db.bug.insert({tag: "foo", date:ISODate("2012-02-09T08:39:20Z")});
db.bug.insert({tag: "bar", date:ISODate("2012-02-09T08:39:20Z")});
m = function(){emit(this.tag,this)};
r = function(k,v){return v[0]};
f = function(k,v){v.date.setUTCHours(0);return v;}

// bug: finalize output is not used, dates still have non-zero hours.
db.bug.mapReduce(m, r, {"out":{inline:1}, "finalize":f});

// workaround: make trivial change to finalize output so that m/r framework picks up the change:
f = function(k,v){v.date.setUTCHours(0); v.x = 1; return v;}
db.bug.mapReduce(m, r, {"out":{inline:1}, "finalize":f});

	"results" : [
		{
			"_id" : "bar",
			"value" : {
				"_id" : ObjectId("4f338638400b97e04a51941e"),
				"tag" : "bar",
				"date" : ISODate("2012-02-09T08:39:20.396Z")
			}
		},
		{
			"_id" : "foo",
			"value" : {
				"_id" : ObjectId("4f338631400b97e04a51941d"),
				"tag" : "foo",
				"date" : ISODate("2012-02-09T08:39:13.820Z")
			}
		}
	],

	"results" : [
		{
			"_id" : "bar",
			"value" : {
				"_id" : ObjectId("4f338638400b97e04a51941e"),
				"tag" : "bar",
				"date" : ISODate("2012-02-09T00:39:20.396Z"),
				"x" : 1
			}
		},
		{
			"_id" : "foo",
			"value" : {
				"_id" : ObjectId("4f338631400b97e04a51941d"),
				"tag" : "foo",
				"date" : ISODate("2012-02-09T00:39:13.820Z"),
				"x" : 1
			}
		}
	],