdocumentcollection-by-itemname

And it helped, but assumed the field value was a string. Here's my updated function:
function getCollection(viewName, key, fieldName) {
   var db:NotesDatabase = session.getCurrentDatabase();
   var vw:NotesView = db.getView(viewName);
   var coll:NotesDocumentCollection = vw.getAllDocumentsByKey(key);
   if (coll.getCount() == 0) return null;
   // Sort the collection by date
   var v:java.util.Vector = new java.util.Vector();
   var doc:NotesDocument = coll.getFirstDocument();
   var tm:java.util.TreeMap = new java.util.TreeMap();
   while (doc != null) {
       var temp:java.util.Vector = doc.getItemValue(fieldName);
       tm.put(temp.elementAt(0).toJavaDate(), doc);
       doc = coll.getNextDocument(doc);
   }
   var tmColl:java.util.Collection = tm.values();
   var tmIt:java.util.Iterator = tmColl.iterator();
   while (tmIt.hasNext()) {
       v.add(tmIt.next())
   }
   return v;
}
Note that the return value is a Vector (java.util.Vector) and not a Notes Document Collection. Luckily, in a repeat (where I was using this), the repeat will just go through all the documents whether the return is a NotesDocumentCollection type object or a java.util.Vector type object.