MongoDB: group data by day (group)

mongo_group_by_day.js

db.link_access.group(
{
    keyf: function(doc) {
        var date = new Date(doc.date);
        var dateKey = (date.getMonth()+1)+"/"+date.getDate()+"/"+date.getFullYear()+'';
        return {'day': dateKey};
    },
    cond: {short_id: "N"},
    initial: {count:0},
    reduce: function(obj, prev) {prev.count++;}
});
//http://stackoverflow.com/questions/5168904/group-by-dates-in-mongodb