Database
[{"_id":1,"timestamp":ISODate("2018-05-27T22:02:07.837+00:00")},{"_id":2,"timestamp":ISODate("2019-10-27T22:02:07.837+00:00")},{"_id":3,"timestamp":ISODate("2019-12-27T22:02:07.837+00:00")},{"_id":4,"timestamp":ISODate("2020-01-27T22:02:07.837+00:00")},{"_id":5,"timestamp":ISODate("2020-02-27T22:02:07.837+00:00")},{"_id":6,"timestamp":ISODate("2020-09-27T22:02:07.837+00:00")}]
Query
db.collection.aggregate([{$project:{month:[{$year:"$timestamp"},{$month:"$timestamp"}]}},{$group:{_id:null,month:{$push:"$month"}}},{$project:{_id:0}}])