I failed to find a documented solution for this problem. However i did solve it by looping through once the layer is loaded and hiding all the items which did not meet my criteria, as below:
dojo.forEach(tmpLayer.items,function(s) //tmpLayer is georsslayer name
{
var pointDate = new Date(s.attributes['pubDate']); //the graphic attribute to filter
if (pointDate > LastMonth) //if successful symbolise the graphic
{
//here i am using another function to set icon based on attribute value;
s.setSymbol(getCategoryIcon(s.attributes['category']));
s.setInfoTemplate(geoRSSTemplate);
}
else //else hide it
{
s.hide();
}
});