Mongo Unique Created At Days

gistfile1.txt

db.getCollection('collection').aggregate([ 
    { $match: { $and: [
            { created_at: { $exists: true } },
            { created_at: { $ne: null } }
        ] } },
    { $project: { 
        month: { $month: "$created_at" }, 
        year: { $year: "$created_at" }, 
        day: { $dayOfMonth: "$created_at" } } },
    { $group: { _id: { "year": "$year", "month": "$month", "day": "$day" } } },
    { $sort: { _id: -1 } }
])