I know there exist a tag for this: "convertDateTime", maybe it could help. This is how it works:
<ui:composition xmlns:s="http://jboss.com/products/seam/taglib"
xmlns:h="http://java.sun.com/jsf/html">
<h:outputText value="#{pivotRow.statusEntryDate}">
<s:convertDateTime pattern="MM-dd-yyyy hh:mm:ss a z" />
</h:outputText>
Here are the specs about convertDateTime
I used it with outputText tag as you can see, but maybe it works in your cell tag.