[{"_id":6,"timestamp":ISODate("2020-09-27T22:01:07.837+00:00")},{"_id":2,"timestamp":ISODate("2019-10-27T22:02:07.837+00:00")},{"_id":53,"timestamp":ISODate("2019-12-27T22:03:07.837+00:00")},{"_id":33,"timestamp":ISODate("2019-12-27T22:04:07.837+00:00")},{"_id":43,"timestamp":ISODate("2019-12-27T22:05:07.837+00:00")},{"_id":13,"timestamp":ISODate("2019-12-27T22:06:07.837+00:00")},{"_id":311,"timestamp":ISODate("2019-12-27T22:02:07.837+00:00")},{"_id":4001,"timestamp":ISODate("2019-12-27T22:02:07.837+00:00")},{"_id":321,"timestamp":ISODate("2019-12-27T22:02:07.837+00:00")},{"_id":713,"timestamp":ISODate("2019-12-27T22:02:07.837+00:00")},{"_id":6011,"timestamp":ISODate("2019-12-27T22:02:07.837+00:00")},{"_id":5011,"timestamp":ISODate("2019-12-27T22:02:07.837+00:00")},{"_id":3,"timestamp":ISODate("2019-12-27T22:02:07.837+00:00")},{"_id":1,"timestamp":ISODate("2018-06-27T22:02:07.837+00:00")},{"_id":15,"timestamp":ISODate("2018-05-27T22:02:07.837+00:00")},{"_id":20,"timestamp":ISODate("2018-05-22T22:02:07.837+00:00")},{"_id":26,"timestamp":ISODate("2018-01-22T22:02:07.837+00:00")},{"_id":4,"timestamp":ISODate("2020-01-27T22:04:07.837+00:00")},{"_id":5,"timestamp":ISODate("2020-02-27T22:02:07.837+00:00")}]

db.collection.aggregate([{$group:{_id:{year:{$year:"$timestamp"},month:{$month:"$timestamp"}},total:{$sum:1}}},{$sort:{_id:-1}},{$unwind:{path:"$_id"}},{$sort:{"_id.year":-1,"_id.month":-1}},])