MongoDB aggregation pipeline: group by date (ISO format)

gistfile1.js

db.users.aggregate(
    {
        "$match": {
            created_at: { $gte: ISODate("2014-09-01T00:00:00Z") }
        }
    },
    {
        "$project": {
            _id: 0,
            "isoDate": { $substr: [ "$created_at", 0, 10 ] }
        }
    },
    {
        "$group": { "_id": "$isoDate", "count": { "$sum": 1 } }
    },
    {
        $sort: { "_id": 1 }
    }
).result.forEach(function(doc) {
    print(doc._id + "," + doc.count);
});