MongoDB: Count item created per year month using ObjectId as timestamp (map - reduce)

mongo_count_per_month

db.accounts.mapReduce(
        // Map function. Convert object id to timestamp then extract year-month
	function(){
		var ac_create_date = this._id.getTimestamp();
		var year = ac_create_date.getFullYear();
		var month = ac_create_date.getMonth()+1;
		emit(year+"-"+month, 1);
	},
        // Reduce function. Just sum up the values from map.
	function(key, value){
		return Array.sum(value);
	},
        { 
           out: "mapred_account_created_per_month", // output result to this collection
           query: { _id: { $gt: ObjectId("555a0c800000000000000000") } // optional: you can remove this. Just addtl condition to query dates greater than object which is just a timestamp converted to object
                  } 
        }
);